Updated PSA#3     Tue 11/1/2011 3:56 PM

Good afternoon everyone,

I guess I managed to scare a few people yesterday by leaving out the exact dates of this storm (although I did mention that it would have been appropriate weather for Halloween, if we had been in a leap year).

The main reason I am writing this update is that the storm looks a bit more potent today than yesterday, with 3/4-1" of moisture now looking like the most likely outcome from Denver north to Fort Collins and points west, based on higher-resolution models.

It also looks more like a cutoff low at lower-to-mid levels for at least a few hours overnight which should help really bring in some deep upslope flow, say, around midnight to pre-sunrise.

So, I am increasing the snowfall amounts to 6" (further away from the foothills) to 12" (close in, say, Boulder west of Broadway) to 12-18" foothills, despite lower precipitation totals than the last storm, but assuming fluffier snow:moisture ratios. All of this should fall within 12h, say, around 8pm to 8am tomorrow, thus giving us snowfall rates of 1-3"/h at the height of the storm overnight.

There may be enough wind (and snow) to bring down some branches that did not fall off from the last storm, leading to some more power failures in lower elevations. Since this storm starts out colder, and looks colder overnight, I would expect less rain at the beginning of the storm at lower elevations, and a nasty morning rush-hour on Wednesday, November 2nd (for those who are still confused about the timing of this).

The Friday night storm looks still to be more of a mountain snow event than east of the divide (except for a few spill-over showers in the foothills).
There is more in the pipeline for next week, so stay tuned!

Happy Snowy November!

- Klaus
 

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PSA#3 - boo Mon 10/31/2011 5:02 PM

If it's Monday, a snowstorm must be on its way - here we go again!

Another storm is taking aim at Colorado's Front Range, trying for an instant replay of last week's storm, or so it seems. We all know that the sequel is often inferior to the original, so let's see what is different this time around:

1. No complicated double-jet streak structure to complicate (and locally enhance) things;

2. A faster moving storm overall, should be done by late morning;

3. BUT: colder air to work with, so a better chance for getting fluffier snow halfway through the event rather than just at the very tail end (last week's last inch was the only one fluffy for me).

So, similar to last week, I expect to see rain changing to snow after sunset (yes, it started a bit earlier last week) in the plains, with snow commencing right away above 8K, and coming in from north to south.
Peak intensity of this storm should be overnight, with a bit more wind (and upslope flow) to work with. By midnight, everybody should see snow, heavy at times, but also fluffier. Drifting might become a problem on the eastern plains (this one will have an easier time sticking to the roads than the last one). The increased wind speed might be a blessing in disguise for the trees and powerlines, since not as much snow should accumulate on exposed branches and lines. Temperatures will drop below 10F in the foothills and 20F in the plains by sunrise.
Too bad this was not a leap year, then tomorrow would be the 31st of October, and we would have our appropriate Halloween weather...

Snowfall totals: 4-8" in the Front Range cities, and 6-12" higher up.
So, this should end up smaller than the last one, but it could end up lasting longer, since the upcoming week will remain mostly cool and unsettled.

Caveat: this could end up being more of an eastern plains storm, given that it moves along so fast. In that case (the NAM is hinting at that scenario), we would end up with half the moisture/snow indicated above. I will update this PSA by tomorrow afternoon, if I see the drier scenario becoming the likely one (or if it the storm slows down to give us a wetter scenario).

The next storm in the pipeline should come in from the southwest about three days later, but it looks more like a dry (wind) event for us, while the mountains could do well with that one, especially the San Juans, Elk and Park ranges. If this scenario changes towards something more interesting for us, I will have to write PSA#4...

Meanwhile, think snow, we have to milk these active stormtrack situations for all they are worth! At least we are done with October - the La Niña handicap is much weaker in November than in October...

- Klaus

P.S.: The last storm did quite a number on lower elevation trees, especially from North Boulder into Larimer County where power was out for up to three days. There was a heavy snowband that kept reforming from northern Boulder County into Larimer County and created the snowfall map given at:

http://www.crh.noaa.gov/bou/precip/precip_map.php

In case you are wondering, "Ward 4.6 NE" and "3 W Jamestown" both refer to my measurements, the former one is the final correct one, referring to my CoCoRAHS reports. Clearly a decent snowstorm by any standard, just not quite the same caliber as the two big El Niño snowstorms of 1997 and 2009.

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PSA#2 - but... Mon 10/24/2011 5:04 PM
Good afternoon everyone,

unless you live in a cave you must have heard about the incoming storm
that some forecasters have been talking about for close to a week. While
this storm has indeed been predicted for quite a while, the expected amounts
have fluctuated wildly, especially if one includes information from non-U.S.
forecast models (guilty as charged!).

Ahead of the storm, Denver managed to hit 80F today, and establish a new
daily record-high for October 24th. The Boulder records for Wednesday
are 19 and 32F for the coldest low and high on October 26th, along with 4.1"
of snow, while the record low for Thursday is only 21F. Since Boulder reports
from 6pm to 6pm MDT, the record-high for Wednesday will probably not be
broken, even though day-time high temperatures may not reach freezing,
but the record-low on Thursday should be 'easy' to break. So, a pretty
impressive temperature drop is coming our way, time to wrap up the garden(hose),
and get those snow tires before it is too late (day-time tomorrow will still
be warm(ish), with no real precipitation until after nightfall).

I will probably update this tomorrow, but I am not quite as excited about this
one as about some others in the past (see P.S. for more on October-ENSO
statistics). For one thing, it does not look like this one will really form a
cutoff low, and the highest precipitation rates will be a function of complex
jet stream interaction (one polar jet streak and one subtropical jet streak
are supposed to line up just right over our state to reinforce each other).
I also do not like the most recent trend of the GFS to shift the peak precipitation
to our south. Temperatures should get cold enough to allow for snow in the plains
by about midnight, a few hours earlier in the foothills.

Quick bottomline: 3-6" in the plains looks fairly certain by late morning on
Wednesday, twice that in the foothills. Lucky folks under heavier banded
precipitation could see even twice that, but I see that more south of us, similar
to PSA #1 that verified best near Pikes Peak...

More tomorrow,

Klaus


P.S.:

At our house, there have been six October snow storms (allowing
for up to two day-duration) since 1990 that have been truly PSA-worthy
(one foot or more), in 1993 (exactly 1 ft), 1997 (2nd biggest on record for
any storm, with over 4.5 ft), 2002 (over 1ft), 2005 (ditto), 2006 (over 1.5ft),
and 2009 (almost 3 ft). Guess what, only one of them (2005) qualifies to
have happened under La Niña-like conditions, all others had mild (1993)
to strong (1997) El Niño conditions. IOW, the deck is stacked against us.

In Boulder's October record since 1949, there have been exactly two two-day
totals of 6" or more during La Niña conditions: 9.2" in 1971 (40 years ago!),
and 10.7" in 1961 - that's it! By comparison, here are the two day totals for
the six storms listed above: 10.5" (93), 29.8" (97), 12.5" (02), Trace (out of 2.11"
of rain in '05), 4.1" (06), and 23.0" (09). You get the picture, it is a lot easier to
get a big October snow storm under El Niño than La Niña conditions - not
impossible, mind you, but a lot less likely!
------------------------------------------------------------- --------
Updated PSA#2 Tue 10/25/2011 4:11 PM
Good afternoon everyone,

well, so much for La Niña tilting the odds against snow in October - it looks like we are on for quite a storm: the starting shot for this one has already been fired in the higher foothills of Boulder County (about two hours early, if you ask me), moisture appears to be plentiful (after some very low humidities yesterday, this was not guaranteed), and temperatures have already dropped to near freezing just upstream in Cheyenne. To top it all off, Grand Junction is now reporting a thunderstorm in the vicinity (thunder also near Telluride), so we are dealing with a potent situation.

The two jet streaks I mentioned yesterday are lining up correctly (think of two airplanes flying in the same direction, but offset by about the length of a plane sideways and back) to give us double-enhanced lift right over our neck of the woods.

Only problems remaining: how long will take it to cool off the plains to allow for snow, and how long will it last to give us the most La Niña snow in October since 1906 (Denver got almost two feet in that one)?

Good questions: based on most models, temperatures should cool off enough for steady snow/frost in much of the Front Range, but it could change over earlier in heavier showers which appear to be plentiful to our north and west (everything is sagging south). So, it might try to change over a few hours earlier. The same models try to keep the storm going for much of tomorrow. In my experience, if an overnight storm does not taper off by late morning, it could very well last all day and into the evening, since we often get a secondary cold surge to come during the afternoon to keep the upslope going. This is not a likely scenario, nor would it add more than a few inches, but it would have a bearing on the evening rush hour...

So, in a nutshell, here is my forecast for the (1) foothills and (2) Front Range
cities:

(1) Temperatures will drop close to freezing above 8K soon, especially in heavier snow showers, not to get above 32F for at least 36h, if not 60h.
While the ground is still quite warm from the last few days, the snow will start accumulating soon on roadways, especially dirt roads. Intensity will fluctuate, but could become quite heavy at times, possibly exceeding 2"/h in the heaviest bands. Starting out quite dense (10:1 or so), it could turn to powder snow (15-20:1) by tomorrow morning to stretch the total precipitation amount into even higher snowfall totals by the time this all ends.
Right now,
I anticipate everyone to get at least 1" of moisture, translating into about a foot of snow for the lowest foothill elevations from Jefferson County northward into Wyoming. It looks like the most favored locations will get 1.5-2"
of H2O,
or 20-30" of snow. This would still be less than just two years ago (never mind 1997), but quite respectable, and possibly enough to last into November near Peak-to-Peak. Temperatures could tumble down close to 0F, if it does not get too windy tomorrow evening.

(2) Front Range cities will see a change-over to snow from north to south this evening (more or less), with enough rain early on to make it hard for the snow to stick before midnight. So, while the liquid amounts should also top 1" for most of the lower elevations, the measured snowfall totals could vary highly, depending on frequency and measurement surface. Nevertheless, 6-12" sounds like a safe bet at this point, with more a foot not out of the question. This could cause tree (branch) damage on deciduous trees and power outages, in addition to a messy rush hour tomorrow morning. IF it changes over to snow well before midnight, and keeps going at 1-2"/h overnight, school closures are not out of the question, although CU will probably be fairly hardnosed about staying open. As mentioned yesterday, I do expect a hard freeze in the 10sF tomorrow night.

One last 'bookmark', the European model shows a very active storm track from the northwest (yes, an early winter La Niña pattern) that should be productive for the northern mountains, and could give us another dose of winter about 7-8 days from now, just past Halloween...

Enjoy the snow (if you can), and keep those snowfall reports coming -

Klaus

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First PSA of 2011-12 season                      Fri 10/7/2011 5:01 PM

Good afternoon everyone,
 after yet another snow-less September, October is apparently trying to make up for lost ground, possibly gracing the higher foothills with a foot of snow, while threatening the lower elevations with on-and off rain/snow through most of Saturday.

Yesterday's storm system that has already raced north into south-central Canada 'unclogged' a fairly stagnant patter over the Western U.S., dropping three feet of snow on Wolf Creek Pass (!), thus enabling them to beat Loveland and A-Basin for the opening date, using all natural snow for a change.

Storm #2 is on its heels, presently carving out a nice Four-Corner's low, and on track to hit us with a good amount of moisture through much of the day tomorrow.  This is about as far the models agree.  The faster (and drier) models then lift this storm out just to our east, giving us some drying and downsloping winds by tomorrow evening, while the slower (and juicier) ones (like the Canadian and European models) take about 12 hours longer to get us out of the precipitation shield.  The difference is about a factor three in the amount of moisture we get (0.5+" vs. 1.5-2").  At 8K and above, over 90% of this should fall as snow, while the plains may see most of it fall as cold rain, with a few flurries thrown in.  Alright, the worst/best case scenario (depending on your preferences) for the plains is some heavy wet snow in the morning to do a number on deciduous trees and (possibly) powerlines, but I don't think it will accumulate on the roads.  More on snowfall amounts below.

Factors to consider: this one brings its own cold air with it from the Pacific rather than Canada, which means that once it lifts out, the air will be free to warm up again (as fast as it can with all this moisture/snow in/on the ground).  SO, if you live low enough to make it through this without a freeze (or only a light freeze) tomorrow night, your garden should be o.k.

next week.  It has been very dry in the wake of storm #1, so this will probably delay the onset of precipitation for a few hours, but it will also allow for additional cooling as the snowflakes fall into the sub-cloud air.  SO, if it snows heavy enough tomorrow morning, it might have less problems to stay in the form of snow all the way down to Boulder than the weather service is allowing for right now.

Bottomline: While some models start the precipitation right around midnight, I think it will hold back until later tonight.  We all might very well wake up to snow falling from the sky, however, with winter driving conditions above 7K or so.  Even if we only realize 0.5" of H2O, this should be good for close to half a foot of snow in the higher foothills, and that much rain in the plains as well (hard to discern a clear-cut east-west pattern with this storm, since the 'upslope' flow appears to come in from the north to northeast, before switching to the north-northwest in PM in the drier scenario).  If it is still snowing hard by early afternoon, we might settle down to the wetter scenario, with sloppy snow in the lower elevations, but continued moderate+ snow higher up (we could see 12h of 1"/h out of this).  So, the upper end totals could be 12-18" above 8K in Boulder and Larimer County, and 3-6" of a wet mess right against the foothills (measured on lawn tables or decks).  Not really

much more above treeline than near Peak-to-Peak highway, but it will not melt as readily higher up as we get into Sunday and Monday.

 Think snow, a 2nd year La Niña winter tends to be drier in Colorado, especially in October, so this could be the last one for a while -

Klaus

SA#5                                                     Wed 5/18/2011 4:11 PM

Good afternoon, y'all -

 Wow, this May is on track to become the snowiest since 1995 for the higher foothills - snow is already occurring higher up, but should become sustained and heavy for several hours this evening/early night above 7-8K elevation.  Don't have time to go into details, but we have already seen much beneficial moisture today that should get more organized into a brief deep upslope event this evening.  Some models are spitting out up to 2" of moisture this evening, so there could be flooding and mudslides in recent burn areas of Boulder and Larimer County in particular.  Heaviest snowfall rates could exceed 2"/h before midnight above 8K.  Totals could reach up to one foot between about 8.5-9K, and more higher up.  Luckily our aspen have taken their sweet time to leaf out, so I don't expect too much tree damage from this (a 17" heavy snow event on the 17th of May '95 wreaked havoc on our aspen, but they had already started to leaf out that year).
 The storm system will loiter just to our north into Saturday, dumping feet of snow on the higher elevations of Wyoming, as well as swinging around into our northwestern mountains that are already showing (near-)record snowpack - this may add another 1-2" of meltwater to that.
 While snow may mix into the heaviest precipitation bands down to the lower canyons this evening, I don't expect to see a freeze in the plains out of this.

 Believe it or not, another storm is possible about a week from now.

 Cheers,

 Klaus

P.S.: Last week's storm verified nicely with a foot or more in many of the higher foothills.
Nice moisture, too, with 1-2" from Boulder westward, and up to 2.5" H2O to our east.
Taking a serious bite out of our moisture deficit the way things are going...

PSA#4 - last call of the season?!                      Tue 5/10/2011 5:16 PM

Good afternoon everyone,

 four weeks after the last measurable snow in Boulder, we are looking at a very good chance for a repeat performance of getting 1"+ of moisture around here. 

Tough call whether any of this will be in the form of measurable snow in the plains, but the foothills above about 7K should get mostly snow out of this. And, yes, I believe there is the potential for more than a foot of snow higher up!

 After the first 'nice' weekend since early April, with near-record highs in the plains (around 85F), it is hard to believe that the "s...-word" is in the forecast, but this is the Front Range, and we have actually had quite a few such events right around the 10th or 11th of May in the last decade, dropping some measurable snow in the plains, and around a foot in the higher foothills just last year!

 A complicated weather pattern is unfolding as I write this, with a first low passing through last night, just giving us wind and a cool-down, but dropping more than 1" of rain Montana, and helping to set up the 'trick-shot' scenario for the next few days.

A second low got separated from the first one, and is tracking through northern Arizona into Colorado by tomorrow morning, actually getting about as close to the Four Corners as any low I have seen this winter which will set us up for nice deep upslope for much of tomorrow, and an extended period of 'back-wash' moisture on Thursday (especially if you believe the most recent model runs).

There are two main factors that can derail this storm for us, first of all we have not really moistened up behind this storm, running around 12% humidity in Boulder right now (which is better than yesterday) and low 20% RH in the foothills. 

So, we will 'lose' some of the upslope precipitation as 'virga' tonight to moisten up the lower atmosphere from above.  Secondly, while it is an encouraging trend that the models are slowing down the progression of this storm, this also means that it will probably come in a tad warmer than expected earlier.  So, the snowline may end up closer to 8K than 7K, especially if much of this falls during the day-time tomorrow.  If heavier bands of snow set up overhead, even Front Range Cities like Fort Collins and Boulder could see a few quick inches, especially tomorrow morning and evening.

 On the other hand, there is often the potential that the upper low 'kicks out' too fast, but as just stated, the trend in most model runs has been to slow this one down, to the point that it might linger well into Thursday, reducing the chances of this one ending too soon, or even 'skipping over us'.

 Bottomline: clouds should thicken this evening and precipitation should commence overnight, starting out as snow above 8K right around midnight.  If everything goes according to plan (best case scenario: early afternoon NAM model that gives us 2" of moisture), it should precipitate all day tomorrow and into mid-day on Thursday, with a chance for a lull tomorrow afternoon if the low tracks close to us. 

Higher foothills

should see 6-12" of fairly dense snow, with potential for twice as much in that NAM scenario, while the plains have a good shot at 0.5-1" of moisture (a few inches of slush?), again, potentially even more moisture, when all is said and done by Thursday afternoon.

 The weather service has been very reluctant to call for more than a few inches of snow in the foothills until this afternoon's update - always better if they don't get too excited...

 Interesting coda to this:  the low that passed through last night and is currently lifting out from North Dakota into Manitoba may actually dumb-bell around our storm later this week and bring prolonged cloudiness and cool temperatures to the Front Range for our weekend, lest you might get used to 'nice' weekends in May...

 Think precipitation, we still need the moisture!

 Snowy greetings,

 Klaus

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PSA# 3, but three days out!                                                    Fri 3/4/2011 6:24 PM

Good evening everyone,

after a remarkable first decade of days in February, here we are in March ("in like a lamb"), with still only TWO full-fledged PSA's on record.  Similar to a couple of earlier occasions, an interesting storm is shaping up for early in the week, forcing my hand in calling for a tentative PSA #3 between Sunday evening and Tuesday (March 6-8). The scenario may 'sound like a broken record' (still need to find a replacement for that analog-analogy), but the ingredients are indeed the same as previously:

 an upper disturbance is coming in from the Pacific, cold air is at the ready to our north (they are having quite the cold winter up in Western Canada, should have had the Olympics during a La Niña winter), and we will try and mix the two ingredients by Monday to give us (a) enough cold air to generate snow rather than rain in the plains, (b) deep enough upslope, maybe even a cutoff low to our south and west rather than our east, and (c) enough duration (say, at least 12h of decent upslope) to give us the biggest snowstorm of the season so far.

 This far out, lots can go wrong, but I believe the odds are at least 50/50 for a PSA-worthy storm, focusing in on Monday evening when the 'eggbeater'

(technical)term for surface high pressure to our north to generate low-level upslope (northeasterlies) and a cutoff-low to our southwest at the higher levels to generate deep upslope (maybe more from the southeast)) could be in full swing.  The weather service is also going for 50/50, but they just talk about precipitation in general, not PSA-worthy amounts just yet.

 Compared to earlier this winter, a couple of seasonal changes to look for:

 1. With rising sun angles this time of year, it is easier to (vertically) destabilize

air during the day-time, giving us higher potential for high precipitation rates with 'thunder-snow' - not a firm prediction, but a much likelier event than, say, one month ago; so that is a 'plus', if you like snow;  this is also one reason why our Marches typically give us twice or more the amount of moisture than an average winter month;

 2. Unfortunately, March tends to be drier than average with La Niña around, and, despite what you might hear elsewhere, it is definitely still  'around'.   Oh well, that is a 'minus', but at least the relationship is not 1:1 (the last big La Niña in 2008 still dropped 30" in March at our house, below average, but nothing to sneeze at)Compared to El Niño, the odds of this storm slowing down and sticking around for a few days is next to nil, sorry to say.

There is potential for some early-bird snow higher up in the afternoon and evening on Sunday (mixed in the plains, if any).  I would count that towards the storm-total. In any case, here is my 'wet' scenario that would give many of us the PSA-worthy foot (again, I peg this at 50%): Higher foothills snow showers on Sunday evening ahead of the main storm, could drop a quick inch or three above about 7K, more potential for mixed precipitation (or melting snow) below that;  deepening upslope by Monday afternoon, resulting in snow all the way down to the plains by rush-hour (will have trouble sticking on the roads initially), could exceed 1"/h at times that evening, dropping into the 10sF higher up and 20s in the plains;  lingering snow Tuesday morning, allow for 'extra time' in your morning commute; snow totals this far out pretty iffy, but 6-12" are quite likely everywhere, with higher amounts higher up, due to both lower temperatures and higher amounts of moisture; would not be surprised if some of the usual suspects hit 18" (psst, I did not write that). The dry scenario would drop the storm too far to the south (or keep it too far to the north, believe it or not both of these alternatives are possible), and drop, say, half of the above amounts, but we should get at least some snow.

 Will try and update on Monday morning,

 THINK SNOW,

 Klaus

 P.S.: Amuse yourself (if you can wade through some pages of National Weather Service output), and look at:

 http://www.crh.noaa.gov/product.php?site=BOU&product=ZFP&issuedby=BOU

 The zone forecast for the plains cities (Boulder/Denver = Zone 39&40) talks about highs near 40F on Monday, with a 50% chance of rain or snow. The zone forecast for the northern foothills (6K-9K in Boulder and Larimer County = Zone 35) gives exactly the same probabilities for rain or snow (50%), with a high between 30 and 40F.

 The zone forecast for the mountains above 9K to the west (Zones 33&34) goes for "likely" snow (60%), with temperatures on Monday between 29 and 39F.

 I did not know that a 1F temperature shift (30-40F vs. 29-30F) is enough to remove the word "rain"

from the forecast.  As far as probabilities are concerned, 50% for the plains this far out is fairly 'gutsy', while 60% for the mountains strikes me as fairly 'timid', my 2c.

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Less snow, more cold (tentative PSA#3 part2)   Mon, Jan 31, 2011 at 10:25AM

Good morning, everyone,

some updates to the unfolding cold snap:

1. Freezing drizzle this morning was a consequence of some shallow
cold air 'sneaking in' early, and some seeding from above ahead of
the main front. It should be done for this event. Hard to forecast,
especially three days out.

2. This arctic airmass is strengthening (cooling) as it is moving
south from Canada. There were lows in the -30sF in Alberta
this morning (Havre, MT, hit -31F), right upstream from us.
I expect our temperatures to remain steady at best, but mostly
continue their drop to hit below 0F shortly in the higher foothills,
and by later this evening in the plains. Looking at Boulder's daily
records, both highs and lows in the next two days have a chance
of beating these (especially for the low tomorrow night). I fully
expect to see subzero highs everywhere tomorrow as the coldest
air will sit right over us. How low can it get? Depends on how
much clearing we will get before the winds kick up again as the
arctic 'blob' slides to the south of us. Boulder should get down
to -15F (tied record low for the 1st), while the foothills (and Longmont)
will probably see -20sF. Back in 1989, Boulder got down to -24F
on the 5th, and the higher foothills dropped into the -35/-45F
range. I don't think we will see lows that low, but it should be
the coldest February temperatures since then, maybe hitting -30F in
some 'favored' locales (Nederland, for instance).

3. While the temperature forecast is more or less in line with the
weather service forecast, I am more pessimistic about snowfall totals,
anticipating 2-6" tops, with most places getting less than 4". <The
weather service goes for 4-9" in Boulder and 6-11" in the foothills
by tomorrow afternoon>

Therefore, I won't call this PSA#3 after all, and reserve that designation
for the next PSA-worthy storm, possibly as soon as next week.

Winter is definitely not done with us yet, hope your water pipes are
well insulated, this will beat most cold snaps we have seen since
December 1990, with higher foothills possibly staying below 0F for
about 60h -

Let's hope I am underestimating the snowfall totals, too -

Klaus

PSA#3 (tentative)                                                    Fri 1/28/2011 8:52 PM

Good evening everyone,

thought I would indulge in a medium-range forecast here since the onset of the next storm will be on Monday (may or may not be able to update then).  Basic setup is classic for mid-winter cold&snowy spell: arctic air from Canada is on its way from the north, with some energy diving actually west of the divide into the Great Basin early next week.  Our neck of the woods will see arctic slosh all the way to the divide by Monday evening.  Latest model guidance (NAM, this evening) shows the coldest air of the season banked against the foothills by Tuesday morning (well below 0F, even in the plains).  Of course, this being 3-4 days out, nothing is guaranteed, but the 1st of February has a history of witnessing such arctic airmasses, for instance back in 1985 when our state record was set in Maybell (west of Steamboat) with -61F (no, I don't expect a new record). If the NAM is correct, the cold air will come in so fast that we won't see a long period of precipitation, but rather just plain bitter cold temperatures, with maybe 1-3" as the cold air comes in.  The more interesting scenario would slow the arctic a bit down to give us some 12-24h of light to moderate snow that could result in 4-8" of snow up and down the Front Range from about Monday afternoon overnight into Tuesday. (and yes, it could be more than 8" for some lucky folks, such as Boulder, yet again). 

Stay tuned, I will try to update this on Monday (or yank the "3" if the drier scenario looks more likely) -

Klaus

P.S.: 1. Last PSA verified better in Boulder than higher up, but it was a snowier storm for many of us.

2. The overall pattern should remain 'fertile' after early next week, leading to one or two more cold surges in the week after that, with potential for PSA-worthy amounts.

PSA#2: La Niña mid-winter action!                                              Fri 1/7/2011 4:43 PM

Good evening everyone,

looks like a replay of our last storm is shaping up for Sunday into Monday, with a similar temperature drop, and similar amounts of snow.  This far out, my forecast is (even) less certain than usual, even though the overall precipitation amounts are quite similar in different models.  However, the timing of the onset is amazingly different in between the NAM and GFS (U.S. models), starting the snow either before sunrise on Sunday, or holding back until sunset.  Since easterly upslope should be in place by Sunday morning, I am siding with the GFS, which would give us a longer period of snow altogether, also starting at comparatively higher temperatures (don't worry, this one should be all snow just like the last one even in the lowest elevations).  Similar to PSA#1, the snow should continue into the night, possibly even Monday morning, whilst temperatures should drop to below 0F in the higher foothills, and below 10F in the plains.  The other main uncertainty about this storm is how long the arctic air will linger in its wake, or, to be more precise, how far east the cold air will erode during the workweek.  Coldest night in the foothills should be Monday night (flirting with -10F yet again), while the plains might break 0F both Monday and Tuesday night.

 Total amounts should be similar to last time in the lower elevations (4-8"), while I believe that the foothills will do better, IFF the early onset scenario verifies, since this would extend the stretch with favorable temperatures for dendrites (yes, the dreaded change-over to arctic needle snow is still on track for higher elevations by Monday morning).  SO, I am going to stick my neck out and go for 6-12", say, between 6-9K elevation.With decent arctic air pooled to our north for what looks like the next 10 days or so, I would not be surprised to see another PSA-worthy storm on the horizon by next weekend, and, yes, as I wrote last time, we are currently in that mid-winter stretch where La Niña favors snow even in the Front Range. 

Think snow, and get your cars washed by tomorrow...

 Cheers, Klaus

 P.S.:    Postmortem #1: Boulder got nailed with more than 8" of fluff, while the foothills ended up with 3-6" in Boulder and Larimer County.  Temperatures dropped to near 0F in Boulder, and below that pretty much everywhere else. In fact, combined with winds gusting to 40mph or higher, New Year's evening's temperatures from -5 to -10F in my neck of the woods reminded me of a Wisconsin winter day.
--Klaus Wolter
University of Colorado-
CIRES Climate Diagnostics Center NOAA-ESRL
Physical Science Division R/PSD1       
325 Broadway
Boulder,  CO  
80305-3328
Phone/Fax/e-mail: (303) 497-6340 / -6449 / Klaus.Wolter@noaa.gov

First PSA of 2010-2011 season         Wed Dec 29 2010 5:11 PM

Good evening everyone,

the weather service just upgraded their winter storm watch to a winter
storm warning, despite only predicting 5-10" for the Front Range, no
doubt partially due to the lack of snow so far this season. In fact, if you
like records, the current seasonal total of 3.3" in Boulder would have been
on track for the lowest snowfall total through December, since the old
record is 5.0" (back in 1910), but it appears that we should pass that
benchmark by this tomorrow...

A storm is poised to hit our state over the next 24-36h that would have
all the ingredients of being just as big as the recent East Coast storm,
IF it would only slow down a bit. It looks like we will briefly see a Four
Corners low, deep upslope all the way to the Continental Divide, and
brutally cold air invading our state, but the window of opportunity for
decent snows around here will close just about as fast as it opens
tomorrow. Snow flurries will tease foothill dwellers probably even before
sunrise tomorrow, but the main upslope surge and more serious snowfall
rates may not kick until after noon tomorrow. Since temperatures will
drop dramatically from noon until midnight tomorrow (hitting subzero
readings higher up by then), whichever amounts of snow will fall should
stick everywhere, not just 'grassy areas'. SO, while we may only see
a few inches by this time tomorrow, driving should be a lot more challenging
in the lower elevations than anything experienced so far this season.

While I don't expect to see more than, say, 3-6" (4-8") in the plains (foothills)
by late evening tomorrow, what may drive the total amount up by Friday
morning is that there may be a period of fluffy powder snow accumulation
overnight, as arctic air keeps pushing up at the Front Range. Higher foothills
will probably transition to dense arctic needles that don't add up to more than
0.1" per hour. By Friday morning, someone in the lower foothills may very
well end up with a foot of mostly fluff (0.5", give or take a 1/10", of moisture),
while higher foothills may not get more than places like Boulder and Golden
(say, 6-10"). Temperatures will reach the lowest values of this season so far
either tomorrow night or New Year's night, below 0F pretty much everywhere,
with -10F quite possible in the usual cold spots (Longmont, Nederland, etc.).

It will take at least a week if not longer before we will see temperatures as warm
as today, so this will be a very timely reminder of what season we are in.

Happy New Year, and THINK SNOW, if only to curb the fire danger in the lower foothills
that is still crazily high,

Klaus

P.S.: As best as I can remember, this is my latest PSA#1 of the season
ever (oh, back to 1992, or so), a sign of the times. I need to track down
what my lowest count for any season was, since we may have a shot
at that one, too.

P.P.S.: This should be a good snowmaker everywhere west of the Front Range
as well, if you are planning on skiing, just allow for very wintery temperatures
(-20F at night should be an easy target by New Year's eve in pretty much all
ski resorts, with highs in the single digits both New Year's eve&day).

P.P.P.S.: January is typically the best month for enhanced snow chances (almost)
statewide under La Niña conditions, so I fully expect to write a PSA or two over
the next five weeks, especially since I will be traveling again...

PSA#10 - at least for the higher foothills             Mon 5/10/2010 4:12 PM

Good evening everyone,

 looks like we will get yet another snowstorm to hit us from Tuesday into Wednesday, with enough cold air even for this late in the season to see rain changing to snow even in the plains overnight.  There were a few forecast runs over the weekend that showed liquid totals in excess of 3", while today's runs have backed off somewhat, with a 50/50 chance of still getting at least an inch of moisture, while some of the high-resolution models still give the foothills a shot at 2"+ of moisture.  Don't have time for details here, but tonight's model update will be critical, since the storm is just now coming onshore, so that we will have a better idea about the exact track of this storm by tomorrow morning.

I won't be able to update this until early afternoon tomorrow, but here are the two most extreme scenarios:

 1) Low-end, majority-of-large-scale models track this storm a bit too far north, giving us less moisture (0.5-1.3") and/or higher temperatures (the latest GFS is still fairly dry but colder again), which would give the plains just a few inches of snow at best, and the higher foothills (especially of Boulder and LARIMER County) somewhere between 8-16" of snow by Wednesday afternoon;

 2) High-end, minority of models, but consistent with El-Niño-typical spring-behavior, including several storms so far this spring would dig this storm a little further south before lifting out, giving us some 12h of heavy precipitation (2"+/h of snow possible all the way down to the lowest foothills, but probably not in town) that could drop 20-30" (2-3" H2O) on a few lucky spots - the highest May snowstorm totals since at least 1995, if not 1983.  But, as the weather service has just announced, 4-8" is a real possibility in Boulder and Fort Collins as well, which would lead to some serious tree damage (more than 1" water content) even down here, although the best chance for that is in the lower foothills were trees have just leafed out.

 Stay tuned, this one bears watching, and, yes, there is a real risk of damaging frost in the wake of this for much of the week (which should remain 'unsettled' - showery and cool thru at least Friday, if not Saturday).

 More tomorrow,

PSA#9                                                        Thu 4/22/2010 5:00 PM

Good evening everyone,
 Interesting storm on its way, lots of moisture in the air, and the weather service as hoisted a winter storm watch for the foothills.  Was in a meeting all week, but this one does remind me of April '99, and last night's widespread rain event has set the stage for a good amount of moisture at the ready.  It being late April, the climatological odds for snow in the plains have dropped off considerably, and this storm is not particularly cold.  On the other hand, with some of the precipitation rates observed last night, we managed to change over to snow near 8.5K both last night and this morning, well ahead of schedule.

Since I only have a few minutes to put this up, I will be uncharacteristically brief: I expect to see another 1.5-3" of rain by Saturday morning in the plains, with the potential for brief change-overs to snow in the heaviest showers, and towards the end of the storm as colder air gets entrained from the north and west tomorrow night.  The higher foothills will probably see mostly snow out of this from here on out, but with temperatures near or above freezing, it will have a hard time sticking for the next 24h.  Nevertheless, I expect to see 6-12" of wet snow by this time tomorrow, to be followed by 'better snow' in the subsequent 12-18h (Saturday morning), so some higher foothill folks could see a total around two feet of snow by then.  I will update this tomorrow, if this forecast becomes too pessimistic/optimistic by then.

 Think snow, we had such a productive season so far, we can't just end it with a whimper!

 - Klaus

 P.S.: A clipper-like disturbance will be dropping in from the north around Monday for smaller amounts that could make it down to the plains, and a second big storm might drop in towards the end of next week, again, reminiscent of April

'99 when

we got three big storms in a just over two weeks...
 

PSA #9 - yes, we can have two PSA-storms within five days!
Mon 3/22/2010 10:28 PM

Good evening, y'all,

 I was holding out until the last possible moment, but I now have to follow the lead of the weather service and issue yet another PSA, after hinting at it already in my last PSA.  The track of the incoming storm is a bit more from the west this time, the involved airmasses will be not as chilly as last time (no subzero lows in the offing), and there is a bit more uncertainty still left in the outcome.

 The surface wind has shifted to the north(north-east) in Boulder just in the last hour, and we are starting to recover from another mid-day 10% humidity, currently at 34% in Boulder, so still a long way to go.  Radar echoes have started to show up (mostly virga) in southern Wyoming, where the storm will probably get going in just a few hours.  Given the jetstream flow from the west-southwest, it will take quite a while, probably more than 12h from now (10pm on Monday) before it will make significant inroads into Colorado, but Larimer County should definitely get hit first, possibly quite hard already by, say, noon tomorrow.  The core of this storm will occur in the PM hours of tomorrow, and it is possible that we will get higher precipitation rates away from the foothills than last time.  Since much of it will fall during daylight, it may have a hard time sticking to roads etc., even though it should change over to snow the moment soon after it gets going (in the plains).  Foothills should see an earlier onset, since it will take less time to moisten up the air underneath the clouds tomorrow morning.

Models are producing similar amounts to what was advertized last time, but with much less of a focus on the foothills, but rather along the I-25 corridor.

This is due to the fact that this storm will produce a cutoff low that will form either overhead or just to our south and east tomorrow afternoon (about 100-200 miles off the last cutoff during its brief lifetime).  There is still the possibility that this cutoff process could occur too late/too far to the east to end up reducing our totals, but even then, we should be good for at least 0.5" H2O, while the high-end scenarios could drop between 1-1.5" of precipitation up and down the northern Front Range.

Bottomline (I need to get going): I expect to see reports of 12-18" (yet

again)

in the foothills by Wednesday morning, but will more water content than last time - I don't expect to see the huge snow:water ratios of the tail end of this storm as we had last time (50:1 or higher was recorded in Boulder and my place by Friday evening); Front Range cities will have a fairly long inefficient period of wet&partially melting snow thru, say, 4pm on Tuesday, cutting down total amounts - this time I would be surprised to see snowfall reports over 12" in the plains, so, let's go for 6-10" down here.

 I was too optimistic about the snowfall chances west of the Divide with the last storm, it got hung up over Elk Range (yet again), but we should see at least some snow west of here, but the brunt of this storm will end up in eastern Colorado yet again.

 This is a very active stormtrack, don't be surprised if we get another storm around Saturday.  I may not be able to send out another PSA before then, since I may be engaged in some in situ sampling of snow conditions around the state (it is spring break, after all).

 Think snow, in fact, let's go for some seasonal records over the next few weeks/ months - Boulder 'only' needs another three feet or so (more like 10 ft. for our house) to break their 100-yr record -

 Cheers, Klaus

P.S.: 20.4" at our house (and 14.6" in Boulder) were higher than expected, although it did reflect some very fluffy dendrites indeed.  There were several 24"+ reports in Jefferson County, yet again the bull's-eye of a snowstorm, consistent with behavior established since December '06...

PSA #8 - a good shot at a foot for everyone, but not a classic El Niño spring-dum

Good afternoon, y'all,

after last weekend's dud (for all but Jefferson County foothills), a well advertised storm is dropping through Wyoming right now to bring us a reminder that our snowstorm season is far from over, despite our brief flirtation with spring warmth this week.

Our winds have already shifted around to the northeast, and should blow from that sector for the next 24-36h, cooling us below freezing in the plains by midnight or earlier, only to continue dropping (or barely remain stationary) for most of tomorrow (the coldest air is being tapped all the way from Canada and will take some time to make it here).

There is plenty of low-level moisture available to our northeast due to the melting snowpack over the western high plains, plus it looks like the storm will tap a little bit into a moisture feed from the eastern subtropical Pacific.  Both a departing jet streak to our northeast and an incoming one from the northwest will help with giving us a bit of extra lift, and there may be brief periods when the mid-level circulation (just above mountain level) will try to cut off to slow things down a bit, but we won't get into a classic Four Corners' Low situation, so a repeat of the big mid-March storms of 2003 or 1998 is not in the cards, alas.

Models have been fairly consistent about spitting out at least half an inch of H2O over us, with some of the finer resolution models giving the foothills an inch or more by tomorrow evening.  This seems reasonable for this time of year, and we have seen plenty of such March storms in the past, irrespective of the ENSO phase. The storm will draw in some pretty cold air for this time of year which will assure all snow when things get serious after midnight even in the plains, and should be good for some good dendrites in the second half of the storm, tomorrow afternoon.  Once we clear out tomorrow night, we could see subzero lows in the higher foothills, and close to 10F in the plains.  If soccer games are not canceled on Saturday, the kids will be pretty miserable, especially if we keep some clouds around for much of the day.

How much snow do I expect?  Fort Collins might get enough snow overnight to make the morning commute 'interesting', although paved roads have had a couple of days to soak up the heat from the first spell in the 60s of the year.  The evening rush hour should be pretty bad everywhere, since temperatures will drop well below freezing by then, even though the heaviest snow might be winding down by then.

All totaled, I would give Fort Collins 6-10" by tomorrow evening, Boulder 8-12", and the higher foothills 12-18".  Just enough to catch us up to near-normal for March...

Think snow -

Klaus

P.S.: Next storm is due to come in around Wednesday next week, has a 50/50 chance of becoming PSA-worthy...

P.P.S.: With all the dynamics and some Pacific moisture being entrained, this storm should be good for all mountains in Colorado, favoring the northern and central mountains over the southwestern mountains (Telluride and south).  IOW, a bit of an equalizer after this season...

PSA #7 - just barely                                                                Fri 2/19/2010 5:15 PM

Good afternoon, everyone,

 After yesterday's 1-4", and current flurries (in Boulder), I have decided to send out a PSA, even though it will take about three days (Thursday into Sunday) to come up with snowfall totals approaching a foot somewhere in the foothills of Larimer (most likely) or Boulder County (2nd best bet).  We are in the middle of an overrunning precipitation event where low-level upslope flow is keeping us in relatively cold air, while moist Pacific air is being brought in aloft with fairly strong west-southwesterly flow.  After yesterday's 0.1-0.2" of moisture, we can expect to see another 0.3-0.7" by Sunday morning in one of those drawn-out snowfall events that have been somewhat characteristic of this winter.  Both the low-level upslope flow and the upper-level flow are not particularly strong, so I do not expect to see "heavy" snowfall at any point (say, more than 1"/h for more than one hour), but we might very well get 1/4"-1/2" per hour for up to 24h from early Saturday thru early Sunday. 

The weather service arrived at the same conclusion in its afternoon update, and points out (correctly) that low-level flow that might veer to the southeast by tomorrow would be more favorable for Larimer County than points further south. They also favor the foothills over the adjacent plains, which reflects better lifting and closer proximity to the divide for the spill-over moisture aloft. Temperature should remain cold enough for all snow at all elevations, but temperatures may be close enough to freezing in the plains to allow for some day-time melting tomorrow. The coldest air of this event should reside over our region on Sunday, as the whole system (very slowly) drifts to the southeast (leaving southern Colorado under the threat of more snow all the way into Monday, while our chances for the white stuff should  drop off considerably on Sunday).

There is some remaining uncertainty about precipitation amounts due to upper level jet streak dynamics that could create some heavier snowfall bands, but I am not too hopeful that this will really pan out.  If you do get one of those heavier bands that could last more than 1h, consider yourself lucky. If we clear out Sunday night, we could easily see another subzero night in the foothills, if not on the plains.

 Watch out for the next PSA, we are getting closer to March when El Niño can dish out some big ones (such as in 1992, 1998, and 2003)...

 Think snow, oh, and this one looks good for all Colorado mountains, but (as usual) especially in the San Juans, Elk Range, and towards the end, the Sangre de Cristos.

 - Klaus

 P.S.: The last PSA verified a bit on the low end, but Boulder (5.9") and yours truly (7.6") ended up close to what was expected.

PSA #6 (just barely)         Fri 2/5/2010 5:35 PM

Good evening everyone,

it has been more than six weeks since my last PSA - blame these doldrums on El Niño!  In any case, I may be tempting the snow gods by contemplating the possibility of a foot out of the upcoming storm, but the combined effect of snow and cold will be quite a change from our benign weather of the last four weeks in particular.

A rather complicated storm system will be setting up on Sunday and Monday, and I don't have the time to really do it justice.  In short, one upper level low pressure system will slowly drift eastward across the Four Corners region, while a second such low will drop southwestward (!) from Canada into the High Plains to eventually absorb the first low, but not before giving us a prolonged period of low level easterly flow along with falling temperature to levels not seen since early January, if not early December (yes, I am talking the possibility of sub-zero lows even in the plains).  Ahead of the first low, snow showers will form in the mountains west of here to give most ski resorts a modest dusting on Saturday, and it will be interesting to see how much of that moisture spills over into the foothills by Saturday night (it would be an early indicator of good moisture availability if it does).  As the first, Pacific low moves across the Four Corners, the San Juans should be good for yet another good-sized snowfall to keep up their (above-)average snowpack on Sunday, while our neck of the woods should see lowering clouds, slowly dropping temperatures, and the onset of continuous snowfall by early afternoon, the latest on that day.  There is still quite a difference between different forecast models about the intensity of this first round of snow for us, but all of us should see at least two to four inches by Sunday evening, with temperatures dropping into the 10sF in the plains and +0sF in the higher foothills late on Sunday.  The 'best' case scenario would involve getting early spill-over already on Saturday, and fluffy snowfall ramping up to 0.5"/h by the afternoon, leading to 6" or more by late Sunday (in the foothills).

 Monday will be very wintery around here, with Canadian air oozing in from the north all day, and steady light upslope snow continuing most of that day (again, there is still quite a range in the models on how much could fall, but another two to four inches are quite likely for all of us.  Temperatures will struggle to rise at all on Monday, most likely staying in the 10sF in the plains and maybe reaching 10F higher up.  There may be lingering snow into the evening rush hour, but amounts should steadily decrease by then.  By Monday evening, I expect to see at least 4-8" in the plains (probably least in Larimer County and more towards Denver), with 6-12" in the foothills. Monday (and Tuesday) night will be the coldest since early January, probably coldest the first night in the higher foothills (they will clear out sooner than below), and coldest the second night in the plains.  I expect to see lows near 0F in the plains and down to -10F higher up by Wednesday morning, nothing too unusual, but a reminder that winter is still with us.

 It will remain somewhat cool and unsettled in the mountains for the next two weeks (but overall drier than in many other Februaries), while the eastern plains will probably see a return of the mid-winter 'El Niño snooze' before the real action resumes in March...

Think snow, and stay tuned for my next PSA,

Klaus

PSA#5 - A wintery Xmas week to round out a wintery December  Tue 12/22/2009 10:11 AM

Good evening, y'all,
Yes, there is snow in the offing, especially on Wednesday, with potential for flurries through Christmas Day, but I am getting ahead of myself.  Similar to the early December event, this storm is dropping in from the northwest, dragging in Canadian air (not the ultracold variety, but cold nevertheless).  There are some issues with the details of this storm, as usual.  For instance, the onset on Tuesday evening is somewhat uncertain, due to the dry air that has been created by several days of downslope winds, and due to the possibility that the upper air circulation will probably drop a bit too far south so as to preempt us from getting a full-blown Four Corners Low event.  On the other hand, we should see a reinforcing surge of the coldest air on Thursday that might give us a total of up to 72h of more or less northeasterly (upslope) flow, as the cutoff low (that tracks too far south and then east of us to give us a direct hit) wobbles from New Mexico northeastward into the Great Plains.  <The Newsmedia will probably go bonkers over this blizzard for parts of Kansas, Nebraska, South Dakota, etc., right around Xmas; for a brief moment, the lower 48 states will have a very impressive snowcover before the eastern seaboard melts out again>.

In any case, here is the bottomline for us: snow should begin tomorrow evening, first in the mountains west of here, then the foothills, and finally the plains.  Heaviest snowfall rates (probably not in excess of 1"/h) can be expected during the first half of Wednesday.

By noon on Wednesday, I would expect to see 4-8" in the foothills, and 2-6" in the plains, with off and on snowfall continuing for another two days or so, becoming arctic snow (ice needles) in the foothills by Thursday (i.e., denser snow after the first round of fluffy snow on Wednesday).  Coldest temperatures can be expected around Thursday night when most of us should see sub-zero lows (yet again).  Daytime highs on Thursday and Friday will probably struggle to get above 10F in the plains and 0F in the foothills (yes, comparable to earlier this month, but now the anomalies are not as extreme).

When all is said and done (Friday), someone will probably report close to a foot of snow (could be the lower foothills as for PSA#3), but most everybody should have received 4-6" to make for a picture-perfect Xmas setting.

Looks pretty boring beyond this storm into 2010, for now, more on that next week,

Happy Solstice!
Think snow,
Klaus

PSA#4 - Belated Update               12/7/2009 10:14 PM
Quick update, despite a lot of hope for this second 'storm', it looks like it is setting up with too much southwesterly flow aloft to be of benefit for most of us. A few inches is all I can promise by tomorrow evening, but it will remain cold into mid-week.
In fact, this is easily the coldest beginning stretch of December since 1972, quite a change from November. It will probably take more than a few days to warm us up back to normal, so whatever snow is on the ground by tomorrow evening will not melt anytime soon.

Got to run,

Klaus
 

PSA#4 - Saturday thru Tuesday evening (5-8 Dec'09) Fri 12/4/2009 6:01 PM

Good evening everyone,

 More winter weather is on the way, with snow mostly from tomorrow evening thru Sunday evening, and Monday evening thru most of Tuesday, the latter being iffier than the first one.

 Temperatures will 'peak' tomorrow morning, with highs above freezing on the plains after a mild and breezy night, only to drop back into the deep freeze by Sunday morning.

 A storm system quite similar to the last one is on track to reach us tomorrow evening, and it should be good for a prolonged light-to-moderate snowfall period of at least 24h.

Depending on the amount of drying with downslope flow ahead of it, it may take quite a while (again) to saturate the air above us before it can start snowing (earlier in the foothills than in the plains).  But this also means that it will cool off sufficiently to be all snow at all elevations.  If the snow commences well before midnight tomorrow, I could see getting 3-6" by Sunday morning in the higher foothills, and 2-4" in the plains, while a later onset could easily cut these amounts in half.  The weather service inexplicably predicts only 1-3" & 1", respectively, even though its own discussion mentions 3-6" by the end of Sunday (http://www.crh.noaa.gov/product.php?site=BOU&product=AFD&issuedby=BOU ), oh well.  It should snow all day on Sunday, but amounts may be hindered by the fact that we should be enveloped in arctic air by sunrise, meaning that we will see less and less dendrites (and more dense ice needles) during the day.  On the other hand, the upslope

(easterly) flow should last pretty much all day, making up for lost intensity with duration compared to the last storm.  All in all, I believe we should see anywhere from 2-5" in the plains in the driest scenario to 6-10" almost everywhere for the wettest scenario.  There is more of a southeasterly component to this upslope which should be of greater benefit to Fort Collins and surroundings than the last storm.

 Lowest temperatures of this event should be reached Sunday night, with another round of subzero lows possible in the plains and foothills, although the air may not get still enough to allow for the sub-minus 10F lows that we saw in the higher foothills yesterday morning.

 The more interesting storm will approach us on Monday with good moisture from the Pacific to mostly benefit the mountains west of here (finally!), especially towards the San Juans, and the potential for significant spill-over moisture for us, depending on the depth of the remaining arctic air that may get held in place by high pressure to our north, and/or whether the actual storm tracks thru central/northern Colorado, or slightly to the south which could give us the additional benefit of deep upslope flow on Tuesday.  Right now, the range of potential outcomes is quite large, but the wetter scenarios could easily duplicate or exceed the amounts that we will receive over the weekend.  The wetter scenario would also imply that we get more or less continuous snowfall (with the least amount of snow on Monday) from Saturday evening thru Tuesday evening!

 Stay tuned, I will update this on Monday when things are (hopefully) clearer.

Think snow, these next two storms could get most of us up to a 'normal' December snowfall total, if not higher!

Cheers,
Klaus

P.S.: PSA#3 verified pretty well, except for Fort Collins, sorry to say.  Most of us received from 4-8" snowfall totals, with at least one report (thanks, Rich!) of 12"

total just west of Boulder, and

the coldest (often sub-zero) lows of the season yesterday morning in the foothills, and this morning in the plains. These temperatures were on the low end of what is possible this early in the winter, and also compared to other El Niño winters.  In a few months, we may well look back at this early December cold snap and find that it was the coldest of the season (this is not a promise,

however).   However, I can promise that it will warm up significantly

once we get past Wednesday!

PSA#3 - marginal snow, but drastic cool-down in the works! Tue 12/1/2009 5:45 PM

Good evening, everyone,

 it has been quite a while since our last significant snow event in mid-November, but it looks like this will be remedied overnight as a decent postfrontal band of snow has developed over Wyoming today that looks poised to hit us later this evening with at least 12h, if not 24h of light to moderate snowfall.  Temperatures should drop by at least 25F tomorrow, and quite possibly another 5-10F by Thursday when we might also get a secondary surge when the coldest air gets in here.  Especially if it clears up tomorrow night and especially Thursday night, everybody has a shot at sub-zero lows, with 'more favored'  locales hitting -10F before it's said and done.  BTW, for an El Niño situation, this would be about as cold as we can expect to get all winter!

 How much snow will we get?  We are starting out with some pretty dry air, and it looks like the upslope level of this event will be confined to the lowest 3-4K ft., so not the kind of deep upslope we enjoyed during the last two PSA-events in late October and mid-November.

There is going to be an overnight period when the airmass will be good for dendritic growth (-10 to -15C), so this snow could be quite fluffy towards the morning, and it will start out as snow even in the lowest elevations.  There should be lingering snow into Wednesday evening, and a second round of (lighter) snow on Thursday, probably around sunset / rush hour.  I mention the timing since the air will be cold enough by that time (low teens F in the plains / near 0F in higher elevations) to render the usual magnesium-chloride treatment useless, if not dangerous (nothing like hitting icy roads when you don't expect it).  If we can get the snowfall going this evening (rather than early tomorrow), I would expect to see 4-8" overnight for the foothills, and 3-6" in the adjacent plains, with an additional 1-2" by tomorrow evening, and another 1-2" by Thursday evening, so some lucky foothill dweller (with a good eastern exposure, probably below

8000') could get

a foot of fluffy snow (maybe, 0.5" of moisture at best), while almost everyone should see at least 4-6" out of this.  If the upslope is too weak thru this event / moisture too slow to get in here, cut these amounts by about a third (if you can see the moon past midnight, we get into that scenario), but still expect to see the cold air, with highs in the 10s F most of tomorrow / all of Thursday, close to the daily record low high temperatures for this time of year.

 There is more snow and cold in the pipeline for the weekend / early next week, but the models are still wrestling with the details, since the timing and interplay of another batch of cold air from the north and new moisture from the Pacific are somewhat iffy. 

However, that storm should definitely hit the mountains west of here, no matter what the details.  It could go a long way towards improving skiing conditions at resorts & in the backcountry, unless it comes too hard and heavy, in which case we could see vastly increased avalanche danger given the weak snowpack west of here.  More on this later this week.

Meanwhile, I hope you enjoy winter weather, the combination of snow and cold over the next few days will be quite a cahnge after this mild November -

Think snow!

- Klaus

P.S.: Looks like PSA#2 verified nicely, but the 12"+ totals were more towards Jefferson than Boulder County.  Compared to my forecast, it took longer than expected to get the powder snow I promised, so the melted amounts were much closer to my forecast than the snowfall amounts.

P.P.S.: For the record, the weather service forecast is slightly on the conservative (dry) side for the upcoming event (2-4" of snow by tomorrow evening in and near Boulder, and 2-5" in the foothills), and about 5F warmer.

PSA#2                                      Fri 11/13/2009 6:22 PM

Good evening everyone,

 after our dump two weeks ago, it is a bit harder to get excited about a smaller storm, but it looks like we should see enough snow even in Front Range cities to justify a PSA for Saturday into Sunday.

 The snowband that has moved thru our area this afternoon is not to be confused with the main event tomorrow afternoon into Sunday which will probably feature a near-classic Four Corners low for a short period on Sunday morning, deep upslope flow, and good moisture all the way from today thru mid-day Sunday (while today's front and upslope surge won't be good for more than a few inches of snow at best, it has 'primed the pump' so that tomorrow night's system won't have to bring in much additional moisture before the upslope flow can convert it into precipitation).  The sun may try to peak thru the clouds late morning tomorrow at higher elevations, but that will be short-lived at best.

 Alas, like several storms before it in the last few months, and unlike the PSA#1 storm, tomorrow's storm will slow down too late, and just sit and spin too far east of here to give us any 'backwash' precipitation past the early afternoon on Sunday. Thus, precipitation and snowfall totals won't come close to what we saw 2.5 weeks ago.  After the initial transition from rain/drizzle to snow this afternoon, it will be cold enough thru Sunday for all snow to fall at all elevations.

In fact, Sunday's snow may end up falling at just the right temperature (about -10C near 10K elevation) to be quite fluffy, yet again.  The stalled low will keep us fairly cool well into the middle of next week, with the potential for yet another storm by next weekend, but that might end up being another PSA.

 Bottomline: most of us won't see much more snow until tomorrow afternoon (I really don't buy the weather service idea of 'likely snow' overnight), then about a 24h period of light-to-moderate snow from about noon tomorrow until noon on Sunday, which could top out over 12" for many locales in and near the foothills (northeasterly exposure would help), but even Fort Collins should be good for 4-8", so I agree with the weather service on the main storm, just not the next 18h or so.  Temperatures should remain below freezing thru the weekend at higher elevations, but may break out into the 30s in lower elevations to melt some of the snow initially tomorrow.

 Think snow,
 Klaus

 Short postmortem on the October 27-29, 2009 storm:

 I guess my "foot or higher" forecast panned out for most everybody, and the comparison to the 1997 storm worked out as well, with very fluffy snow in the latter period just as in that legendary storm.  As best as I can tell, Jefferson County's foothills took the brunt of this storm, with several reports over 40" of snow, and over 2" of moisture, second only to October 1997. Both Fort Collins and Boulder ended up with one of their top-10 snowstorms since 1950 (almost 22"

and 23", respectively), very respectable.   If you want to see a

map/listing of

storm reports, I recommend:

http://www.crh.noaa.gov/news/display_cmsstory.php?wfo=bou&storyid=35794&source=0

However, it missed out on some reports from northwest Boulder County (ahem!), so I consider that map somewhat preliminary, even though it did include a fair number of CoCoRAHS reports.

This was the 6th biggest storm at our house (just over three feet), but the biggest one that I have ever missed (and almost all of it melted over the next two weeks, unlike 1997 when a bigger storm actually started my winter snowpack that lasted right into May).

PSA#1 - Tuesday night into Thursday Mon 10/26/2009 10:36 AM

Good morning everyone,

this will be one of my shorter PSA's, need to blast out of here in a few minutes.
There is a storm coming onshore that will hit us mid-week, with the potential for over a foot up and down the Front Range. Enough differences in the details between model (runs) to hedge the exact amounts, but almost everyone should get at least 1" of moisture by Thursday. As always, there are scenarios where we could end up with substantially more (I have seen runs up to and over 2"
of moisture), or less (below 0.5" would be a real surprise), but the most likely scenario would give us upslope rain changing to snow in the plains by midnight tomorrow (snow in foothills all along), with the best chance for heavier snows on Wednesday, probably midday (which would lessen the impact on warm roads after two warm days). It could linger well into Thursday, but it looks like things are moving along too quickly for that.

There is a passing resemblance of this storm to the October '97 storm, but it should remain weaker than that one...

Gotta run, this snow should stick around into at least Saturday...

Think snow,

Klaus

--
Klaus Wolter
University of Colorado- CIRES Climate Diagnostics Center NOAA-ESRL Physical Science Division
R/PSD1 325 Broadway
Boulder, CO 80305-3328
 

Postmortum PSA # 7 Sun 4/19/2009 4:32 PM

Storm total: 4.33/37.7".   Heaviest snow from about noon on

Updated PSA#7 - Ouverture is just about over, time to batten down the hatches (above 7K)!        

Thu 4/16/2009 4:42 PM

Good afternoon, y'all -

Act I of this storm is on track in terms of timing, but may turn out to be a real crapshoot in terms of precipitation amounts around here.  At this point, I blame the southeasterly upslope, right off the Palmer Divide, for keeping Boulder and adjacent foothills relatively dry for now, even though the radar is trying to fill in from the east.  Once we get over this hump, we should be good for more or less continuous precipitation well into Saturday, and most of it should fall as snow in the foothills.  However, there may be long stretches with only slushy snow in the plains.  But I am getting ahead of myself.

Compared to my initial PSA, I am glad to report that models other than the NAM have come around to giving us at least a good-sized storm (2"+ H2O seems to be the lowest amount advertized), but temperatures during the storm will be not very much lower from where they are now (no strong Canadian high 'plunging down' to bring extra cold air into the fray).  In fact, while the system brought exceptionally cold air with it when it hit the desert southwest yesterday, it will be slowing down as it moves east, and we will have overrunning warm air coming in from the southeast to balance out the cooling from precipitation processes overhead.  Therefore, I am not nearly as convinced as the weather service (they just upgraded to a winter storm warning with 10-24" in the plains and 18-36" for the foothills (no change there)) that this will be such a big snowstorm on the plains.  In fact, there is enough of a range in the different models' temperature forecasts that one can find justification for keeping it almost all rain in the plains throughout the storm.  On the other hand, the NAM is one of the colder models and looks like it will allow the switch to snow by around sunset in the plains, and should stay cold enough right into Saturday to keep at least the falling precipitation in the form of snow down here even if it has a hard time sticking to the roads.

Meanwhile I don't think anymore that there will be much of break into two parts ("Acts"), in fact, the latest NAM is now showing some of the heaviest precipitation tomorrow morning when we were going to see that original break.  Always tough to time the most intense precipitation periods during a prolonged storm like this, so I won't belabor this any further.  However, most models now advertise a prolonged backwash period into Saturday evening as the upper low is supposed to park itself over southeastern Colorado.  Oh, and before I forget, there is plenty of moisture around, in fact, the abundance of moisture upstream (dewpoints around 45F on the eastern plains) is one of the reasons that I don't see that much of a cool-off coming with this storm (wetbulb effect should be minimal).

Here is my forecast for the next two days, first for the foothills, then for the plains, with some caveats thrown in at the end, so we don't get too excited:

FOOTHILLS (above 7K): Occasional snow showers should become more widespread this evening, favoring Larimer County for now.  With strengthening easterly upslope, I would expect to see a steady overnight snow to bring us about 6-15" total by tomorrow morning (highest in higher (south-)eastfacing locales).  From tomorrow morning thru Saturday morning, we should have more or less continuous moderate to heavy snow (0.5-2"/h), you can do the math yourself - it looks like one of those classic spring upslope events that has been sorely missing for a few years. If the more optimistic models come thru, we should be good for a prolonged tapering off period on Saturday that might add another 4-8" for us.  Totals by Saturday evening: two to five feet, and I am not saying this offhand, some of the higher resolution models have been advertising more than 4" of moisture for several runs now (even if we were to miss out on much precipitation this afternoon).  Enjoy it while it lasts, there really is not much hope for at least a week after this one!  If the higher amounts verify, it could beat the April 1997 storm totals that still constitute my April storm record (since 1991), but it will probably stay below the most extreme totals of March 2003, March 1990, and April 1986.

FRONT RANGE CITIES (below 6K): Boy, this is a tough one, and I wish I could share the optimism of the weather service. However, their own forecasts already give a hint at the problem, they are predicting highs in the mid-30s tomorrow and low 40s on Saturday, with snow "likely" falling both days!  First of all, if that backwash snow on Saturday really materializes, I don't think it will be in the low 40s, but even high 30s would still be too warm to get much accumulation going.  On the other hand, there are thunderstorms just east of us that could give the plains the kind of heavy snowfall rates that could accumulate really quickly despite near- or even above-freezing temperatures during the daytime.

Bottomline: if you insist on measuring snow only at the end of the storm, you may not have much to show for it, but, say, measure-and-scrape tabletop measurements could indeed come up with more than a foot, especially if you get some 'thundersnow' or good snows overnight.

In fact, you should wake up to some snow on the ground both tomorrow and on Saturday, and a well-time early morning snowburst could wreak havoc with the rush hour tomorrow, but in general this storm should not be that much of a traffic hindrance compare to, say, the March 26 storm.

Before I forget, if you live in the lower foothills, just split the difference between the two discussions above.

Caveats: any time you have a lot of thunderstorms thrown into the mix, you can get extra moisture (or get short-changed) depending on where you are with respect to the storm (this mostly affects today); the overall system is on a near-perfect track to give us the big moisture amounts described above - any departure from it, even a slight acceleration towards the end could knock down the precipitation totals from, say, 2-4" down to 1-2"

pretty easily, so let's just

hope that this won't happen (and I don't see any indication right now that it could).  If the storm were to slow down instead, precipitation totals could be higher, but then the lack of cold air entrainment from the north could result in even lower snow:rain ratios in the lowest elevations.

This may be the last PSA of the season where I have to mention the lower elevations, but I sure hope it's not the last one for the rest of us - even if end up measuring four feet with this one, I would still just catch up with a normal snow season rather than being dead last as I was three weeks ago.

Think snow, we still need it!

Klaus

PSA #7                                                                      Wed 4/15/2009 5:26 PM

Good afternoon everyone, 

a very big storm coming in over the next 2-3 days, and I won't have time to do it justice for now.  Things to note: 1. a drawn-out event, potentially lasting for two days or more; 2. while currently about as cold as it gets for mid-April, this storm will probably modify as it slowly approaches, and lose some of the low-elevation threat for snow that the models are trying to 'sell us'; 3. while the drawn-out nature and size of the storm virtually guarantee a good soaking for all us, there is the possibility that would give the Front Range foothills the biggest snowstorm since March 2003, but only one model (the NAM) has been latching onto this 'best-case'  scenario, dropping 3"+ of moisture on the foothills from Larimer down to Jefferson County.

While the main storm will slowly pass thru the Four Corners region tomorrow evening, a leading 'spoke' of this system will pass thru our area, and could produce a first round of snow for the foothills, while the lower elevations may see rain changing to snow after sunset.  Amounts are very iffy at this point, since it will be quite warm ahead of it, and since it might be quite showery (thunderstorms?!) as this portion of the storm crosses over us.  Best case scenario: 12" of thundersnow for some lucky foothill dweller - and this is ahead of the main storm!

The second 'business-part' of the storm should impact us from about midday Friday until midday Saturday, but it will require for the upper low to 'rebound' from a southern track thru central New Mexico into SE Colorado and SW Kansas, where it would have to stall to optimize the moisture plume from the Gulf of Mexico, as well as keep us under deep upslope thru the duration.  If we really get heavy snow for 24h or so, several feet of snow would be possible in the foothills, while Front Range cities could get about a foot (at least on grassy areas).

I will try and update this tomorrow morning, before Act I opens, but I am also trying to juggle TWO workshops in town that are happening at the same time. 

Hey, great jinx factor to have a bunch of meteorologists in town trying to get out tomorrow evening and on Friday!

So, there you have it - anything from a trace of snow in the plains for the worst case scenario to several feet above 8K is in the cards (although I will be disappointed if we get less than 1.5 ft. up high).  Good snow producer for the mountains west of here as well, initially on south-facing slopes (San Juans), along with another dose of dust(?), then more for the northern mountains to give everyone a shot at about a foot or two...

Looks like a break for a few days after this, but I don't think that the season will be over just yet (for the foothills) -

Have fun in the snow - Friday could be dicy even in the plains -

Klaus 

Klaus Wolter

University of Colorado- CIRES Climate Diagnostics Center NOAA-ESRL Physical Science Division

R/PSD1        325 Broadway

Boulder,  CO   80305-3328

PSA # 6        Fri 4/3/2009 5:07 PM

Good afternoon everyone,
well, April came in like a lion for some of us, and last week's storm set the stage for a nice little wet spell around here.  As if on cue, La Niña appears to be collapsing as well, but that would be another audience (and this could be one of those 'bear market rallies' as well).

In any case, another storm is taking aim at us, it is coming in from the west as I write this, it will initially produce mixed precipitation or even rain in the plains, and there are some fairly large uncertainties regarding the final snowfall amounts, depending on the track and speed of the system.  I don't have time to go into all of these caveats, but here is my 'middle-of-the-road' scenario (the weather service is a bit more conservative, after more than one day of 'back-pedaling' by various forecast models):

Look for more showers this evening, especially after 9pm; these will gradually change to all snow by midnight even in the lowest elevations, while the higher foothills should change over to snow by sunset, especially in heavier showers.  A surface cold front should sweep in from the north and east in the next few hours, shifting our winds from southwest to an easterly direction all the way up to about 10K later this evening. 

Moisture seems to be in good supply, judging from rain showers in Boulder already 3h ago, so I don't think we need to worry about that.  As the storm comes into Colorado, it should form a cutoff low close to the Four Corners, an almost ideal spot, but it will keep moving along to the east-northeast to allow only for a short window of opportunity for the highest snowfall rates overnight.

I would guess we should get some 12h of decent snowfall overnight into tomorrow morning, with a very prolonged tapering-off period, as the upper air low should slow down once it gets east of the mountains.  These 'backwash' situations are often quite meaningless for the immediate Front Range (while blizzard conditions are actually forecast in far eastern CO), but there may just be enough instability and moisture left behind tomorrow to give us a few decent showers all the way to sunset tomorrow.

All in all, I would expect to see 12-24" in favored foothill locations (I expect less of a 'barrier jet' to rob us of our typical enhancement compared to the plains that we saw a week ago), and about 7-15" in Front Range cities (western Boulder should get a foot, but probably not as much as a week ago).  The total H2O should reach more than 1" for the foothills, and 0.5-1" for the plains. This forecast is slightly more generous than the weather service, since I expect a slightly more favorable track than they have picked for now.  I also believe that the forecast details are not as critical this time of year compared to mid-winter.

Alright, quick list of a few caveats: for the plains, there is the potential for thundershowers this evening (wasn't that fun to hear thunder with heavy snow in Boulder?!) which could (a) bring a substantial amount of rain to the plains before changing to sloppy snow, and (b) could affect the strength of the upslope enough in its wake to give the plains a long pause before renewed precipitation falling as snow; for the foothills, the main threat to getting more than a foot of snow would be that the upper low moves out to the plains too fast. 

While forecast models have been teasing us with the idea that the storm could end up tracking too far to the north (favoring southern WY over us), I don't see that happening right now, and this has been a typical model bias for many years (apparently hard to fix) so that I am not too concerned about that (that's why the wx service is more conservative in its forecast than I am).

On the other hand, if we are lucky (like some of us on Wednesday?!), the low might just stall long enough (and close enough) to our east to produce an additional 4-8" of backwash in addition to the 2-4" that I foresee right now.  Oh well, one can always hope...

Think snow,

Klaus

PSA#5         Wed 3/25/2009 7:28 PM

Good evening everyone, 

sorry about the delay, but I was out of town until yesterday, and was still overcoming a heavy dose of skepticism after Monday's dud.  But it really looks as if (almost) all of the required parts of a decent snowstorm are coming together for tomorrow - at last! It probably helps that I will try to fly out of DIA in the middle of all of this... Before I get going, here are a few remaining caveats: this is a fast mover, I really don't think we will see much snow after sunset tomorrow, so 12-18h of moderate to heavy snow will have to suffice.  If there is a delay in moistening up our boundary layer from our present anemic values (dewpoints have started to inch up but are still below 20F around here), that will reduce the efficiency of this storm.  A second caveat: while the pressure rises to the north correspond nicely to a surface high over Alberta that is building in from the north, the pressure falls to our south appear disorganized for now.  There is still the possibility that this storm may end up further south than currently expected which would shorten our preciously short upslope period even further.  Between these two factors, I could see getting half of what I will describe below.  If we get even less than that, we really are in the midst of one of the worst dry spells I have ever witnessed, rivaling 2001-02, enough said.

 So, the most likely scenario for tonight and tomorrow is that the current spill-over snow will be replaced by widespread light snow before sunrise tomorrow, depends a bit on how fast the upper low consolidates over SW WY/NW CO tonight, as well as on the speed of the surface cold front that still needs to cross most of WY. By sunrise, we should see deep upslope flow in place with the heaviest snows near the divide, but working their way down to the plains by mid-day the latest. 

Temperatures will be cold enough for all snow for everyone, but the warmth of the recent week or two will be sufficient to delay serious road problems on the plains until after the AM rush hour, UNLESS the storm comes in a bit faster and stronger than expected right now.

Temperatures should drop most the day and reach the 10sF in the plains by the evening, and about 10F lower in the higher foothills - a very wintery storm indeed, and good for fluffy powder snow during the bulk of it (might be getting too cold near the end in the foothills to maintain those dendrites).  Given predicted moisture totals of at least 0.5" in most model runs, with a sizable minority reaching over 1" in and near the foothills, almost everyone should see at least half a foot of snow, with many one foot+ reports possible.  Northeast-facing foothill locations might reach 2"+/h during the height of the storm, totaling two feet+ if everything comes together as expected. 

This should easily become the biggest snowstorm of the season, attesting more to the wimpiness of the season than the strength&duration of this storm.  Speaking of which, if it is still snowing at a good clip about 24h from now, there is always the possibility that the storm might actually slow down enough to give us an extra half to one foot, but I put the odds for that below the odds of getting another dud instead... 

The overall stormtrack should remain in our favor, with one or two more PSA-worthy storms possible next week, but let's get this one under our belt first.

We have a long way to go to make up for a 'lost season' -

 Think snow, Klaus

P.S.: If we clear out tomorrow night, northern foothill locales could see below 0F before the winds pick up as usual, quite cold for this time of year.  The next potential storm could hit as early as Sunday night - will try and update/issue another PSA by Saturday if needed (won't be back from my trip until Friday night).

Update to PSA#4 - tonight    Thu 2/12/2009 12:58 PM
Good afternoon -

after a decent little snow two days ago (for many of us), we are going to see another round of snow tonight, with more unsettled weather into Sunday.

Looks like Larimer County has a better chance for snow than places further south (both placement of low and upslope flow more from southeast than east).  Don't have time to do a more thorough analysis, but 1-3" should be widespread from Boulder northward, and slightly more in the foothills tonight.

If the rest of the weekend, looks more promising, I will try and update by tomorrow afternoon, otherwise, enjoy the seasonal weather for a change -

Klaus

Update PSA#4 - best chances for snow in next four days
Mon 2/9/2009 4:03 PM
Quick update on current PSA:

1. spill-over event for higher foothills did not bring much (if any) snow to us today;

2. next shot of snow will be tomorrow afternoon, along with a brief(<12h) period of upslope - air is probably too dry to give us anything but flurries at best, but the most recent models give a bit more reason for optimism (say, 1-3" best case scenario by tomorrow evening) than (a) earlier runs, and (b) this dud of a weekend;

3. speaking of duds - the wind storm that materialized today (stronger than expected) looks is on its way out, mainly due to colder air moving in at higher elevations which will help to 'de-focus' the windfield away from the Front Range - the rest of the week should be relatively wind-free (say, under 40mph wind gusts); 

4. the next chance for snow will be Thursday night into Friday - still has the potential to rescue this PSA from being a false alarm altogether - more on this later this week -

At least the mountains west of the divide are finally cashing in on this situation... Think snow (as opposed to grass fires!) -

 Klaus

PSA #4 - totals over next week or so     Fri 2/6/2009 5:52 PM

Good afternoon, y'all,

 given that my last (non-)PSA from two weeks ago ended up underestimating that event for the foothills (up to and over 10" above 8.000' at least in Boulder County from January 23rd - 27th), I decided to be more optimistic with the pattern-change coming starting tomorrow. There are some similarities and difference between that event and the one coming our way currently - a surface high will build down from Canada, but it will be quite a bit weaker than its predecessor, a Pacific storm system will come at us from the southwest in bits and pieces, but this one should be followed by at least two more storms in its wake, leaving us more 'unsettled' and wintery well into next weekend. 

As usual, the devil is in the detail, and total snow/precipitation amounts for this weekend will depend critically on the amount of upslope at lower levels, 'spill-over' moisture, and on how fast we can overcome the initial dryness at lower levels to keep falling precipitation just evaporating before hitting the ground.  The initial upslope surge from the north is just getting ready to cross from Canada into Montana/North Dakota and should get by around noon tomorrow.  Moisture from the Pacific storm has been hung up in California so far, but should start moving into the Four Corner's region by tomorrow night, while the upslope surge in conjunction with 'advantgarde-moisture'

from the southwestern storm should allow for an early shot of snow in the foothills of Boulder and Larimer County in particular by tomorrow evening.  Will be interesting to watch when the clouds will lower/snow will begin (if at all) tomorrow evening, because that will be an early indicator of how much moisture is available for this system.  There have been model runs that put down 3-6" overnight into Sunday for some lucky folks in this region, but the latest runs have backed off from that scenario, for now.  Little if any moisture should spill-over all the way into the plains, and would have a hard time sticking to the surface after this warm week.

After a lull in the action, Sunday afternoon will see the transition of this storm system from Arizona into New Mexico from where it will lift northeastward into the Great Plains.  We should stay in a weak southeasterly upslope situation until Sunday evening before we get onto the backside of this storm as it passes to our south and east.  Tough call on how much snow (and possibly rain in the lowest elevations) will fall out of the sky by then - the weather service is hedging with 0-3" in the foothills and a 40% chance of rain/snow mixed in the plains.  No matter what, the plains will probably be too warm during the day for it to stick, and by the time it cools off, the precipitation should be over (late on Sunday).  Interestingly, the weather service gives the foothills a range of 1-11" for the full time frame Saturday evening until Monday morning, I guess they have also noticed the large amounts of snow we saw two weeks ago...

I will update this on Monday, if the next storm (on Tuesday) looks promising, and storm #3 looks like it will get here about a week from today -

Have a good weekend, winter is far from over -

Klaus

Updated outlook for Xmas week
Mon 12/22/2008 11:33 PM

Quick update to our current situation:

 1. Incoming storm looks like yet another good one for the mountains WEST of the Divide, especially the Elk Range and San Juans.  We will have our shot at snow tomorrow, but even the most optimistic model has now backed off on the amounts, so we are looking at yet another 'Trace-2" non-event around here, and at least a replacement of the airmass, it was getting a bit stagnant today;

 2. Xmas eve/day (Wed-Thu): in between storms, starting out cold on Wednesday, Thursday now looks like the warmest day of the week again, should break freezing almost everywhere below 9,000'.

 3. Friday-Saturday and beyond: another quick moving storm will hit us from the west, dropping most of its moisture west of here, and dragging down some pretty cold air from our neighbors to the north (-west) - this looks like the last hurrah of this whole sequence, both in terms of snow and cold,  - another T-2" event for us, and near 0F lows Friday night, followed by mild weather into the New Year (or so it seems right now - I won't be able to send out an update later this week).

 4.  Bigger picture: the storm that will hit us tomorrow will bring a major rainstorm to the east coast by about Wednesday night, after dropping another load of snow on the Midwest; if you want to escape all this weather, southern Texas sounds like a pretty safe bet, and southern California by about Friday.  The Pacific Northwest will moderate over the course of this week, with lots of rain on top of the snow that has fallen this month, kind of depressing if you like snow, but a relief for regions with 'low snowplow density'...

 Happy Holidays, at least, December has tried to make up for a 'lost' fall season - Klaus

Postmortem on last nine days or so: plenty of cold weather, but not too much snow after the initial push, a bit of a disappointment I have to admit!  Impressive inversions in the wake of both weekend arctic incursions - high foothills were 10F or more warmer than the plains today (and a week ago)!

From Klaus / Fri 12/19/2008 4:44 PM
------------------------------------
I will be briefer than usual. Another arctic front is working its way thru Wyoming right now, quite similar in intensity to last weekend's version. Less moisture to work with this time, but a drop of 30F in a few hours (as observed in Cody and Buffalo earlier
today) is nothing to
sneeze at either. While the weather service had been backing off on snowfall chances and even temperatures thru this morning's forecast, they have now come around to giving this arctic front its due. This translates into a 40% chance of up to 1"
both tonight and tomorrow
for places like Boulder, and a few inches for the higher foothills.
While the initial front
will arrive tonight, the coldest air may not get here until tomorrow evening. Sub-zero lows are possible/likely in the plains/foothills by Sunday morning.

The coldest air will retreat eastward from Sunday to Monday, making way for yet another storm that will come in from the west by Tuesday. This one might be good for more snow for all of us, but it is being hurried along by yet another storm on its heels that is now trying to get here by Xmas. Until last night, forecast models kept holding out for a true warm-up on that day instead, so this is a new development. Stay tuned, I will update this/issue a PSA#4 on Monday, if this looks more promising than right now.

One more note: the cold snap earlier this week was more severe (on the
plains) than
expected, in fact, Denver set (or equaled) three daily (low) temperature records in a row, something we have not seen in a while. The arctic air surging back in by tomorrow looks to be of similar intensity as the originally forecast arctic air one week ago. So, don't be surprised if we plunge below currently advertised values by Sunday morning, pretty much everybody has enough snow on the ground to allow for that to happen, and these shallow arctic airmasses have been 'misunderestimated' several times in the last few months...

Happy Holidays, snow-covered ground is virtually guaranteed until at least Christmas eve, and maybe more from me by Monday -

 

Update to PSA#3 Mon 12/15/2008 4:49 PM
Good afternoon, y'all,
I hope your winter gear has been in working order the last two days, very impressive temperatures in the plains, especially east of Boulder, but cold everywhere.  Have not had a chance to check this, but today might have been the coldest December day since 1990 for quite a few places. Snowfall totals with the arctic front were just about what I expected, with widespread 2-6" amounts.

There is another 'upper air disturbance' taking aim at us, very impressive jet streak with 180mph or more shifting across us tonight, allowing for a lot of lift in the right places - the San Juans might get three feet of snow out of this by tomorrow evening!

Unfortunately, the general wind direction aloft will be from the southwest, while surface winds will be from the southeast.  This is a drying combination for most of us, but could be good for a surprise in Larimer County.  Models are having a hard time 'spilling' the associated moisture with the incoming storm across the divide, and it is almost a leap of faith to assume that the strong dynamical forcing will overcome this handicap.  The strong jet streak may also induce some fairly narrow southwest/northeast snowbands that could favor some places over others by an order of magnitude.

The weather service is now going for smaller amounts than in its earlier forecast renditions - 1-2" for Boulder (1-3" for Fort Collins), and 3-6" for the foothills, vs. 8-18" west of Peak-to-Peak highway.  I am not going to quibble with the lower elevations amounts - except for Fort Collins, where I think that 4" are possible, but the higher foothills could see more than 3-6", if one of those snowbands ends up sitting over some lucky recipients...

I think that the highest reports from the foothills could reach 8" or higher by tomorrow evening (speaking of which, I always welcome snowfall reports, especially if you are not reporting to CoCoRAHS already).

Then a break until the next round of snow for us around Wednesday night/Thursday, too early to get too excited about that one.  An impressive reinforcement / reestablishment of our arctic siege is lining up for around Saturday, but more on that later this week.  Unless the mid-week storm looks to beat the next 24h-event, I won't update this until then. The mountains will continue to see off and on snow all this week, and quite possibly beyond, yeah!

 Cheers, Klaus

 P.S.: The arctic air will erode somewhat over the next five days, but places like Greeley, and even Longmont (and DIA) will remain in a much deeper freeze than Boulder.  I don't think we will see temperatures like last night until this weekend, but there is potential for even lower temperatures around that time - stay tuned!

PSA#3 - a foot possible totaled over next week
Fri 12/12/2008 3:47 PM

Good afternoon, y'all, as mentioned in my last PSA from Monday, we are looking at a good-sized arctic airmass marching our way, with a strong cold front to hit Saturday night into Sunday. This initial front is not modeled to produce more than a few inches of snow for us, but my experience is that such arctic fronts are sometimes stronger (wetter) than predicted, and they tend to produce pretty fluffy snow in the lower elevations, if the upslope lingers.  The temperature change will be drastic, with temperatures having a hard time staying above 10F on Sunday (in the plains) compared to 40s tomorrow.

After the initial snow is over, we might clear out enough over fresh snow to get the coldest temperatures of the season so far - below 0F in the higher foothills, and close to it even in Boulder.  Monday through Friday have no big storm in the offing, but we should continue to see occasional spill-over snow from the west, with the arctic air firmly entrenched thru the period, albeit warming a little bit each time we get some of the westerly flow to break thru and warm us up.  Right now I think that the best bets for additional snow are on Tuesday and on Thursday, with the latter having the potential for more snow than the first one.  Tough call on amounts or even temperatures thru the period since models always have a hard time getting the details right east of the mountains in such a situation. 

Bottomline: I expect to see 6-12" of (mostly fluffy) snow from Sunday thru Friday, very little melting, and more snow in the foothills than below.  Did I mention that the mountains west of here are going to get clobbered?!  If you have a chance to go skiing, try to get there before Sunday, the roads will be tough going, and it should more or less continuously almost anywhere in the Colorado Rockies - the weather service is calling for 1-2 feet with the first storm alone, and the follow-up storms should keep it going thru the week (with milder temperatures than here in the East). If I see cleared amounts on the horizon early next week, I will update this, but don't count on it. Think snow, we are still playing catch-up -

 Klaus

Postmortem on PSA#2:
 this time the fortunes were reversed - Fort Collins and vicinity barely got an inch or two, while 8-12" were common from Boulder southwestward into the foothills of Jefferson County.  All in all I underestimated the moisture with this one (many places had 0.5" or more), but at least I was more optimistic than the wx service.

Speaking of which, the latest forecast just came out and goes for up to 6" in the foothills (from Saturday into Sunday), and up to 4" near Boulder - fair enough.

PSA#2 (just barely, yet again) Mon 12/8/2008 4:04 PM
Good afternoon, y'all, while this storm will hit harder to our south, I believe it will be quite messy from about 5pm to around midnight. Due to the dry boundary layer earlier today, we are currently waiting for the falling precipitation to moisten up the air closer to the ground, but this should happen rather quickly within the next few hours. This is a complicated system, with a piece of energy that snuck in from the southwest (hence the 8" reported from Silverton Mountain this afternoon), a good solid coldfront that just came in from the north (hence the 4mb pressure rise in the last 3h at Foothills Lab), and the prospect of decent northeasterly (upslope) flow for the next 8h or so.  Midlevel temperatures are dropping rapidly, but should reside in the favorable -10C to -15C range to give us good crystal growth to make for fluffy snow this evening. So, even though this will be a quick storm, and I would be pleasantly surprised to see more than 1/4" of moisture out of this around here, I believe that the southern foothills do indeed have a shot at close to a foot of fluff (maybe 0.5" of H2O), and we might see a few hours of moderate-to-heavy snowfall rates this evening.  Bottomline for Front Range cities: 2-4" near Fort Collins, 3-6" near Boulder, and more as you go south;  Foothills: 4-8" in Boulder (and Larimer) Counties, 6-12" to the south. Next chance for a little snow: Thursday into Friday (not PSA-worthy), bigger potential: this weekend into early next week, with some arctic air thrown into the mix - watch out for PSA#3! Think snow - the mountains should be good for at least the same as the foothills!

- Klaus

P.S.: And here are the weather service totals, hot off the press (3:49pm officially): 

FCL: 1-3"
 BOU: 2-5" (= Denver)
 BOU/LAR foothills: 2-4" (beats me!)
JEFCO&south foothills: 4-8" 

Quick postmortem PSA#1/quick heads-up for Monday, potentially another PSA-worthy event 
Fri 12/5/2008 5:42 PM
Good afternoon, y'all, while the initial cold front did not bring much moisture for us, the 2nd stage (yesterday) delivered the goods almost everywhere, with the area west of Fort Collins doing the best (there were some 12"+ storm totals!), and Estes Park barely reaching the 4" I predicted. Almost everyone received at least1/4" moisture which was sorely needed. Another storm is taking aim at us for Monday (into Tuesday), with lots of uncertainty surrounding the exact track and timing this far out, but it should be good for snow up and down the Front Range, and I would be disappointed if we ended up with less than a few inches either.Watch for an update/PSA#2 on Monday morning, think snow! - Klaus

2008 PSA#1 (best case scenario) for December 3rd&4th
12-02-2008

Good afternoon, y'all,

 the weekend "storm" ended up distributing the snow very unevenly, with some of us getting less than 1" total, while others wallowed in more than half a foot, and this is from north of Golden to Fort Collins (not to mention almost two feet near Castle Rock and almost four feet at Loveland!).  Sad to say, the 'snow-shadow' of Longs Peak proved too much for my neck of the woods, so northern Boulder County foothills did very poorly.

 As advertized last week, the next two days are shaping up for some more winter weather, with a very dramatic temperature drop from today to tomorrow (it will be more than 30F colder in the plains than today!), and at least some moisture/lowlevel upslope flow/upper level dynamics to play with.  Big differences between different models, with today's information being more 'optimistic' (snowy) than earlier runs.  What makes this interesting is that there are TWO chances for snow in the next 60h - first the front tonight (which has a good history of dropping a few inches at low elevations upstream (Montana, and now northern Wyoming)), and then a prolonged low-level upslope/upper level westerlies 'tug-of'war' that should bring us more snow on Thursday.

 I just saw the weather service forecasts, and they give themselves a very wide range to play with (this is for foothills between 6-9K): 1-2" tonight, "up to 3" tomorrow", "up to 2" tomorrow night", and another "up to 3" on Thursday - so, expect anywhere from 1-10" before this is done!  Joking aside (and noting the natural tendency to make a really good forecast for the LAST storm), here is my take on this:

 1. Good coldfront is moving our way, and should hit late evening/overnight.  This should be good for all snow from the get-go, even in the plains, and should get cold enough by morning to drop some nice fluffy snow around that time.

With 0.1"-0.4"

of moisture possible, I would expect reports from 2-4" in most places (more than the weather service forecast), and some nasty road conditions after today's warmth.

 2.  Tomorrow evening should have clear enough skies to freeze the remaining snow into a solid 'base' that should protect the new snow from melting much from below.

I would not be surprised to see upper 10sF in the plains with this, and maybe 10F lower in the higher foothills.

 3. Next round of snow should commence overnight into Thursday and should bring just about as much snow as the first one: 2-4" for most, with a slight preference for the foothills (in both cases).  All in all, 4-8" is in the cards with this one.

 4. West of Peak-to-Peak highway should see less of an interruption of snowfall tomorrow evening, and remain windier throughout - avalanche danger will remain high for several days.  This is definitely a system that should favor the NORTHERN Front Range, so we should not see big snow reports from the Palmer Divide, etc.

If the snow stays fluffy throughout, some lucky folks could see a foot of snow by Thursday evening, especially since we could have some snowbands setting up and remaining stationary for a while, apparently not in the same places as last time (wind direction is different). Too hard to call where they should set up, however.

 5. Caveats: after a near-record dry November, it might just take a little more upslope to moisten us up than usual - if the initial surge/front fizzles out before getting to us, that initial hump could really hurt us tonight.  And Thursday's 'overrunning' event could still set up more to our north&east, as the weather service was still playing it yesterday.  So, there is potential for another dud, but I believe the odds are in our favor this time around.

 Beyond Thursday, nothing on the horizon until early next week, so the lower elevations will probably lose their snow again by then.

 Here we go, tomorrow could be a messy commute - think snow, we have a lot of catching up to do -

 Klaus

2007 PSA#9 - mainly above 8,000 feet! Fri 3/23/2007 5:40 PM
Good evening everyone,
I can't believe it has been almost two months since the last PSA (31jan), but while February remained cold, the snows just 'nickeled and dimed'  us towards a near-normal month, and March has been an unmitigated disaster so far (from a snow lover's perspective). This is about to change for folks higher up, flat-landers will have to deal with a good amount of rain instead!  A storm that had dropped into the Baha region over the last few days is now 'yoyo-ing' back towards us, and should pass right over us within the next 24h.  The first (big) December snow was a bit like that, with the difference being the warmer temperatures that one gets three months later, but the subtropical moisture source, and the forward speed of this storm is quite comparable. Low-level easterlies have already started to blow towards the Front Range, and a lot of moisture is coming towards us.  The main low is still sitting in southern Arizona, but a piece of this storm is quickly moving our way along with a jet streak that will put us right under the left front quadrant - very favorable for enhanced upward motion! The rumble outside attests to the decent vertical instability associated with this storm system, so a good amount of rain (0.5-1") could fall with a few evening thunderstorms that are drifting in from the south. Almost all the cooling with this storm will come from the precipitation itself (and associated evaporation from falling precipitation), since no cold air to speak off is within a long distance from here - no real cold front in sight!  Deep upslope should remain in place overnight until the upper low tracks either right over northern Colorado (towards the northeast).  It will weaken as it crosses over us, so the main precipitation will take place by tomorrow morning.Night-time cooling should slowly lower the snow line from about 10K right now to possibly 7K by tomorrow morning. While the weather service (and various forecast models) has been waffling this week on how to call this storm, they have now issued a winter storm warning for 6-12" in the foothills, with all rain expected in the plains.  I have no overall disagreement with this forecast, except that the amount of liquid that could fall over the foothills (Larimer County in particular) could translate into up to two feet of snow in the highest (east-facing) elevations.  I fully expect to see up to 2" of moisture with this storm, and the earlier it transitions into snow, the more you going to get.Ironically, the amount of moisture in the air (dewpoints in the 40s just east of here) is reducing the "wetbulb-cooling" effect that will bring temperatures down - if it had been drier to begin with, the cooling might have been dramatic! Enjoy the first thunderstorm of the season/finally a spring storm, and, maybe, a foot or more of heavy wet snow above 8K - it will warm right back up early next week, ahead of the next storm that looks like bora right now (should be good for powder west of the divide, though, most likely on Wednesday) -Klaus
 P.S.: El Niño is dead, long live theanti-correlation tendency of wet Aprils to follow dry Marches, such as in 1995, 1997, 1999, 2004, and 2005!

PSA#8 (=1-2 punch + extreme cold on Friday) Wed 1/31/2007 12:22 AM
Good morning,
I will keep this one short.  Today (Wednesday) looks like yet another "little snow storm" day that will nevertheless pack a punch since it will be cold enough to stick all day even in the plains.  Temperatures will be dropping all day, with the potential for sub-zero readings by midnight/early Thursday in the higher foothills, and just above that in Boulder et al.  Most of the moisture that we will wring out from the clouds should fall in the form of fluffy dendrites. Therefore, I am going for 3-6" even in the lower elevations, with the potential for a few inches more in the foothills with good north(eastern) exposure. The more interesting situation will set up for Thursday night with a true arctic cold front that will usher in sub-zero temperatures for most of Friday both in the plains and foothills (yes, I am going colder than most other forecasts out there).  Depending on a couple of factors that are not settled yet (how much spill-over of the arctic air to the west slope, how much moisture is left in same region to be blown over the dome of cold air that will set up for a day or so, how fast the front rushes in & through), we might wake up on Friday to not only sub-10F readings in the higher foothills and sub-zero in the plains, but also another 2-6" of snow. Right now it looks like the cold air will only hang in for a day in the foothills and maybe two days along the Front Range (Saturday should be warmer in any case, although the wind chill may make this warm-up miserable higher up), with warmer (and dry) conditions into next week, but the models have been quite fickle about this, so the amount of warming will depend very much on how much westerly flow we will get to flush the cold air eastward (say, to Greeley).  Would not be surprised if we stay at or just below normal early next week rather than a true Chinook situation. What a winter! And, yes, this arctic snap is NOT typical for El Niño winters (despite a couple of precedents in '72-3). Meanwhile, might as well cheer on our attempt to set all kinds of seasonal snowfall records (more on that in my next PSA/postmortem) -

 Klaus

PSA#7 (don't blame the messenger!) Thu 1/4/2007 5:03 PM

I will keep this one short, too busy to do this one justice, and it won't be as big as the last two. Another eight days have passed, and it is time for yet another storm. This one is a fast mover, it has a very impressive jet streak (>150mph) associated with it, and we should have a few hours under the left front flank of it later tonight (good setup for heavy snow showers). A cold front is trying to slip right in from the north, there is plenty of moisture around (recycled from melting snow in recent days?!), and we should have at 6-12h of decent upslope starting right around midnight.
Bottomline: except
for jet-induced showers that could streak out onto the plains, we should keep most of the snow right against the foothills, more or less like the last storm. I am expecting 6-12" from Boulder westward, with some lucky (?) folks measuring up to 18" by tomorrow around noon. If you get 2"+/h sometime after midnight, you might sit underneath one of those heavier snow bands, and end up with those higher amounts.
Since this will be mostly overnight, and fairly cold air should get entrained by tomorrow, there won't be any rain this time.

Happy Snowy New Year!

Klaus

P.S.: No time for long postmortem, but here is a nice web site to see snowfall totals for the last two storms (yours truly is NOT the snowiest location on these maps, believe it or not).

http://www.crh.noaa.gov/bou/?n=winter2_storms

P.P.S.: We are on record-pace for seasonal snowfall totals, in many locales exceeding the '97-98 pace in particular, and already ahead of last year's seasonal totals, and we are not even at the usual half-way point!

 

PSA #6 - on record snow&PSA-pace...     Wed 12/27/2006 3:48 PM
Wow, here we go again, just eight days after the last HISTORIC December snow, here is another storm knocking at our door, and it has the potential to give us almost as much snow as the last one! If you can't get 'ready' today, you may have most of the morning tomorrow to run last-minute errands. Given the state of our roads right now, it will take less snow than last time to create poor to impassable road conditions.
Hopefully, DIA has gotten some of the cobwebs shaken off, and is better prepared for the next storm...

This storm is coming on a more traditional, if slightly southern, track at us, including a (south-of) Four Corners low at higher levels, and with slightly less moisture to begin with (although a moisture plume from the eastern Pacific appears to be feeding into it already). It will start fairly mild tomorrow morning (if you see any precipitation in the plains in the morning, it might be mixed rain/snow!), but will wrap in colder air than in the last storm by Friday. What makes this storm quite unusual (aside from being a spring-like storm in December!) is that a second short wave will drop in from the north on Friday, which should slow down the progression of the main storm to the point of feeding yet another moisture plum from the Gulf of Mexico back into our region by Friday evening, giving us a good 'back-wash' scenario.
Of course, this
scenario, while quite possible, is not 'nailed down' just yet. In fact, these 'Fujiwara'
situations (of two lows circling each other) are notoriously difficult to predict, but the potential for a drawn-out snow storm into Saturday night is quite real, and the fact that the southern storm is much bigger than the northern wave dropping in argues that there may be less uncertainty than usual.

Bottomline: foothills might see some early snows starting tomorrow morning, but the heavier snow should hold back until around (or even after) noon.
There should
be 12h period or so of moderate-to-heavy snow into tomorrow night, with a fairly pronounced focus on the (northern) foothills and adjacent plains rather than far out east - we will have less moisture to work with, and there appears to be more of a direct (easterly) upslope component. This first batch of moisture should be good for at least a foot of snow from Boulder westward, with some favored spots approaching two feet of fairly fluffy snow by Friday morning. Friday should see lingering snow showers while we wait for the 'backwash' to kick in. Even if this second part of the storm fizzles/misses us, the snow from Thursday into Friday morning should be sufficient to make this the snowiest/wettest December since 1913. If the prolonged 'backwash'
into Saturday evening comes through on top of this, we could see another foot or so to round out a December that might rival or even displace December 1913 as the snowiest and wettest winter month (Dec-Feb) in recorded Front Range history.

In terms of impacts of tomorrow's snow, I am concerned that less snow than last time might make for comparable impacts - the roads are not completely cleared, and while the winds won't be as strong as with the last storm, drifting snow on the eastern plains might create big snow drifts anchored by remaining snow berms along the roadways.

I may not 'get around' (literally) to update this tomorrow, so enjoy this one (if you can), and think of the good skiing conditions we should see well into spring (in the higher foothills) after this storm!

Happy New Year('s snow) -

Klaus


P.S.: Almost everybody had two to three feet of snow in the last storm, rivaling or even exceeding (Boulder northward) the Xmas'82 benchmark. Only 1913 saw more snow in December! Larger variations in melted totals than snow fall totals, and I can think of two reasons for that: 1. There was a lot of wind on the plains (and even some foothills) with the last storm, so smaller gages in particular were 'undercatching' the snow; 2. If you tried to follow the 'Nolan Doesken approach' of measuring as few times as possible per day, you had to deal with crowning of the gages, or even overspill. And then there was DIA, reporting UNDER 1" of liquid from a 20" blizzard - no way! But this keeps their ranking as one of the driest years on record going...
--
 

Update #2 - PSA5 - biggest December snow in 19 years?! Tue 12/19/2006 11:24 AM
Good morning, y'all,

while I am not 'seeing the white in the eye of the storm' just yet, indications are very good for a rare December 'dump' in the Front Range. This is actually perfectly consistent with El Niño, despite some of the media reports you may have been
hearing: we often get 'schizophrenic' winter months with El Niño, where nothing happens for weeks on end, only to be followed by big snow storms even in mid-winter (spring is another story, for another PSA). It has been a while since we have had such a big 'El Niño storm' in December, but we saw two of them in the '80s, the infamous Xmas blizzard of '82 being the first one (two feet even in Denver, but almost nothing in Fort Collins), and just after Xmas in '87, when more than a foot fell in and around Boulder, in a storm eerily similar to the one that I am expecting for tomorrow (yes, weather history can repeat itself!).

The storm that battered the Pacific Northwest last week swept through Colorado on Sunday, dropping up to 9" around Estes Park, 2-4" from my neighborhood to Allenspark, and next to nothing in Boulder and Nederland, is currently reorganizing over Arizona.
It is expected to 'yo-yo' back into our state by tomorrow morning, with the potential for a deep upslope storm from tomorrow morning into Thursday morning. Ahead of it, we may see a few snow showers drifting in from the south today and tonight, but it should not amount to more than a reminder to get your errands done today. There is a slight chance that heavier snow band will pass through later this evening, but it should not be mistaken for the main storm.

Moderate to heavy snowfall rates (1"+/h) may hold back until late morning tomorrow, but could start as early as before sunrise, sorry about that uncertainty. It should last for at least 12h, but has the potential to continue well into tomorrow night, depending on how slow the cutoff low will leave the 'sweet spot' in SE Colorado, which in turn will depend on the next storm's speed and track upstream. Backwash snow bands should keep us at least cloudy with occasional flurries on Thursday, before the next round of snow comes in some time on Friday.

How much are we going to get? Consensus of most models is now from over 0.5" of moisture (respectable for any December snow) to almost 2" in the 'best-case' scenario, with strong upslope flow through at least 18h.
Thus, everyone in NE Colorado should see at least half a foot all the way out to Kansas, but Boulder/Denver/DIA (!) have a good chance of seeing a foot or more. Flights in and out of DIA may see minor impacts through tonight/early tomorrow, but might get bogged down by noon or so. 'Sweet'
spots in our foothills (northeastern exposures should do best) have a shot at measuring two feet by Thursday morning. If all the ingredients come together, with lingering upslope and just the right temperatures for fluffy dendrites near the end of the storm, I would not be suprised to see even higher amounts.

What could go wrong? Yesterday's U.S. models advertized the 'evil twin' version of this storm, with placement too far to the east, and deepening low pressure giving us the dreaded northwesterly downslope flow pattern.
But even that scenario gave us 3-6"
of snow (which is still the weather service line this morning, watch them upgrade this storm to much higher amounts later today...).

No really cold air to entrain, so plain 'vanilla'
cold through the rest of the week, the
plains might even see above freezing when the sun peaks through off and on, but no real melting either. Given the school break, foothill roads may remain barely plowed through Xmas (and we won't seen any melting at all up there, just plain settling).

If I don't send out another PSA before Xmas, expect only snowfall amounts through early next week, guaranteeing snow on the ground, and even a small chance for some snow falling again Sunday/Monday as well.

Season's greetings,

Klaus
 

Update #1 to PSA#5 Sat 12/16/2006 4:15 PM

Good afternoon,

looks like "Stage 1" of this storm is going to be a bit late, and weaker than hoped for yesterday. I still think that everyone on this PSA-list will see some snow between about midnight tonight (northern foothills) and early Monday morning.
My best guess
right now would be 1-3" in lower elevations, and 2-5" in foothills, enough to make it look wintery again. Due to the banded snowfall patterns with this storm, some lucky folks could still see more than that, again, more in the foothills of Larimer County than further south.

Then a lull in the action, but remaining chilly (with a little day-time melting at lower
elevations) on Monday and Tuesday, while we are waiting for the main storm.

While Stage 1 looks somewhat diminished, Stage 2, the cutoff low to lift out through Colorado around Wednesday is still on track! That one could be good for some fairly intense snowfall, watch for Update #2 , hopefully with PSA-worthy forecast amounts!

Another storm appears to be taking aim on us just under a week from now, but it is too early to get excited about that one. However, if all of the above pans out, a White Xmas with more than half a foot of snow on the ground is in the cards for the plains, even though it does not look like any fresh snow will fall after next Saturday.

Think snow!

- Klaus
 

Tentative PSA #5 Fri 12/15/2006 5:51 PM

Good evening,

interesting weather situation unfolding for the next five days or so. Very wide range of possible outcomes, anywhere from just a few flakes to more than a foot all the way down to plains by next Thursday. The wettest scenario bears more than a passing resemblance to the Xmas'87 storm, for those that remember that one.

The same storm that has pounded the Pacific Northwest in the last two days is working its way inland, and is starting a complicated sequence of 'ripples in the flow' that will impact us both this weekend and early into next week. In fact, we have a decent chance to get hit by the same storm twice, since a lot of energy gets dropped into the bottom of the trough after the initial short wave passes over us (or, unfortunately, to the north, as the latest models want us to believe), leading to a cutoff low near Baha (!) that will later on lift out, and could hit us again around Tuesday/Wednesday if the ECMWF track holds up.

So, the broad-brush scenario is that a cold front will sag south over the next 48h, giving the lower elevations upslope flow by tomorrow evening, but it will have a hard time getting deeper, fighting the 150mph winds at the jet stream level will blow from the southwest from tomorrow evening through most of Sunday. This pattern often results in more or less stationary precipitation (snow) bands that give one area much more snow than another one, generally favoring Larimer County over Boulder County, for instance. Too early to tell whether we will just a few flurries by Sunday evening, or somewhere in the vicinity of 4-8" as last night's models were predicting. Cold air will cover us early next week (below freezing day-time highs in the foothills, but hardly anyone will see below 0F). Then the suspense about the cutoff low track - will history repeat itself as on December 26th, 1987, with a good snow storm on Tuesday/Wednesday, or will it miss us? Stay tuned!

Hope you enjoyed today's warm weather, not to be repeated at least past Xmas - I will update this forecast on Saturday if I see clearer which way we are heading -

Think snow/season's greetings!

- Klaus

P.S.: PSA#4 verified nicely with about 6-18" up and down the Front Range. A more reliable start to the winter snowpack than the October snows. Low temperatures dipped down to near 0F in the plains, and near -10F higher up. If this El Niño winter continues true to form, those cold days in late November/early December might have been the coldest for the season!
--

PSA # 4 Tue. Nov 28 2006  1:31 PM
Good afternoon,
hard to believe it has been more than a month since my last PSA, but even my 'trigger-happy' pre-Thanksgiving forecast self did not find it worthwhile to mention a "very small chance for flurries" last week. The incoming storm will be more noteworthy for its wintery character than snow totals, although there is another uncertainty/smaller-scale jet dynamics to give us an outside chance to reach the magic foot by tomorrow. Cold air has been "festering" over central Alaska for most of November, in an odd repeat of last year's pattern (despite El Niño), and has finally shifted into the Yukon and points south over the last few days. This is an exceptionally cold air mass for so early in the season, with numerous -40F to -50F reports (well to the north of the border). Sub-zero air has now spilled all the way through Montana, single numbers cover northern Wyoming, while we are still hanging on to above-freezing temperatures for a few more hours. While the arctic high air pressure is starting to nose into our state, a surface low is now setting up over southeastern Colorado (close to Pueblo), and our wind has shifted to the northeast. Unfortunately, there is a batch of really dry near-surface air being brought in that will reduce our snowfall chances (in lower elevations) for at least another 3-6h, but the deepening upslope flow (not as intense as I would like to see) should overcome this handicap, while higher-level moisture should keep us cloudy for the rest of the day. While the coldest air of the season is waiting to spill into our backyard, it will have to come across a pretty long fetch of bare ground (most of eastern WY), and the actual duration of upslope will be 1-2 days at best, opening the door for modification starting already tomorrow night.

Meanwhile, much of the cold air has spilled across the divide and dropped snow down to the sea level in places like Seattle and Portland, feeding into a decent snow storm in our mountains, with highest ski resort reports from the San Juans (Silverton: 18" of powder!) as of earlier this morning. As the storm tracks eastward, mountain snows should start to favor northwestern exposures more than the current southwestern exposures, so that everyone west of the divide ends up with more than a foot of powder, followed by bitter cold, well below zero.

Bottomline for the Front Range below 9K:  spill-over snow coverage will increase in the foothills this afternoon, and should become steady moderate snow by sunset. Should last through much of the night, but could wind down before sunrise, depending on how fast the cold air rushes in. Most of the foothill dwellers should measure 6-12" by this time tomorrow, with a few lucky folks seeing even higher fluffy totals.

Temperatures will drop the rest of the day, reaching sub-zero lows in all but the lowest foothills to the south of here. Would not be surprised to see close to -10F in higher northern foothills, possibly the coldest of the season in this young El Niño winters. Boulder, Fort Collins, etc., may not see sticking snow until sunset, but should have lingering snow into the morning, to the tune of 4-8" is my best guess right now. Due to cloud cover, low temperatures may not get much below 10F by tomorrow morning. This cold snap will modify tomorrow night, as we might see a bit of a bora situation, with Boulder hitting its low close to 5F early in the night, while Longmont and other protected lower elevations could slip down below 0F, given that there is more than a trace of snow on the ground.

Another short wave will drop in around Saturday, looks weaker for now, but should reinforce the wintery character of our weather (higher foothills might stay at or below freezing for more than a week from today), followed by another, potentially PSA-worthy storm about a week from now.

Enjoy!
- Klaus
P.S: Temperatures may drop low enough by tomorrow morning to render the liquid de-icer useless, so watch your driving tomorrow morning in particular (hope you have not forgotten to drive in snow after our long dry spell).
P.P.S.: The PSA3 storm in late October delivered in spades, lots of 1'-2' reports, with the highest numbers from Nederland southward. It capped an exceptionally snowy October, surpassed only by 1997 in higher foothill elevations. Too bad we had such warm and dry spells in November, otherwise this could have been a record-early onset of winter snow pack below 9K!

Clearly, we will see at least 0.5" of moisture tonight, but it is not so clear who will get the most - the upslope component has been advertized as more northerly than easterly, and if the surface low deepends radially over SE Colorado, we could see enough downsloping wind from the NNW to keep Boulder/Foothills on the low end, while DIA/Longmont could see more moisture (9 March 1992 and another one in early January 1992 were both a bit like that, although the difference in Boulder was between a trace in January and over a foot in March). In any case, the latest tendencies in the models tell me that this storm is not bringing in any extra cold air from the north, so that much of the cooling will have to come through evaporation (from falling snow) and vertical overturning as the cold air makes its way across the mountains from the west. So, I would be surprised to see snow in the plains before midnight, and it might have a very hard time sticking to the roads. Trees are another matter, however, so there could be some pretty hefty tree damage (power outages?) where the snow is heavy enough to bring down branches. The good news is that it also seems that the winds might not be quite as strong as earlier expected - the system is coming in just a tad slower, and will be just a bit slower kicking out as well, making for slightly lower wind speeds from the north at the height of the storm.

Bottomline: expect precipitation to commence in the foothills first this evening, could be light drizzle off an on in the lower elevations, but could also hold off until midnight. Between about midnight and noon, the bulk of the precipitation should fall at a good clip (some places will get more than 0.1"/h, translating into snowfall rates up to 2"/h in the higher elevations). It will be all snow above 7K, but could remain mixed until the early morning hours in the lowest elevations. Total snowfall hard to call - if the upslope remains favorable for more than a few hours, I would expect to see more than a foot in the higher foothills, but some lower elevations may have trouble measuring more than an inch or two (typical location for that would be Lyons, for instance).

We should start warming up again by Friday, so this is about as quick as it gets around here. Roads around DIA could be messy tomorrow morning, since this storm could be more intense to our south and east! What an exciting month - keep it coming!
- Klaus
P.S.: 9" from the non-PSA last weekend, a great start to the season continues...

Updated PSA#1 - looks like the "dry" scenario wins!  Thu 9/21/2006 6:34 PM
As I was saying - it is almost never a good sign when the weather service jumps the gun... Wave #1 has passed and dropped up to an inch of rain just south of Denver, leaving Boulder and Larimer County with 0.2", more or less, and no snow below 9K.

Wave #2 is taking aim at us, and it looks like the upslope scenario is becoming less and less likely. A few spill-over snow showers higher up and certainly another cool down is on tap for Friday and Saturday, but no big dump below 9K.
Don't get me wrong, I could see some areas (especially with northern exposure) getting a few inches of snow between now and Saturday morning (there is enough jet dynamics around to allow for a few northwest-southeast oriented stripes of heavier precip), and the snowline should lower to 8K by tomorrow morning, and about
6K(+/-) by Saturday morning, but the amounts just aren't there to get too excited about this. Different story west of Peak-to-Peak highway where the mountains should get a decent spill-over event with a foot of snow or more by Saturday morning.

Oh well, it is very hard to get a decent snow storm during El Niño Septembers (last 2"+ September snow of this type in Boulder was in 1965!), quite contrary to what often happens in October (remember 2002 and 1997?!). Should have trusted my statistics rather than the (more optimistic) forecast models...

But we are going to have yet another cold (early) weekend, and there is a distinct chance for a freeze both Saturday and Sunday morning down to the plains.

Until the next PSA, hopefully before November -

 

First PSA of the 2006-07 season  /  Wed 9/20/2006 8:35 PM
Good evening, after a few flurries above 8K last Saturday, it looks like the higher foothills may get some measurable snow both late tonight as well as from late Thursday into early Saturday, with the latter storm lowering the snow levels to the lower foothills.
The weather service has just come out with a winter storm watch for the second period in the higher foothills, never a good sign so early in the game...

Complicated weather pattern evolving right now, with a leading wave coming from the west, very similar to last weekend - good for snow WEST of the divide and some spill-over to the east, aided by good vertical (in)stability and a brief potential period of upslope (northeasterly) flow tonight.
However, we are starting out with a very dry airmass, and a surface low that might deepen too rapidly over southeastern Colorado, thus tweaking our upslope flow to a more northerly direction (ending up with less moisture). This might be one reason that there is a large spread in the model forecast precipitation for tonight, depending on when the model run was started (four per day) and which model is used.
IFFF we are lucky, this could be good for a few hours of moderate to heavy snow showers (above
8K) and get us off to a good start.

The second, more important storm is dropping in from a more northerly direction, dragging in colder air, and leading to a more prolonged period of precipitation for us from tomorrow night (or so) into Friday, maybe Saturday. Still lots of question marks on that one as well, not least because it could interact (negatively) with the first wave - if the first one deepens too much and slows down, we could get a longer period of downslope winds tomorrow, reducing our moisture potential yet again.
However, it has the potential of producing deep upslope flow from the northeast (not quite a full cutoff low circulation for us, since the main low parks itself over the norther plains), which could lead to 24h or so of moderate to heavy snow above 7K or so.

Since I probably won't have time to update this until tomorrow evening, here are the two bottomline scenarios for now:

1. DRY: occasional sprinkles tonight, with a few flurries late above 8K, gusty winds from a northerly (to northwesterly) direction later tonight, as the cold front passes. Blustery Thursday, but mostly dry all day. Thickening clouds tomorrow evening, but only weak upslope (if any). Mixed rain/snow showers on Friday above 7K, totaling less than half an inch of moisture from now until Saturday morning. No snow in the plains or lower foothills.

2. WET: good showers with front tonight, rapidly changing to snow above 7.5K, with some damage to weaker fully-leafed aspen (have no idea how much the recent aspen disease outbreak has weakened the affected trees, but almost all leaves are still up below 9K). Some lucky and intrepid observers could measure half a foot by tomorrow morning (not on open ground, just grassy areas). Still mostly dry around midday tomorrow, but cool enough for some of the snow to survive the day (in the shade/where the ground is not too warm). Lower elevations might get some good rain showers, even with the potential for a late evening thunderstorm with the front.

Tomorrow night things could get interesting: if the precipitation gets heavy reasonably early AND we have good upslope flow, snow might mix in down to the lowest elevations by Friday morning (although the odds are much better for Friday night). Heavy snow is possible for the higher foothills, a real foot of wet snow being not out of the question, especially by Saturday morning.

More by tomorrow evening, hopefully not a cancellation...

As always in this first PSA (Public Service
Announcement) of the season, feel free
to let me know if you want to be taken of this mailing list, and report any significant snow measurements back to me - or tell me which CoCoRAHS observer you are!

Think snow, this season can't be as bad the last one, especially with El Niño back in the game -
 

PSA#8 - better late than never!  Sun 3/19/2006 2:28 AM
Good morning, y'all,
today's moisture (apparently close to half a foot of heavy wet snow in some foothill locales) was just the harbinger of things to come: a genuine spring snow storm is coming to town, just in time for the March 2003 (and 1998) anniversary! While many models anticipated the general idea of a storm hitting Colorado this weekend, the details remained murky until Saturday, and last evening's precipitation was actually quite underforecast, so there is still quite a bit of uncertainty.

Basically, we have a low dropping in on us from the northwest, while a blocking high is residing near Hudson's Bay, providing us with the necessary cold air to turn this into snow by tomorrow evening. Moisture is ample, both from the Pacific aloft, and from the Gulf near the surface. There is a good chance that this storm will indeed cut off near the Four Corners, giving us about 12-18h of good upslope flow tonight into Monday, although some of the highest precipitation totals may fall on the eastern plains (the models are still waffling on that one). One inch H20 (or more, if you trust the Canadian meso-scale model) are in store for us, on top of what already fell last evening. In the foothills (above 6K or so), snow should accumulate off and on through Sunday AM, with the heaviest snow commencing in the afternoon. In the plains, it might not be before later in the afternoon that the snow will start to stick (it should fall as snow almost everywhere). Snow totals by Monday rush hour: easily 8-12" in the foothills, 4-8" in most of the plains (Boulder probably right in between). "Best case scenario" could drop 12-18" in favored locations (northerly exposure should help), including possibly the Palmer Divide, but that is outside my region of interest.

I will be out of town from Monday afternoon until Friday, but there is an interesting little storm that should pass through about Wednesday. While undercutting the upper level ridge that is trying to build over us, there is the possibility that it could briefly cut off and pass to our south which might give us another chance at more than a few flurries... Compared to last week's storms (which dropped a total of more than 18" and 1" of moisture on our house), this one is not as cold (no sub-zero lows!), and the snow will be a lot 'heavier' (snow/H20 ratio) this weekend than last.

Think snow,

Klaus

PSA # 7  Sat 3/11/2006 5:58 PM
Good evening,two bouts of snow later, we are still looking at another snowy period from tonight through tomorrow evening. The snow that is trying to materialize right now stems from one last ripple in the flow ahead of the main trough lifting out right over us tomorrow. Does not look like strong upslope flow during this period, but decent cold air aloft (allowing for good showers during the day-time tomorrow), good moisture at all levels, and some dynamical forcing due to the jet stream to our south should allow for off and on moderate snow to continue for more than 24h.

Since a fair bit of this snow will be forming at favorable temperatures for dendritic growth (-10/-15C), we could also see some pretty fluffy snow (yesterday morning's snow was 40:1 in many foothill locations) thrown in. The joker in this set-up is that as the low lifts out tomorrow, we might get some good backwash in the PM, which combined with very cold air aloft could allow for some heavy snow-showers as well. While it probably won't really clear out Sunday night, we could see a repeat of near-0F in the higher foothills and low 10s in the plains.

The weather service is going for 3-6" in Boulder and places north, and 5-15" in the foothills, with northern locales being favored. I lean towards slightly higher totals in the plains (it should remain cold enough that untreated roads will be a mess tomorrow morning/evening), but would agree that the foothills would be lucky if a foot accumulates. This goes along with about 0.25-0.5" of moisture, so all in all not quite the moisture relief that I had hoped for earlier this week.

However, the rebound into milder temperatures should get interrupted mid-week with yet another system passing thru and affecting mostly the mountains. Speaking of the mountains, this storm set-up has been kind to the southern mountains for a change, with Wolf Creek boasting 42" in the last 48h, and they should get at least another foot by tomorrow evening. The mid-week 'storm'

should revert us to the familiar "northern-central mountains being favored" pattern. If you can avoid avalanche-prone regions, back-country skiing tomorrow should be about as wintery as it can get around here (the late calendar date notwithstanding).
Enjoy!

Klaus

P.S.: PSA#6 rendered a total of 0.60/11.1" for me from Wednesday thru Friday, while Niwot Ridge (at C-1) had 0.85/15.6" (thanks, Mark). This is the most snow I have had since the October snow!

PSA#6 - Update Thu 3/9/2006 6:02 PM
Good evening, y'all,this is a quick update on tomorrow's storm: we are still on for another bout of day-time snow around here. Main differences to yesterday's snow (which underperformed somewhat in the foothills but dropped close to 0.5" liquid on Boulder and other communities close to the foothills): the ground has been cooled down considerably, so the roads should become snow-covered much faster than yesterday, the air mass will be colder than yesterday as well, so that this time there is NO chance of rain, it will be all snow (we had some rain in the AM below 6K yesterday), and right now it looks like a smaller event, say, from 0.25-0.5" H2O rather than twice the amounts I expected for yesterday. This being spring-time, who knows we might be lucky and get more than expected for a change. In any case, tomorrow's totals should reach 2-4" in the plains and 4-8" in the foothills, with a slight preference for northern locales. The next event after tomorrow will come in a bit faster than expected, say, around Sunday morning, and might be good for another PSA, stay tuned! Think snow!- Klaus

P.S.: Since people are reporting less and less to me ( and I encourage you to report any reasonable snow fall amount you measure, regardless whether I predicted it or not), you can check county by county snowfall (and moisture) amounts at the following web site:

http://www.cocorahs.org/

Just click on "Maps", and pick what you want to see. For instance, this morning's reports for Boulder County showed quite a few 0.5" reports from Boulder, but only two reports at 0.40" for the foothills (the northern one is mine).

PSA # 6 Tue 3/7/2006 5:12 PM
Good afternoon y'all, snow is on its way, and it will feel like winter (again) for at least the next seven days! There is a storm brewing west of us that should track more or less right over us tomorrow, and will bring a good chance for moderate to heavy amounts over much of Colorado (the southeast corner will get the least). Interesting details to worry about: heaviest precipitation will fall during the day-time for us, which may leave paved roads in the plains just wet (it should all fall as snow, however); it looks like a fast mover right now, but it involves a short-lived cutoff circulation which, if it stalls, could easily double the expected snow amounts; models are still all over the place with the exact placement of the heaviest precipitation (anywhere from southeastern WY to the Palmer Divide), but someone could easily wind up with an inch of moisture out of all this; today's down slope winds brought in some awfully dry air (yet again) that needs to be replaced/moistened up which will partially accomplished by seeding from above ('virga' = wasted precipitation that evaporates into the air below) and from low-level flow from the north and east that will bring in moister (and colder) air by tomorrow morning.

Bottom-line: expect snow to begin tomorrow morning (the earlier, the better in terms of how long it will take to moisten up the atmosphere) and should fairly showery most of the day; heaviest snow bursts could occur just before evening rush hour, making for highly variable road conditions. Total amounts should be highest in the east-facing foothills (the higher, the less wastage to melting from below) and reach at least 3-6" in the plains and easily twice that higher up.

Would not be surprised to see 2"/h for some of the better pockets of snow, giving totals of up to 18" in best locales. In terms of moisture, most people should see half an inch or more, giving us a nice break from the fire danger.

The pattern will remain cold and unsettled through the weekend, giving us at least two more chances for several inches around Friday and early next week (if I see a chance for another foot, watch our for PSA#7), while temperatures should remain below freezing through the period above 8K, and with fresh snow lows could reach near 0F once again. If it all pans out, we might end up with a normal March's worth of moisture by this time next week!

Think snow, those wild fires have been flaring up way too early!

- Klaus

P.S.: I hope you still have your gloves handy, snow tires on your cars, and remember to disconnect the garden hoses from their outside faucets.

PSA 5 Update Fri 2/17/2006 5:29 PM
Good evening, minor updates for tonight and this weekend:
1. This air is quite a bit colder than predicted, especially on the plains. Expect to see lows at or below -5F tonight in the plains and below -10F higher up.

For Boulder and Denver, the daily records are just a few degrees below 0F, so we might see a few record lows around here.

2. Today's cold push also came in earlier than advertised (keeping temperatures in the plains steady or dropping all day with a stiff (feeling) northerly breeze).

It has now filled up to at least 9000' feet, but Niwot Ridge is still 'basking' in the 10s right now, and may not drop below zero at all tonight. Since the cold air came in so early, we have less of an upslope push to work with this evening, but there still is moisture coming in aloft from the southwest that should be good for at least a dusting tonight - just don't expect several inches. Mind you this subtropical moisture fetch is giving Wolf Creek Pass heavy snow right now - one of the best storms of the season so far!

3. The last bout of snow and cold on Sunday now looks weaker than expected two days ago, but should keep everyone below freezing and with a final upslope surge give most of us some light snow.

4. Downsloping flow should return by Monday, but may not be strong enough to scour out the arctic on the plains (especially in places like Greeley and Longmont). Of course, the mid-February sun is going to do its part to help us warm up a few degrees every day. Since there still is the chance for another cold surge from the north around Wednesday, a rebound into the 50s may have wait for quite a while.

Let's hope that my snow forecasts are too conservative this time for our region.

The change to west-northwesterly flow by Monday should resume the favorable regime for the northern and central mountains next week, although the moisture fetch does not look too impressive right now.

Have a nice and wintery President's weekend -

Klaus

P.S.: Two day totals so far are in the 4-8" range from Boulder northward, with some lower foothill locations slightly above that.

PSA # 5 Wed 2/15/2006 6:18 PM
Good evening, y'all -better late than never, here is tentative PSA for the next five or six days that promises no big dumps around here, but plenty of winter weather, including snow off and on through the period, and continuous below freezing temperatures, reaching close to 0F even in the plains.

Tonight, a storm that has been brewing to our west, and brought overrunning precipitation to southeastern Wyoming for the last 24h (up to 18" north of Cheyenne!!!), will make its move across the mountains and give us a few hours of moderate snow overnight. Since there is a strong jet streak involved, some east-west oriented heavier bands of snow could bring twice the amount of snow to some lucky folks, but pretty much everyone should get at least a couple of inches. The whole storm should also sag from north to south as it translates across the divide, but northern foothills should on average get more snow than southern ones (say, south of Boulder). So, by tomorrow morning, some reports could be up to 10", yet again (although the highest amount I saw last time was "only" 7.5", with widespread 5"
amounts in western Boulder County - where were the reports from Boulder Heights, Coal Creek Canyon, and around Estes Park???), while others may only measure 2". However, it has been cold all day, and should remain cold enough for any snow to stick around into at least early next week.

A second surge of cold air and upslope flow should materialize on Friday (PM) and be good for another couple of inches (possibly more in the foothills). And, finally, the last piece of energy trapped as a cutoff low over the Great Basin should eject across Colorado in the Sunday/Monday time-frame, and be good for another round of snow, possibly more than just a few inches. Since I will be out of town over the weekend, that one will probably be the biggest one...

Considering that arctic air from the Hudson's Bay area has been moving steadily southwestward and is projected to reach Montana over the next few days, where it will get well below zero (possibly into the -20s and -30s after highs in the 50s just three days ago), our expected lows near 0F (o.k., -10F higher up, if it clears out long enough) are pretty puny in comparison.

Enjoy our return to winter over the next week, it should remain relatively calm as well, which is a nice change of pace from more than two months of almost relentless downslope winds!

Think snow -

Klaus

P.S.: If I see a big change in this scenario by Friday, expect an updated forecast by then!

PSA # 4 Thu 2/9/2006 4:46 PM
Good evening, y'all -winter is knocking at our door again after a few weeks of warm/windy weather (although my last PSA verified o.k., especially from Nederland south). We had a pretty impressive frontal passage around noon, with a 15F drop in a few minutes and wind gusts up to 40mph

(NCAR) from the northwest. It has taken another four hours or so to cloud over, and the moisture situation is a bit underwhelming, but should be sufficient for snow to start in the next few hours.

This is going to be a fairly quick affair, with most of the snow falling between about 6pm and midnight, and anywhere from 0.1" to 0.5" of moisture, depending on your locale (eastern plains will get the least, northeastern exposures the most, probably in lower foothills rather than higher foothills) and which model you believe. This translates into an inch or two for the least favored locations and up to 10" of fluff for the lucky winner. There will be little spill-over into Western Colorado with this storm, for a change. All models do agree on a very fast temperature drop to near 10F (0F) in the plains (foothills) by tomorrow morning, and little warming during the day tomorrow. In fact, the air may be unstable enough to allow for some scattered snow showers to fall during the afternoon tomorrow as the coldest air passes overhead.

With fresh snow on the ground, the plains should see its lowest temperatures tomorrow night (close to 0F), while the higher elevations may get too breezy to drop much below 0F.

Since the air aloft is going to be the coldest since mid-December, some protected spots higher up could drop below -10F nevertheless.

After a slow warm-up over the weekend (especially if we get more than a dusting), there is potential for another PSA-worthy event in about a week that could make tonight's event look wimpy by comparison.

Stay tuned, it looks like winter is not going to remain AWOL for the next week or two!

Think snow -

Klaus

P.S.: With all the recent warm weather, I would anticipate some nasty icing to form by tomorrow morning's rush hour, even on treated surfaces (since the air temperature will be so low).