Category Archives: Winter Storms

A Storm with Big Winners and Big Losers

I can't recall a storm in which the winners and losers were more defined than the one that hit us Monday night through Wednesday.  In part, this reflects our meager snowpack, which makes the vulnerability to warmth and rain even more obvious than when we have a more robust low-elevation snowpack.

The winners were clearly the upper-elevations of central Wasatch, above perhaps 8500 or 9000 feet.  The central Wasatch have a great deal going for it, including elevation, and it paid off in spades this week.  Data from the Snowbird SNOTEL shows a nice uptick in snowpack water equivalent. 

Source: NWS
The storm represented only the fourth since mid November, but it finally brings us to something close to mid December snowpack for this aspect and elevation

The losers?  The storm was pretty much a disaster near or below 7000 feet where almost all the precipitation fell in the form of rain.  I was glad to find late yesterday that Mountain Dell still had enough snow to skate ski, but the situation there and along trails in the Park City area is now critical or beyond critical.  Such a shame. 

The Ben Lomond Trail SNOTEL is at 6000 feet and it illustrates the dire situation down low.  Keep in mind this is an extremely snowy location — typically with a much deeper snowpack than found at comparable or even higher elevations in the central Wasatch or Park City area.  There was a bit of a bump in snowpack water equivalent at this location during the storm, with no net change.  I suspect what happened is that the snowpack soaked up the rain initially, causing an increase, afterwhich it experienced net melting and loss.  The site now sits at about 25% of median. 

Source: NWS
Looking forward, we will see some snow showers at times through Friday.  Then, the zombie apocalypse returns with a high amplitude ridge developing over western North America.  


The positive?  It looks like a beautiful Martin Luther King weekend.  For many out-of-towners, and what should be excellent conditions on the groomers, it will be the stuff that dreams are made of.  I expect there will be big smiles at the Cottonwood Canyon resorts.  

The Warm of the Storm

Let's start with the good news.

If my math is right, storm total water equivalent at Alta-Collins through 7 am (9662 ft) is 1.20", producing 9" of base-building Cascade concrete.  Snow, possibly heavy at times, will continue through much of the morning.  The HRRR has things winding down late morning.  The NAM keeps a few snow showers around in the afternoon.  I'll go with another 2-5", mainly from 7 to 11 AM this morning.  We'll do a bit better than that if we can a good burst going or snow showers in the afternoon are more productive than expected.  These numbers are lower than the NWS Cottonwood Canyons forecast, but looking at the radar and the HRRR, I don't see us doing much better than that.  Even the "5" in 2-5 was a stretch.  Hope I'm wrong and they are right.

Now, let's shift to the bad news.

The prefrontal southwesterly flow was even warmer than I anticipated and temperatures overnight have cooled more slowly than expected.  A look at the temperature and precipitation graphs for Alta Collins shows temperatures hovering from 31-32ºF until yesterday evening and then only cooling very gradually to the current (7 am) value of 29ºF.




Thus, while we've added base builder in the upper elevations, the mid and lower elevation snowpack has taken a serious hit.  Further, the snow level remains high.  Even now, it is 34ºF at 8500 feet and the highest traffic camera in Big Cottonwood Canyon, which is near 7500 feet, shows only wet roads. 

I hate to say it, but get out and enjoy the high elevation snow today.   We may get a bit of windy "dirty ridge" snowshowers Thursday and Friday, but it won't add up to much.  Then the ridge returns.

Beggars can't be choosers.

Addendum @ 10 am

5 inches and .48" of water at Alta Collins since 7am.  Storm just exploded.  Wonderful! 

Wahoo, a Storm!

We have a bit of moisture coming into Utah for the first part of the work week, which I suspect everyone will be glad to see.

The situation this morning features a complex trough off the Pacific coast with a broad region of moist air with high precipitable water (color contours) affecting the entire California coast.

Observed radar, IR satellite imagery and NAM precipitable water and 700-mb wind barbs valid 1500 UTC 8 Jan
By 0600 UTC 9 January (11 PM MST Tonight), the coplex trough is broken into two pieces, one moving over the Pacific Northwest coast, the other approaching California.  Northern Utah is under the influence of bands of moisture moving into the western interior in the southwest flow.  

NAM 500-mb heights, 700-mb wind, satellite imagery, and 3-h accumulated precipitation valid 0600 UTC 9 Jan
That situation persists overnight and for much of the day tomorrow as the two troughs move inland and the 700-mb flow over northern Utah gradually veers (turns clockwise).

NAM 500-mb heights, 700-mb wind, satellite imagery, and 3-h accumulated precipitation valid 2100 UTC 9 Jan
Then, late Tuesday night and early Wednesday morning, the northern trough and accompanying cold front push into northern Utah and the flow switches to northwesterly.  

NAM 500-mb heights, 700-mb wind, satellite imagery, and 3-h accumulated precipitation valid 1500 UTC 10 Jan
So, this will essentially be a three-part storm.  The first part will feature the southwesterly flow and precipitation in waves for the mountains.  The second part is the frontal passage late Tuesday night or early Wednesday.  The third part is the post-frontal period thereafter.  These stages are evident in the NAM time-height section below (remember time increases to the left in this plot — more on how to interpret these diagrams here).  


The blue line in the plot above is the freezing level, which looks to be fairly high during the southwest flow portion of the event.  The freezing level should not be confused with the snow level as those are two different things.  The freezing level is the latitude at which the temperature is 0ºC, but the snow level is below this level.  How far depends on how temperatures change with height in the atmosphere, the relative humidity, and other factors.  Complicating the snow level forecast for the southwesterly flow period is the cold, dry air currently in place in the valleys.  That could help keep the snow level low initially, but it will eventually warm enough that we will be looking at snow levels between 6500 and 8000 feet until the front approaches Tuesday night.  Snow levels will drop with the frontal passage.  

The 0600 UTC NAM generates 1.1 inches of water equivalent at Alta Collins from 7PM tonight through 9 AM Wednesday morning.  Nearly all of that precipitation falls in the southwesterly flow and front periods, with little post-frontal.  Downscaled SREF plumes for Alta Collins show anywhere from about 0.75" to nearly 3" of water.  There is strong clustering by the model used, with the NMB members producing less precipitation in general than the ARW members, which is typical.  A few members to for some significant post-frontal precipitation.  

Looking at things spatially, the downscaled SREF shows high (>90% probability) of 1" or more of water at most high elevations in the Wasatch Range.  Higher elevations regions that typically do well in southwesterly flow, including Ben Lomond, Snowbasin, and Timpanogos Peak have > 80% chance of 2" of water.   Most of this precipitation falls through Wednesday morning, although there are a few members that get higher precipitation totals due to precipitation Wednesday or Wednesday night.  Thus, if you want odds through Wednesday morning, cut those numbers a bit.  

By and large, this looks like a decent wet storm for the high elevations, with high density snow at times tonight through Tuesday night, and snow levels (and snow densities) dropping late Tuesday night or early Wednesday morning with the frontal passage.  Mid elevations will depend on snow levels.  The base of Snowbasin and Park City for example, could see rain at times during the southwesterly flow period.  

I was going to say "tip of the week is to call in sick Wednesday morning" as the right-side up nature of the snowfall should make for great turns in the upper elevations, but the red light will be on for backcountry skiing on or below steep terrain.  At the resorts, keep your friendly ski patrollers happy as they will have their work cut out for them Wednesday morning.  

A Remarkable Nor’easter

Let me provide a couple of observations that will blow your mind this morning.

At 6 AM MST, it was 31ºF on Alta's Mt. Baldy (11,066 ft.).

At the same time, it was 30ºF in Tallahassee, Florida, with ice pellets and snow.

That's right.  SNOW.  In Florida.

A remarkable weather event is unfolding as a nor'easter develops along the eastern seaboard.  It is a textbook event, of the type I watched for as a kid viewing the evening news.  The loop below shows the classic high-amplitude wave setup with a ridge over western North America and a deep trough over eastern North America.  In addition, there is a short wave trough moving through the long wave pattern.  You can see it moving from near Wyoming to the Golf coast in the loop below.  


In the loop above, you can see how the large-scale ascent associated with the upper-level trough is generating clouds and precipiation as it moves over the southeast United States.  However, that trough will also spark cyclogenesis as it rounds the long-wave trough and is able to move over warm, unstable, water-vapor-laden air over the Atlantic Ocean and along the Gulf Stream.  


The end result is a very powerful nor'easter.  The NAM forecast initialized at 0600 UTC deepens the system from a weak, inverted trough along the Florida coast with a minimum sea level pressure of about 1013 mb at 1200 UTC this morning to a 961 mb low off the coast of North Carolina by 1200 UTC 4 Jan and then 953 mb by 0000 UTC 5 Jan over the Gulf of Maine.  




The GFS isn't quite as deep (not shown), but the 0000 UTC had similar numbers to the NAM with central pressures of 961 mb and 948 mb at 1200 UTC 4 Jan and 0000 UTC 5 Jan respectively.  

Using numbers from the NAM, that is a fall in central pressure of 52 mb in 24 hours and 60 mb in 36 hours, easily meeting the criteria for what is known as explosive cyclogenesis, or rapid cyclone development.  

The structure of the NAM-forecast storm off the North Carolina coast is consistent with an intense marine cyclone.  The cold and warm/occluded fronts are oriented at right angles, forming a frontal T-bone.  The temperature gradient along the cold front weakens near the warm/occluded, a characteristic known as the frontal fracture (there is some uncertainty in the exact cold front position in this forecast and one might place it further west than I have).  The 925-mb temperature contours (black) become quite concentrated along the occluded front near the low center and its back-bent extension south of the low, where the strongest 925 mb winds are found (color fill).  This area of strong winds is referred to as the poisonous tail of the back-bent occlusion by Norwegian meteorologists.  Cold air has encircled the low center, cutting off a pocket of warm air known as a warm-core seclusion.  


The severity of ompacts along the eastern seaboard will depend strongly on storm track as a slight shift to the west or east will make a difference in the amount of precipitation, which  and severity of winds along the coast.  One need only look at the watches, warnings, and advisories map from the Boston National Weather Service Office to realize what a mess this storm could bring.  

If you are in the area, monitor official forecasts.  

Steenburgh Effect This Week

I am currently on travel, which is good news for you, because it means the Steenburgh effect is en force.  I haven't had time for a proper look at the models, but did notice that they are looking active for this week, as evinced by the NAEFS ensemble plumes for Alta.

As we mentioned a week or two ago, it ain't over until it's over.  Keep the skis waxed and get on it before it's baked.

Last night I received the Charles Hosler Alumni Scholar Award from Penn State's College of Earth and Mineral Sciences for contributions to the development of science through teaching, research and administrative leadership. Dr. Hosler served his country in WWII and Penn State as Prof. of Meteorology, Dean of Earth and Mineral Sciences, VP of Research, Dean of the Graduate School, and Provost. He also signed my acceptance letter to Penn State in 1985. It was an honor to sit with him and receive the award from Dean Bill Easterling.


I'm pretty shocked about the whole thing as the College of Earth and Mineral Sciences has a ton of excellent alumni and I'm really a ski bum with a Ph.D.   Despite the fact that Pennsylvania is Austrian for "ski purgatory" I was very fortunate to attend Penn State and learn from so many great faculty.  I'm also fortunate to have a wife and kids, family and friends, students and post-docs, and colleagues and collaborators who have helped me go places I could never go by myself.

It’s Not Over Until It’s Over

This weekend's storm provided a wonderful exclamation point for the ski season.  Really, I was feeling depressed about how the season was going out with a whimper rather than a bang.  From March 5th to April 8th, there was only one day during which Alta recorded 10 inches of snow (March 27th) and we suffered through the hottest March on record.  Corn is fine, but I'd rather be skiing powder.  For April, Sunday was as good as it gets, and I liked it so much that I did a dawn patrol yesterday morning to get in some more turns before class.

My season now feels complete, although it's never over until it's over.  I've skied good (not great) powder on Memorial Day weekend in the Wasatch.  It happens.

And, while we're talking about it not being over until it's over, we turn our attention to California.  The models continue to show this prolonged period of southwesterly large-scale flow over the eastern Pacific and western US in which California continues to see storm after storm.  For the next seven days, our downscaled NAEFS ensemble product is producing mean water equivalents reaching over 3 inches.

Such numbers are not remarkable compared to wet periods this past winter, but they show winter hasn't yet quit in the Sierras.  In addition, it will likely do the job for breaking the wettest water year on record in that region.

I really need to take a trip over there to check out the snowpack.

Looking Back at Yesterday’s Epic Deluge

Yesterday we were in clear outlier mode in Salt Lake City as we broke the all-time record for precipitation on a calendar day in March and experienced one of the wettest days ever.  I mangled some of the numbers in the previous post, so we'll take a little bit of time today to look back on the event and set the record straight.

The primary large-scale feature driving the event was a mid-level trough that moved very slowly across Salt Lake City and stalled very near the Wasatch Crest for most of the day.  This trough was a large-scale feature extending northeastward to the Canadian border, and served as the locus for precipitation development along much of its lengths.  The image below shows the scene at 2000 UTC (2 PM MDT).  Wind barbs are at 700 mb (10,000 ft).  Gaps in the radar echo coverage in portions of Wyoming reflect both poor radar coverage and some orographic effects.


Cloud and precipitation bands of this type are sometimes called "wrap-around" because the wrap around the backside of the surface or low-level trough.  Indeed that was the case yesterday, as shown below with the sea level pressure analysis, although I'm not entirely satisfied with that classification of this system for a variety of reasons I won't get into here for lack of time.


The National Weather Service reports that the daily precipitation was 1.97".  Note that this is for the midnight to midnight calendar day based on Mountain Standard Time.  We are currently on Mountain Daylight Time.  If the calendar day was defined based on daylight time, the daily total would have been an even larger 2.07 inches, based on surface airways observations provided by the airport.  This highlights an important aspect of meteorological records.  Calendar day records are not the same as 24-hour records, since the latter can use arbitrary start and end times.  Often, major precipitation events straddle calendar days and are broken into two smaller pieces.  If you are using statistics of 24-hour precipitation based on calendar day records to design your storm-runoff system, this is an issue to consider!  Nevertheless, many meteorological records are based on calendar days, I suppose for historical and practical reasons.  For example, many of our meteorological records are collected by volunteers who provide daily observations of maximum temperature, minimum temperature and precipitation amount.

Below is a summary of the 50 largest calendar day precipitation events at the Salt Lake City airport and, prior to the creation of the airport, downtown.  1.97" makes yesterday the 6th wettest calendar day on record and the wettest March day on record.

Source: NOAA Regional climate Centers
The precipitation, however, was not evenly distributed during the calendar day.  It was heaviest prior to about 6 AM MDT, when over an inch fell in the prior 6-hour period, and ceased at 5:40 PM MDT (at the airport, showers lingered on the east bench).  Radar imagery at about 4 AM MDT (0957 UTC) shows that the area around the Airport and portions of the I-15 corridor to the north experience very heavy precipitation with areas of radar reflectivity greater than 35 dBZ (yellow) and localized pockets above 40 dBZ.  Although I'm not showing a loop, these areas of heavy precipitation were quasistationary for much of the night and very important in pushing the precipitation amounts at Salt Lake City to record levels.  

Source: NCAR/RAL
As wet as it was at the airport, there were locations that were wetter.  Tooele recorded 2.49 inches of precipitation and the Rocky Basin Settlement SNOTEL in the Oquirrh Mountains recorded 2 inches of precipitation water equivalent and 22 inches of snow from midnight to midnight MST.  

The central Wasatch didn't do as well because the strongest precipitation along the band was just to the west, along with the northwesterly flow behind the mid-level trough.  As a result, the large-scale precipitation dynamics in the central Wasatch yesterday were weaker.  If one looks at the wind time series on top of Alta's Mt. Baldy, one can see the slow trough passage from 0000 MDT on the 23rd to 1800 MDT on the 23rd with the wind shifting very gradually from south to north-northwest.  Note, however, how weak the winds were for much of the period, especially from 0000-1200 MDT.  It wasn't until the trough had passed late in the day that the northwesterly flow intensified, but by then, the larger-scale precipitation dynamics were dying.  

Source: MesoWest
It wasn't a total disaster for the central Wasatch.  For the midnight to midnight MST period, Alta-Collins picked up about 9 inches of snow and 0.82" of water.  However, this is less than observed in the northern Wasatch and far less than observed in the Oquirrh Mountains.  

The spring pattern continues tonight and tomorrow with a cold front moving in and bringing mountain snow to the central Wasatch starting early tomorrow morning.  The NAM meteogram below tells the story pretty well.  This looks to be a quick hitting event, dumping several inches of snow in the morning, tapering off quickly to snow showers afternoon.  

Source: weather.utah.edu
Last night's NCAR ensemble produced anywhere from 0.6 to 1.6 inches of water for Alta-Collins.  Most members are between 0.6 and 1.1 inches of water.  

Probably 4-8 inch totals lie in the most likely range, but lets keep our fingers crossed we can do a little better.  

A Night of Extreme Change

The "Steenburgh Effect" was in full play this weekend as I was out of town skiing in Jackson Hole and of course missed out blogging about the best frontal passage we've had in months.  Nevertheless, I saw what was coming and we ended up getting off the mountain at 1 PM yesterday, and racing home via Evanston, experiencing the full force of the pre-frontal southerlies, but arriving home just ahead of the post-frontal snowmaggedon.

I took a quick gander through the wind reports over northern Utah and it appears that nobody escaped the full force of the winds.  Gusts over 70 mph were reported at all elevations and in many areas that you might think of as being "sheltered."  Here are a few selected peak gusts, all of which were recorded in the pre-frontal environment.

Mountains
Arrowhead Summit (Sundance): SSW 110 mph (0010 UTC)
Windy Peak (Uinta Mts): SW 97 mph (0100 UTC)
Mt. Baldy (Central Wasatch): SSW 96 mph (0000 UTC)
Big Mountain Pass: SW 96 mph (2220 UTC)
Ogden Peak: S 89 mph (1530 UTC)

Mid-Elevation Hills and Canyons
Eureka (6584 ft): W 95 mph (2240 UTC)
Vernon Hill (5761 ft): SE 90 mph (2230 UTC)
Deer Creek Dam (5429 ft): SSW 83 mph (0110 UTC)
Mayflower Summit/US-40 (6929 ft): S 81 mph (0120 UTC)
S-Turns Big Cottonwood (6235 ft): W 78 mph (2230 UTC)

Salt Lake Valley
SR-201/I-80: WSW 82 (2310 UTC)
Olympus Cove (4960 ft): SW 73 mph (0030 UTC)
Kennecott Tailings Magna (4440 ft): SW 72 mph (0000 UTC)
Draper (5052 ft): W 68 mph (2140 UTC)
University of Utah: S 64 mph (2315 UTC)

The driver of these strong winds was a strong cold front that developed over the Great Basin and moved across northern Utah yesterday afternoon.  The intense surface trough accompanying the front featured strong pre- and post-frontal pressure gradients.


Although the strongest gusts were generally observed ahead of the front, the winds behind the cold front were nothing to laugh about either.  Here at the University of Utah, the peak gust of 64 mph occurred about 90 minutes ahead of the frontal wind shift.  Although there was a temporary drop in the wind speeds following the frontal passage, they picked up again, with a prolonged period of W-NW flow and gusts above 30 knots, as the frontal precipitation band came through.


The lag between the surface based front, as indicated by the pressure trough, and the precipitation band was clearly evident in the analysis for 0200 UTC below.


The combination of heavy snow and strong W-NW flow made for a quick coating of white at our place in the upper Avenues (about 5000 feet) by about 8 PM MST.  It's always hard to take night-time pictures of snow, especially with an old cell phone, but the combination of snow and wind made for about the nastiest weather I've seen in the valley this winter.


I suspect this morning provided a rude awakening for many as temperatures on campus are around 24ºF with NW winds gusting from 15-20 mph over the past couple of hours.  Throw in icy sidewalks and you have conditions best described as "sporting."  After a high on campus yesterday of 59ºF, it is safe to say that it was a night of extreme change.

Also this morning in the post-frontal environment we have some beautiful orographic clouds draped over the mountains, such as these over the Oquirrh Mountains as I walked to the bus at about 7 am.


Thanks to the post-frontal convection, graupel has been falling at times on campus.


Enjoy this taste of winter.

The Valley Doth Blow as Hard as the Mountains

Here's an interesting non-alternative fact about our current wind storm.  Peak gusts over the past 24-hour period ending at around 8:30 AM are quite similar in the valleys and on the mountain ridges.

Three sites have reported gusts in excess of 70 mph, two upper-elevation sites near Snowbasin (83 and 73 mph) and one in the western Uintas (77 mph) and these are the highest gusts so far in northern Utah.  So, technically one can find a couple of mountain sites that have the highest gusts in the region.

However, if we move a bit closer to the Salt Lake Valley, we see that peak gusts in the valleys are comparable to those in the mountains.  Here's a selection from MesoWest:

SR-201/I-80 Junction: 69 mph
Park City Ridgeline: 69 mph
Mount Baldy: 68 mph
Stockton Bar: 68 mph
Great Salt Lake Marina: 67 mph
Mid-elevation Mt. Timpanogos: 66 mph
Sandy: 66 mph

So, what gives?  Why is the wind blowing so hard in the valleys, especially overnight?

The peak gust in the Salt Lake Valley occurred at 1200 UTC (0500 MST) this morning at the intersection of SR-201 and I-80, but really at that site it blew quite hard all night long, picking up at around 0200 UTC (7 PM MST) yesterday evening.

Source: MesoWest
The valley winds are being driven by a strong pressure gradient between high pressure centered in the Four Corners area and low pressure over the Pacific Northwest.  The sea-level pressure gradient from southeast to northwest Utah is around 12 mb and concentrated in particular near the Salt Lake Valley. 


This pattern favors pressure-driven channeling within the lower-elevation valleys, so much so that winds are comparable to those experienced on the highest peaks.

The big blow is not over yet.  Hold on to your hats today as a cold front approaches from the northwest and is expected to arrive in the Salt Lake Valley around 7-9 PM tonight, when our flirtation with spring thankfully ends.  

The Jackson Windstorm

On the evening of Tuesday, 7 February (MST), strong, damaging winds struck the Jackson, WY area, snapping 17 steel power poles along the Moose-Wilson Road and, cutting off power to Teton Village and Jackson Hole Mountain Resort.  

Source: Lower Valley Energy, via FirstTracksOnline.
After several days of closure, Jackson Hole Mountain Resort reopened yesterday (Monday).

I thought we would take a look today at the events conspiring to wreak this havoc, although I confess I have more questions than answers at this time.

There were no wind observations provided to MesoWest in the immediate area of the damage.  As such, for this preliminary write up, we are forced to rely on observations from the vicinity.  The first is from the Jackson Airport, which reported sustianed winds reaching 40 mph and a peak gust just above 60 mph at about 0100 UTC (6 PM MST) before observations ceased. The wind direction during this period was S-SW.


The other nearby site is at the summit of Jackson Hole Mountain Resort.  The peak gust there was just above 75 mph at just after 0000 UTC (5 PM MST, note the differing time axis).  Note that at this elevation, the sustained winds and the gusts during this period are not much stronger than those observed in the late afternoon and evening of the prior day.  This contrasts with the Jackson Hole Airport where winds were much strong, indicating that something happened to cause anomalously strong valley winds.


At 1800 UTC 7 February, about 6 hours prior to the wind event, regional analyses from the GFS show a broad, low amplitude ridge across the western United States with the ridge axis centered over Nevada.  An intense jet, with wind speeds of over 75 m/s (150 knots) extended across far northern California and Nevada.  At 700 mb (10,000 ft above sea level, roughly crest height near Jackson Hole Mountain Resort), the flow was southwesterly and featured strong warm advection (i.e., winds transporting warm air into the region from the southwest).  Basically, Jackson was in a warm-frontal zone.


The influence of this strong warm advection is clearly seen in the temperature time series from both the Jackson Airport and the summit of Jackson Hole Mountain Resort (note the differing time axes).  At both sites, temperatures climbed prior to the wind event.  The climb at the summit of Jackson Hole Mountain Resort (bottom image) was steady throughout the day, whereas at Jackson Hole Airport, it was more abrupt and occurred in the 2 hours prior to the wind storm.  This often occurs with warm-frontal passages in which the cold air remains more persistent at low levels, especially deep mountain valleys.   Note that the peak wind gusts may have occurred at the time of peak temperature, although the power outage precludes any chance to confirm that.




At 0000 UTC 8 February (5 PM MST 7 February), just prior to the wind event, an intense zonal (westerly) jet extended across northern California, Nevada, and Utah, just south of Jackson Hole.  Temperatures at 700-mb had warmed significantly and Jackson was basically in the "ridge" of warmest air, with cold advection just upstream over Idaho.  The surface trough, which is poorly analyzed at this resolution, was also very close to Jackson.  


By 0600 UTC 8 February (11 PM MST 7 February), the upper-level flow over Jackson was veering and becoming WNW in response to the building upstream ridge and Jackson was in the wake of the ridge of warm air at 700 mb and experiencing regional-scale cold advection.


Analyses from the High Resolution Rapid Refresh (HRRR) show a surge of strong southwesterly flow up the eastern Snake River Plain just prior to the winds storm.  Note in particular the "break-in" of strong southwesterly to westerly flow into the Jackson Hole region from 2300-0100 UTC below.

HRRR surface winds at 2300 UTC 7 February
HRRR surface winds at 0000 UTC 8 February
HRRR surface winds at 01 UTC 8 February
There is evidence of of a band of convection sagging through the Snake River Plain during the wind event.  The lack of echoes over Jackson, however, is somewhat meaningless due to its remote distance from the radars, which means the coverage is terrible.

Source: NCAR/College of DuPage
Source: NCAR/College of DuPage
So, given the limited data and the limited time I have to investigate this event, let me provide some early speculation.  I suspect that an old occluded front and accompanying tongue of warm air passed across the Jackson area at the time of maximum winds.  This feature was accompanied by a surge of strong SW-W flow.  This is fairly consistent with my forecast that morning in which I warned of the potential for strong winds in the Wasatch Range (The Big Bad Wolf Is Coming to Town).  What is less clear is whether or not the strongest winds were produced by some sort of terrain-driven circulation as that feature moved through, or potentially some sort of precipitation-induced feature if the convection over southeast Idaho extended into the Jackson area.  

I am inclined to suspect the former, but only a careful investigation of this event in which we examine the high-resolution radar data, all surface wind observations, and perhaps do some numerical modeling, will allow us to determine if that hypothesis holds water.  If you were in the Jackson area, please share your observations of the weather on that evening as they could prove useful.