Are Major Winter Storms Increasing in Frequency and Intensity?

After major winter storm cycles, especially those that lead to long Interlodge periods in Little Cottonwood Canyon, I get a lot of questions about whether or not snowstorms are getting bigger due to climate change.  Many people assume this is the case, but it turns out to be a difficult question to answer, especially if you are talking about a specific location like Little Cottonwood Canyon.  

Let's talk first about what we know.  Globally averaged temperatures have increased about 1˚C and Utah has warmed at almost double that rate.  Recent studies indicate that most if not all of the warming in recent decades is due to human activities, especially greenhouse gas emissions.  

Precipitation is a messier variable, however, as it exhibits greater regional variability than temperature.  It is also not sampled as well, with fewer stations and often lower quality measurements.  Finally, there are two questions that one might ask.  First, is the frequency of extreme precipitation events changing (i.e., are they becoming more or less common). Second, is the size (or intensity) of extreme precipitation events changing (i.e., getting bigger or smaller).  

The answers to these questions vary regionally.  It is likely that there are more land areas where the frequency of heavy precipitation events has increased rather than decreased and it is likely that there has also been intensification of the heaviest precipitation events.  For example, the total annual precipitation produced by the largest (top 1%) precipitation days has increased throughout much of contiguous United States, but has remained relatively steady over the southwest.  

Source: https://www.globalchange.gov/browse/indicators/heavy-precipitation

Major winter storms are even more difficult to assess and project trends for and they have an additional confounding factor because in a warming climate, some precipitation events will feature rain instead of snow.  The US National Climate Assessment released in 2017 concluded the following:

“Projections of winter storm frequency and intensity over the United States vary from increasing to decreasing depending on region, but model agreement is poor and confidence is low.”

I have never felt very confident saying that winter storm intensity is increasing in northern Utah for several reasons.  First, snowfall records are much more limited than temperature records, both in their historical coverage and their spatial density.  This makes detecting trends in rare events even more difficult.  Second, our computer models struggle more with precipitation than temperature and, if we're talking about what happens in Little Cottonwood Canyon, don't even account for topographic effects or lake influences.  Additionally, those models don't exhibit a great deal of agreement in how annual average precipitation will change in northern Utah, let alone extreme events.  

Some people may say that we should be able to try theoretical thinking toward this problem.  The challenge with that is many factors affect winter storm intensity.  In a warming climate, the water holding capacity of the atmosphere increases, so storm intensity should increase.  If we are talking about global statistics, that may hold, but regionally that effect could be countered by other changes.  For example, if the storm track shifts northward or ridging becomes more common, we may see fewer storms, so in our area, the frequency of big events goes down.  At some elevations, the "big" storm in a warming climate could be a rainstorm instead of a snowstorm.  Tough luck there. 

So, I can't confidently determine if major winter storms are increasing in frequency or intensity in northern Utah or Little Cottonwood Canyon.  I'm comfortable saying that we don't know, but that we have good evidence that other things are happening to our snow climate that should concern us, especially in the lower elevations where we are seeing a decrease in the fraction of cool-season precipitation that falls as snow, more frequent mid-season snow-loss events, and a decrease in the amount of wintertime precipitation retained in the snowpack at the end of the snow accumulation season.  These trends are less dramatic at upper elevations, but will amplify in the coming decades, especially if greenhouse gas emissions continue.  

Last Storm of Winter...

Thought that headline might get your attention.

Meteorological winter covers December, January, and February and ends this Sunday, the last day of February.  Thus, the last storm of meteorological winter will be this Friday night and Saturday as a broad trough moves through northern Utah from the northwest.  Below is the GFS forecast for 0600 UTC 27 February (11 PM MST Friday) showing northern Utah in the moist, unstable flow.  

The time-height section for Salt Lake City shows this is a storm cycle that's a bit less organized and less juicy than last week's.  There's a weak trough passage late Thursday night/early Friday Morning and then another late Friday night/Saturday morning, followed by a moist post-frontal period.  

The GFS generates a total of 13.5 inches for Alta between 1 AM MST Friday through 7 PM MST Saturday night, the NAM 7.7 inches.  The SREF mean is around GFS levels, with about 0.75 inches of water and 12 inches of snow.  

Thus, this looks more like a Goldilocks storm than last-week's monster, although it will come in pieces.  

Enjoy this last storm of meteorological winter.  If you haven't noticed, the sun is getting higher and the days longer. 

Giving Day

The University of Utah's Annual Giving Day is this Tuesday and Wednesday, February 23-24.  Yes, I know that's two days, but we try to make everything in higher education as complicated as possible.  Technically, the Giving Day is an 1,850 minute fundraising effort, with 1850 being the year of the University's founding.  See, it all makes sense in the end.

If you are a fan of this blog, please consider making a donation.  My fellow faculty and I will match the first $3,050 in donations and once we've reached $3,000, Dave and Johanna Whiteman, who started our Mountain Meteorology Fund, will contribute an additional $5,000.  

There are two options for donations.  The first is our Mountain Meteorology Fund, which supports interdisciplinary research and education to improve the understanding and prediction of weather and climate processes in complex terrain.  In other words, the things I write regularly about on this blog.  To contribute to the Mountain Meteorology Fund, click here and follow the directions at upper left.  

The second is the Atmospheric Sciences Scholarship Fund, which is more critical than ever for our undergraduates.  To donate to the Atmospheric Sciences Scholarship Fund, click here and then click "give" in the upper right.   

If you are reading this on Monday, just before Giving Day, no worries, your donation will still be matched!

Thanks in advance for any contributions.

Jim

State of the Snowpack

It's pretty incredible that with all the snow we received over the past 10 days or so, we are still near or below median at most northern Utah SNOTEL stations.  

Source: NRCS

I think the only site that is above median in northern Utah is Dry Fork in the Northern Oquirrh Mountains.  The recovery there was pretty incredible, with a net gain of about 5 inches in our most recent storm cycle, enabling it to claw out from near the bottom to 101% of median.  

The fattest snowpack in terms of total snow water equivalent is at the Ben Lomond Peak SNOTEL, which now sits at 24.1 inches, which is still only 80% of median.  

Source: NRCS

The Snowbird SNOTEL is a bit behind in total SWE sitting at 22.4 inches, which at that location is 90% of median.  

Source: NRCS

So, the skiing has improved immensely, but the snowpack is still below median! I'm suspect water managers are thankful for the latest storm cycle, but still hoping for more.  

Comparison of the graphs above (note that the scale for Dry Fork is different than for Ben Lomond and Snowbird) shows three salient characteristics of the snow climate of northern Utah.

First, the area around Ben Lomond Peak and the North Ogden Valley is as I like to say "pound for pound the snowiest in northern Utah."  For its elevation, there's no region that I'm aware of that gets more snow.  Eventually, the median snowpack at Snowbird overtakes that at Ben Lomond, but this reflects elevation and aspect.  The Snowbird SNOTEL is on a north aspect at 9177 feet, whereas the Ben Lomond SNOTEL is on an east aspect at 8000 feet.  Thus, the snow accumulation season at Snowbird is longer, enabling the median to eventually peak higher in late April, but through April 1, the median snowpack water equivalent at Ben Lomond is greater.  

Second, the snowpack in the Oquirrhs is typically about half of that in Little Cottonwood.  At least that's my rule of thumb.  The Dry Fork SNOTEL sits at only 10", whereas Snowbird is at 22.4.  However, Dry Fork is quite a bit lower (7147 feet), so that's not really a fair comparison. Another option is the Rocky Basin Settlement SNOTEL at 8702 feet, which sits at 13.8", or about 61% of Snowbird. On the other hand, that is a skiff above Thaynes Canyon on the Wasatch Back (12.0"), which did relatively poorly in this recent storm cycle.  In fact, that SNOTEL still sits well below median.

Source: NRCS

Which brings us to the third characteristic, and that is the remarkable contrast in natural snowpack between Little Cottonwood Canyon and the Wasatch Back, which we discussed in the previous post.  Right now, the snowpack at Thaynes Canyon is only 54% of that at Snowbird.  Usually this time of year the contrast isn't that large (the Thaynes Canyon median is 65% of the Snowbird median), so so far this year, Park City has gotten skunked even more than usual relative to Little Cottonwood.

Why There's a Huge Snowfall Contrast from Alta to Park City

The snowfall contrast between Little Cottonwood Canyon and Park City Mountain Resort over the past 24 hours is quite dramatic.  

In upper Little Cottonwood Canyon, Alta reported 30 inches of snow. 

In upper Big Cottonwood Canyon, Brighton reported 22 inches of snow.

On the Park City Ridgeline, Park City Mountain Resort reported only 12 inches.  

The distance between Alta the top of the Jupiter chairlift at Park City Mountain Resort is less than 5 miles!  That's a huge contrast in snowfall.  How can this happen?

One of my late-career dreams is to have the University of Wyoming King Air research aircraft flying through a storm like this one with it's world-class cloud radar.  This instrument takes cat scans in storms.  One thing I would like to have it do is fly directly down the northwesterly flow and examine the storm structure from northwest to southeast.  Here's why.

We often generalize the western side of the Wasatch Range as the "windward" and wet side and the eastern side as the "leeward" and dry side.  That, however, is an oversimplification.  In reality, the terrain felt by the flow varies depending on wind direction and because the Wasatch range isn't linear, but contains complex ridges, canyons, and inflections.

For example, the late-afternoon sounding from yesterday showed that the 700-mb (crest level or 10,000 foot) flow was from 315˚.  If you are in Little Cottonwood Canyon or the high terrain surrounding it, flow from this direction (i.e., along the thick black line below with the topography along that line in profile at the bottom) moves across the relatively low Salt Lake Valley and then is forced rise rapidly over the Cottonwood Ridge and Alpine Ridge.  These two ridges are over 11,000 feet high and represent the highest terrain in the central Wasatch.  During unstable flow, this results in persistent initiation of clouds and precipitation just upstream and over Little Cottonwood Canyon.  

Source: caltopo.com

As you move northeastward to the Park City Ridgeline, the situation changes.  Not only is the Park City Ridgeline lower (highest peaks 10,000 feet, it is also oriented parallel to the flow.  In addition, because the Wasatch near North Salt Lake City and Bountiful just westward, the flow encounters mountains well upstream of the Park City Ridgeline.  Instead of rapid ascent over about 6,000 vertical feet, the flow ascends more gradually, encountering small ridges along the way.  

This is simply not a great situation for heavy snowfall on the Park City Ridgeline as the terrain-forced ascent is less intense.  Indeed, snowfall reports from Bountiful are in the 10-14 inch range, comparable to that reported by Park City Mountain Resort.  

There are probably some other factors involved as well, but I think these differences in the shape and scale of the terrain felt by the flow from the northwest are a contributor.  Perhaps more on other factors in future posts, but I'm going to have to get some real work done soon.   

It's Still Snowing...

Wow, what a storm cycle.  

It continued to snow overnight in Little Cottonwood as well as much of northern Utah.  More on the latter in a minute, but let's first take a look at Alta.

The tabular output below includes hourly observations over the 24-hour period ending 0700 MST this morning.  The interval board was wiped after 0700 MST yesterday morning, when there was 15 inches on the board, and after 1600 MST yesterday afternoon, when there was 10 inches on the board.  It now sits at 20 inches.  Thus, the total for the 24-hour period from 0700 yesterday to 0700 today is a remarkable 30 inches.  

Looking at the entire storm cycle, total snow depth since 1100 MST 11 February has increased abou 30 inches (ignore the eroneous 136" measurements).  If you add up the interval board measurements at times of peak snow depth, you get a snowfall of 82".  

I guess it goes without saying that everything is coming unglued from a snowpack perspective.  The Town of Alta is in Maximum Security Interlodge this morning as avalanche mitigation efforts begin.  

TOA Max Security Interlodge Go to low levels away from windows, If you have a protocol for your building reply with head count when ready

— Alta Central (@AltaCentral) February 17, 2021

As a meteorologist, I love extreme weather, but these are times I get a deep pit in the stomach and I hope all turns out all right.  Our technology can help mitigate but not necessarily eliminate hazards and we need to remain cognizant of that fact.  

In the backcountry, the Utah Avalanche Center issued, for the first time that I can remember, an extreme avalanche danger for all aspects and elevations in the central Wasatch Range.  

This is essentially a red alert for all winter recreationists as avalanches in these situations can be long running and go into unusual areas.  

There's a lot of talk about this being a lake-effect storm. I want to take a little time here to say that is an oversimplification.  

The past 24 hours have featured remarkably moist, unstable, northwesterly flow.  Precipitation features have been developing across the region, not just downstream of the lake.  One example is the radar image below.  Areas circled in yellow are not downstream of the Great Salt Lake.  For example, you can see radar echoes over the Raft River Range northwest of the lake, over and upstream of the Wellsville Mountains, Cache Valley, and Bear River Range north and northeast of the lake, and over and upstream of the Stansbury Mountains southwest of the lake.  

If there was no Great Salt Lake, it would still be snowing.  

An argument can be made that the frequency and intensity of radar echoes over and downstream of the lake, in the area I've circled in red, is greater.  Thus, I think if you want to say this is a lake enhanced event, that's fine.  Calling it a lake-effect event is a convenient but overly simplified narrative convenience.  Yes, I know these are semantics that only scientists care about, but perhaps the people in Tremonton are wondering where all this lake effect came from. 

Regardless, this has been an incredible storm cycle and it's still snowing...

It's Snowing...

Winter took a while to arrive this year in northern Utah, but it is now delivering in spades.  

If I'm doing my math right, since 0600 UTC 12 February (2300 MST 11 February) the Alta-Collins interval snow stake has measured 54" of snow through 1400 UTC (0700 MST) this morning.  The Utah Avalanche Center site apparently has had enough this morning as when I went to pull it up, I got the dreaded "Too Many Connections" error.

Eventually I got through to find they have hoisted the rare black flag for extreme avalanche danger in the central Wasatch and western Uintas.  

Sadly that was the only page I was able to successfully load, so I will need to wait for more details.  

The radar image for 1457 UTC (0757 MST) this morning shows the long anticipated unstable northwesterly flow.  I didn't bother doing a loop this morning, but if I did it would show transient snow showers moving from northwest to southeast in the lowlands and more persistent stationary echoes over the mountains, consistent with orographic enhancement.  You can see some of this enhancement over the Stansbury, Oquirry, and Wasatch Ranges below.  

Little Cottonwood got raked overnight with 15 inches of snow since 1600 MST yesterday and overnight snowfall rates of up to 2 inches and hour.  The Alta-Collins snow depth is now up to...wait for it...107 inches!

Anyone want to pick the over-under for the opening of Little Cottonwood Canyon?  

Some sites that don't always do great in northwesterly flow were also blessed.  For example, the automated sensor at the Snowbasin-Boardwalk site is up to 16 inches since 1030 MST yesterday.  One reason for this is persistent banding that developed in that area yesterday evening, although they also did well overnight.  

Overnight snowfall on the Park City Ridgeline looks to be around 10 inches as automated sensors at Jupiter and Summit at Park City Mountain Resort look to have gotten to about 10-11 inches since 1600 MST yesterday.  I suspect snowfall is lower than that in town, but will let the locals comment. 

When will this end?  Given the scattered nature of these snow showers, there could be some breaks at times today or tonight, but snowshowers, at least in the Cottonwoods, look to continue into tomorrow.  The right hand violin plots for Alta-Collins suggest odds for the the heaviest snowfall are higher this morning and overnight, with a lull later this afternoon and tapering tomorrow.  

That's based on downscaling, however, rather than simulations of the processes that generate precipitation in these storms.  We will see how things play out.  

It looks like we'll get a break in the action on Thursday before the next storm on Friday.  

Storm Cycle Update

 So far so good.  The Utah Avalanche Center reports that yesterday's storm produced 13" of snow and 1.36" of water in upper Little Cottonwood Canyon.  Storm #2 of the cycle moved in late last night and with warm advection occurring in advance of the upper-level trough, the radar coverage is currently as widespread as it can get.  The only areas without returns are areas with terrain blockage that the radar can't see.  

In the central Wasatch, I expect continuous snowfall this morning.  The trough at crest level is forecast to pass in the early afternoon and should be accompanied by a shift in crest-level winds from southwesterly to northwesterly.  Snowfall will be heavy at times and looks to continue through evening.  The Utah Avalanche Center anticipates 8-14" through early evening, and that's aligned with my thinking.  The radar and model forecasts all point to a significant event.  

If you are looking for something to give you a forecast ulcer, then consider the forecast for the Salt Lake Valley.  My house is at 5000 feet and I'm currently seeing rain with a bit of snow mixed in at times.   MesoWest observations indicate that the freezing level is around 5700-6200 feet (e.g., Mountain Dell @ 5700 ft and the Big Cottonwood S-Curves @ 6235 ft are both 32˚F).  The snow level is usually a bit below the freezing level, but can fluctuate with precipitation intensity.  This is why I'm sometimes seeing some flakes mixed in at my house.  

Will that change?  If we look at the NAM time-height for the airport, for example, the freezing level (blue line) sits at about 875 mb through 0Z Sunday (5 PM MST today), maybe a smidge lower than where it is now.  The NAM generates a trough structure that "tips forward" with height, so while it produces a crest-level trough passage in the afternoon, the valley trough passage and a more dramatic cooling trend doesn't occur until evening.  

The GFS generates a fairly similar forecast, with perhaps a slightly earlier surface trough passage (note that the GFS surface is lower than the NAMs and closer to reality at the airport).  

On the other hand, the HRRR is a bit faster, with the valley-level trough through and northwesterly flow over the Salt Lake Valley at 2200 UTC (1600 MST) this afternoon. 

Thus, I'm anticipating that the valley will see rain and the benches rain or rain mixed with snow this morning.  Things will change to snow first on the benches and then in the valley later this afternoon or evening.  This is in-line with official NWS forecasts and the NWS info graphic below.  

As usual, snow forecasts evolve, so continue to monitor official forecasts at https://www.weather.gov/slc/.

Batten Down the Hatches!

It took me a while to dig out from the latest problems with the National Weather Service server from which we obtain most of our model data.  So it wasn't until this afternoon that I took a look at the model guidance.

Wow.

I hesitate to throw gas on the inferno of hype currently burning through social media, but on paper, things are looking good.

I like large-scale patterns in which the flow is progressive with "open wave" troughs, and this is what we have coming.  This means the waves are lower amplitude.  Think of them as ripples in the flow rather than North Shore monsters.  They move quickly, but come in succession, and give us moist flow with a strong component across the Wasatch Range.  Ideal for orographic snowfall generation. This is what we have coming over the next six days [defined here as 0000 UTC 12 February (5 PM MST Thursday) through 0000 UTC 18 February (5 PM MST Wednesday)].

Just look at the GFS 5-day time-height below.  Lots of low level moisture, occasional deep moisture, frequent low-level instability, and long periods of southwesterly to northwesterly flow.  Snowfall won't be continues, but there's a great series of storms.  

Translation: STORM CYCLE.

Numbers?  For Salt Lake City, the European ensemble mean precipitation (water equivalent) for the six-day period is about 1.25 inches, with a range of 0.85-2.0 inches.  Those are pretty good numbers for the airport, which averages 1.27 inches for the entire month of February. 

For Alta, the GFS produces 3.97" of precipitation and 71.7" of snow. The NAEFS, shown below, has a mean of almost 6" of precipitation an 100" of snow and ranges of about 3-8" and 50-140" (more on the extreme snowfall in a minute).  

For some climatological context, the average precipitation and snowfall at Alta for a six-day period in February are 1.35 and 16 inches, respectively.

Let's talk about those snowfall numbers.  One reason that they are so high is large snow-to-liquid ratios.  In other words, low-density snow.  The average snow-to-liquid ratio at Alta is about 13:1, which means 13" of snow from an inch of water.  For most of this forecast period, our algorithm is going for values at or above that.  

I've been worried this year about our snow-to-liquid ratios being too high.  They've been too high in bigger storms.  Still using an average of 13:1 gives you some big numbers.  

Regardless, it appears the ingredients are in place for a major storm cycle.  However, for a forecast that goes this far out, it's difficult to go all in and a look at the plumes above shows the biggest of the three forecast storms is the last.  There's greater uncertainty at those lead times.  Thus, a "low expectations are the key to a happy life" forecast for Alta would call for 2.5-5" of water and 33-66" of snow for the six-day period.  I wouldn't be surprised if we came in above that.  I would be surprised if we came in much below that.  

Now is the time for me to add the caveat that forecasts change, you should monitor official forecasts from the National Weather Service, and, if you are considering venturing in the backcountry, monitor official forecasts from the Utah Avalanche Center.  For backcountry recreation, this has all the hallmarks of a period during which the best options may eventually be to "cower in the trees" or watch the Alpine World Championships on Peacock.  Personally, I hope this storm comes through, everyone lays low, and we get a big natural cycle to clean out some of the slide paths that have gotten choked up with pucker trees and spindly aspen in recent years.  

Avalanches and Climate Change

Questions about possible linkages to climate change are bound to arise whenever something unusual or exceptional happens, and this year's weak snowpack and high number of avalanche fatalities is no exception.  

Let's start with what we know and what we can say with some confidence.  We are in a period of climate warming, driven by human activity, and this is changing the snow climates of the western United States.  Weather and climate variability give us ups-and-downs even in a changing climate, but we are seeing a shift to warmer mean temperatures, more winter warm spells, and fewer winter cold spells.  

Trends in snowfall and snowpack measures vary, however, depending on the measure, regional climate, and elevation.  In northern Utah, at elevations below about 6500 feet, we are seeing declines in the fraction of wintertime precipitation that falls as snow, more frequent mid-season snow-loss events, and a decrease in the amount of wintertime precipitation retained in the snowpack at the end of the snow accumulation season.  

To date, these trends weaken with elevation and are either small or non-detectable in colder, upper-elevation regions.  However, as warming continues, we will see these trends continue and become more detectable and obvious at higher elevations.  Continued warming appears to be unavoidable through about mid century.  What happens after that is strongly dependent on future greenhouse gas emissions.  

Predicting trends in future avalanche conditions is more challenging.  Snowpack characteristics depend on many factors (e.g., temperature, humidity, precipitation type, precipitation frequency, precipitation amount, long-wave radiation, short-wave radiation, vegetation, terrain, aspect, yada yada yada) and weather variations during the season.  There are many different types of avalanches, each of which is going to be affected differently and in different ways depending on the regional climate, terrain, aspect, etc.  Hard data on past trends is limited.  Current computer models do not simulate fine-scale effects of terrain on snowfall, clouds, radiation, etc., and there hasn't been a great deal of effort put into developing tools to predict how avalanche hazards will evolve in the future.  

Thus, we still rely to a large degree on expert judgement.  As an example, Wilbur and Kraus (2018) surveyed 240 North American avalanche practitioners, receiving 53 responses.  Below is a graph summarizing the responses from experts concerning their impressions for recent and predicted trends in avalanche types, elevation-dependent avalanches,  and other avalanche-relevant variables.  These practitioners reported increases in wet, glide, and upper-elevation avalanches and expected to see those trends continue in the future.  Any recent changes in dry and low-mid elevation avalanches were not statistically significant, although decreases were anticipated for the future.  

Source: Wilbur and Kraus (2018)

Future trends in avalanche conditions are of course important.  They could be considered for the design of highways and avalanche mitigation systems, buildings, development, etc.  However, it is also important to consider cultural and societal trends.  We are in the midst of an incredible upward trend in the number of backcountry recreationists in the Wasatch (and elsewhere).  This is resulting in an increase in the diversity of backcountry recreationists and shifts in backcountry social dynamic.  We are seeing many more dawn patrollers after storms, more recreationists at all times, and more competition for powder.  The weather and snowpack conditions will ebb and flow, but are we trending toward a future in which accidents become more common or less common?  This is a question for people smarter than me.  

Arctic Surge or Not?

This is my first post since Saturday's tragic avalanche in Wilson Glade.  My heart goes out to the victims and their friends and family.  Let me also express my gratitude to all of the first responders, rescue and recovery personnel, and Utah Avalanche Center staff for all of their efforts.  

I've decided to focus on temperature for this post rather than snow.  Often temperature is an "easier" forecast variable than precipitation or snowfall, but that's not the case for this weekend.  

A look at the GFS forecast for 1800 UTC (1100 MST) Saturday shows northern Utah in an arctic airmass that has surged across the Continental divided, dropping 700-mb temperatures in the Salt Lake Valley and Wasatch Mountains to -20˚C or colder.  

In fact, the GFS forecast 700-mb temperature for the Salt Lake City International Airport is -21.4˚C at that time.  

Readers of this blog know that I get excited about 700-mb temperatures above 20˚C or below -20˚C as they are relatively unusual.  The chart below shows the distribution of 700-mb temperatures observed by upper-air soundings collected at the airport.  The thin blue line shows the lowest recorded each day.  You will noticed these brief "tooth like" minima from time to time during the winter months.  These were produced by extreme cold surges.  They don't happen every year and are most common in December and January.  

Source: NOAA/NWS/SPC

From 7 February to 25 February, a 700-mb temperature below -20˚C has only been observed at two sounding times, 1200 UTC 15 February and 0000 UTC 16 February.  This isn't because there's anything special about mid February, it's just that these are unusual and episodic events and we just haven't seen a cold surge that extreme during this 2.5 week period.  

Now that I've attracted your interest, let's take a look at the large-scale setup. Below is the GFS forecast for 1800 UTC (1100 MST) today.  There is a deep upper-level (500-mb) trough centered over southern Canada with an upper-level ridge to the north in the high latitudes.  This produces strong easterly flow between them with and embedded short-wave trough indicated by the red line.  

This short-wave trough migrates westward in the easterly flow and reaches the coast of British Columbia at 1800 UTC (1100 MST) 11 February.  

It then rotates southward and eventually southeastward around the large-scale trough reaching Utah at 1200 UTC (0500 MST) 13 February (Saturday).  

A closeup with 700-mb temperatures shows the accompanying cold front (blue line) is farther downstream in southeast Utah and northeast Colorado.  The coldest air at 700 mb is east of the Continental Divide, but 700-mb temperature in northern Utah are still unusually low and near -20˚C at the airport (they eventually bottom out at -21.4˚C six hours later).  

This is all a wonderful story, but the GFS forecast may be one of the colder ones out there in the model suite.  We don't regularly produce ensemble 700-mb temperature products at weather.utah.edu (maybe we should, but who has time...), so below is the 700-mb GEFS mean temperature and spread from eldoradoweather.com (there is a NOAA logo on this graphic, so I suspect NOAA is the original source). The average 700-mb temperature of the GEFS members is only -12˚C at the airport and the spread is quite large and maximizes near the Continental Divide region. 

https://eldoradoweather.com/current/models/North-Pacific/00-GEFS/gefs-00-700mb-temp.php

All of this indicates that there is a great deal of uncertainty in the temperatures for Saturday as much depends on the characteristics and location of the upper-level trough and whether or not cold air can penetrate across the Divide.

In fact, the ECMWF IFS keeps the cold air near and east of the Divide and northern Utah in the westerly flow at 700 mb, with a 700-mb temperature of -7˚C at 1800 UTC (1100 MST) Saturday, which would be close to average for this time of year.  

Source: Pivotal Weather

I'm not going to pick a winner here.  Instead, I'm going to say that we are playing blackjack with a full deck for Saturday and a wide range of possibilities are possible.  The GFS solution appears to be one of the colder possible outcomes.  It's a possibility, but not the most likely outcome.  More likely is the coldest air doesn't get here and we don't feel the full brunt force of the arctic surge.  Be aware of this possibility and we'll see what time ultimately tells.  

Big Snow and Big Blow

Yesterday's storm delivered with Alta, Snowbird, and Brighton all apparently colluding to report 15 inches this morning.  Poor Solitude reported only 10 inches.  

It's a wonder anyone could measure anything yesterday given the wind.  I was out during the relatively peaceful mid-morning hours and caught some beaten up snowflakes on my ski pants.  Look closely and you can find a few pristine ones, but otherwise it was mostly fragments.  

Later in the day, as Gordon Lightfoot once wrote about the storm that wrecked the Edmund Fitzgerald, "the wind in the wires made a tattletale song" as a few gusts of over 100 mph were recorded at one mountain top observing site.  Snowfall rates also increased, maximizing in the late afternoon, resulting in the temporary closure of Little Cottonwood Canyon.  

The storm persisted, however, for a few hours longer thanks to the so-called "Alta Cloud." If you ever wondered what the Alta Cloud looks like, below is a radar image showing radar returns in the post-frontal northwesterly flow concentrated over the high terrain surrounding Little Cottonwood Canyon.  Sorry Park City.  No soup for you.  

The snow has ended and there's some blue sky out there now, but expect the wind to be a tormentor on the ridges the next couple of days.  We are wedged between an upper-level ridge off the coast of California and an upper-level trough over northwest Canada, leading to strong northwesterly flow at upper levels.  The NAM 300-mb (roughly jet-stream level) wind vector and wind speed forecast for 5 PM MST this afternoon (0000 UTC 7 Feb) shows very strong flow extending through the Pacific Northwest to Colorado.  Maximum winds along the Montana-Idaho border are in excess of 70 meters per second (140 knots).  

This morning's sounding at the airport has a peak wind speed of 120 knots at about 250 mb. 

That's pretty high (9 km above the airport) and winds at mountaintop level are a more sane and reasonable 25 knots, but overall the pattern is one favoring strong ridge-top flow during the period with some waxing and waning of flow strength as weak disturbances move along the jet.  The middle left panel below shows estimates of wind speeds at 11,000 feet for Little Cottonwood Canyon derived from the NAM and you can see values near 70 mph at times through Monday morning.  

Conditions down in the canyons are more difficult to predict.  Sometimes they are decoupled from the ridge-top winds.  Other times the strong winds scour down to low levels.  Add your observations from today in the comments below as I'm doing the WFH thing today.  

A Storm About Orographics

This morning dawned as expected with grey skies and a bit of light snowfall over the Wasatch Range, providing an illustration of the orographic processes that will play a major role in the storm for today and tonight.  

Orographic means related to mountains.  Meteorologists use words like orography, orographic, orographics pretty regularly in places like Utah because the mountains have such a dramatic influence on our weather.  

Orographic precipitation is precipitation that is generated or enhanced by the interaction of moist flow with orography.  If one looks at the 24-hour GFS accumulated precipitation forecast for northern Utah through 1200 UTC (0500 MST) tomorrow, one can see to some degree the influence of the mountains with precipitation greatest along a north-south corridor that is roughly aligned along the Wasatch, with the precipitation maximum in the central Cottonwoods.  

The corridor of precipitation is, however, much wider than the Wasatch Range.  This is because the orography in the GFS is very smooth.  The GFS uses grid cells that are approximately 13-km on a side, and there are no variations in topographic height within each of those cells.  Someday we will look back on models like this and laugh, but for now constraints like this are necessary as computer resources do not allow for smaller grid cells.  As a result, the mountains in the GFS are very smooth.  The GFS mountains start over the Great Salt Lake with a smooth slope up all the way to the western Uintas.  One of my students used to call the GFS (and NAM) mountains the "Uintasatch mountains" because they were all smoothed together.  

We can, however, use statistical techniques to adjust that precipitation into something that reflects what we might expect based on climatology.  This is one of the things that we do to produce the fine-scale snowfall forecasts on weatehr.utah.edu.  Below provides the "downscaled" GFS precipitation forecasts for the same time period.  The pattern is closer to what one might expect, with a maximum over the Wasatch Crest.  

Finally, we can go a step further and use some simple approaches to determine the precipitation type and snow-to-liquid ratio and you have a high-resolution snowfall forecast from the GFS.

This storm is expected to feature a strong increase in snowfall with elevation for two reasons.  First, the large-scale "dynamics" in this event is relatively weak.  There is no strong front or trough moving through (there is, however, a weak one and so there are some dynamics to help with large-scale precipitation development).  Second, there is strong and moist northwesterly flow, leading to flow over the topography.  If one looks at the GFS precipitation for one period of the storm, it looks a lot like the GFS topography.  

This is not to say that there won't be any valley precipitation.  There is a weak trough moving through this afternoon that will probably produce some valley snow, but for the most part, this is a storm that should feature a strong orographic precipitation gradient, with snowfall increasing with elevation. 

The downscaled GFS forecast above is the snowier of the various models I've consulted this morning.  The SREF has somewhat lower numbers with precipitation heaviest this afternoon, an average snow total at Alta by 1200 UTC (0500 MST) tomorrow morning of about 15 inches, and a range of 8-22 inches.  

That range reflects differences in the characteristics of the flow over northern Utah and the techniques used to simulate the fine-scale processes used in the various members of the SREF to generate snowfall.  

I lean toward something in the 9-18" range for that period at Alta.  This looks like a good day to go up for afternoon laps as the storm intensifies with time and probably reaches peak intensity in the afternoon.  

Something I Don't Understand

There are some recurrent things that happen during storms over northern Utah that I don't understand.  Here's one of them.

Early this morning in the pre-frontal southwesterly flow, precipitation developed downstream of Mount Timpanogos and the surrounding Wasatch Range in the area circled in red below.  

The precipitation pattern was somewhat disorganized, but exhibited some banded structures at times.  Echoes clearly were forming near and downstream of the Wasatch Crest and extended well downstream into the western Uinta Mountains.  

I have seen a few examples of leeward precipitation over the years, all typically associated with strong cross-barrier flow.  I have some hypotheses for how this happens, but have never been able to explore them.  In this case, I think it is especially interesting that the echoes appear downstream of the range, suggesting that this is not a case of precipitation forming on the windward side and being carried into the lee by the prevailing flow.  Instead, it appears that something is happening dynamically to spur ascent and precipitation growth downstream of the Wasatch Crest.

Somewhat similar features have been documented in the pre-frontal environment on the northern side of the Alps.  Below is a radar image presented by Siedersleben and Gohm (2016) showing a case with southerly pre-frontal flow across the Alps with banding forming on the leeward (north) side and extending downstream over southern Germany.  

Source: Siedersleben and Gohm (2016)

The generation of those bands is related to how the flow interacts with small-scale topographic features in the Alpine topography, combined with atmospheric instabilities (for brevity, I'm not going to get into those instabilities).  A smooth mountain range doesn't produce these features, and even with a rough one like the Alps, these features don't form in every storm.  

The precipitation pattern observed downstream of the Wasatch Range last night featured banding on a somewhat small scale than that pictured above.  Further analysis is needed to evaluate this hypothesis, or to refine and come up with a better one.