It's Better to be Lucky than Good

Yesterday was one of those days when if you were skiing at Alta in the afternoon, consider yourself blessed or, alternatively, it's better to be lucky than good. 

From 1100 to 1600 MDT, Alta-Collins picked up 9" of fresh, including 3" in an hour from 1200 to 1300 and then again from 1400 to 1500.  Water equivalent was .46", so this was 5% water content.  Winds on Mt. Baldy during that period never guested over 10 mph.  

I took a look at forecasts from the 12Z models on the prior day (2 April) and the GFS was going for nothing.  The HRRR .15" water and 2.7" of low-density snow.  Even yesterday morning, expectations were low.  The Utah Avalanche Center Forecast that morning called for 0.5 to 1" of snow.

This isn't to throw them under the bus as they do a great job, but just to illustrate that yesterday's snowfall was pretty unexpected.  It wasn't handled well by the models or the forecasters.  So what happened?

Well, morning broke with not much happening other than a few scattered snow showers.  At 1459 UTC (0859 MDT), there were some light returns on radar, but nothing to get excited about.  

However, the flow was light and the airmass unstable, and with a little surface heating, convection began to get going.  By 1857 UTC (1257 MDT), during an hour in which Alta picked up 3" of snow, localized convective snow showers had developed over portions of the central Wasatch, especially around Little Cottonwood, and the high terrain down to Mt. Timpanogos.  

Even then, the radar wasn't all that impressive, but the relationship between radar reflectivity and snowfall rate is not a good one.  Low-density snow of the type that fell yesterday often doesn't light up radar screens.  This is why it's so valuable to have weather cams and automated snow depth sensors to monitor actual conditions at the ground.  

The development of the first snow showers over high terrain was probably favored by the light flow and unstable conditions, with daytime heating yielding upslope flow and convergence over the mountains.  

The convection became more widespread with continued surface heating as evident in the 2033 UTC (1433 MDT) radar image from another period when Alta got 3" in an hour.  Nevertheless, snow showers persisted over upper Little Cottonwood.  

It's very difficult to reliably predict the location and intensity of these snow showers.  We can anticipate their development, but questions of where, when, and how intense are hard to answer reliably.  Neither our current models nor human cognition are very good at distinguishing a situation like the one yesterday from one where the snow showers are less productive and maybe provide light accumulations. The processes are simply too small in scale and too sensitive to small changes in the atmospheric stability and moisture content.  Basically, yesterday really was a tough forecast, at least with lead times of more than a couple of hours. 

I suspect if you were skiing yesterday afternoon, you probably didn't care.

About That New Ski Resort in the Oquirhhs...

Hopefully most of you have figured out that the previous post, New Ski Resort to Open in the Oquirrhs, was April 1st foolery.  

A good April fools joke needs to be somewhat believable, so let's break down that post a bit more.

First, the idea of development and possibly a ski resort in the Oquirrhs is quite believable.  Surely as the Wasatch Front metro area expands and the Salt Lake, Tooelle, and Utah Valleys are paved over, there must be developers with an eye on the undeveloped island that is the Oquirrhs.  In fact, Kennecott Land once spoke quite seriously about building a ski resort on their property in the Oquirrhs.  If a ski resort can be built in the snow desert and scrub oak of the Mayflower area, eventually one will probably come to the Oquirrhs.  

Is snow as plentiful in the Oquirrhs as the Little Cottonwood?  No.  The Rocky Basin Settlement Snotel in the southern Oquirrhs at 8700 feet has a median peak SWE of 24 inches compared to 43 inches at 9100 feet at Snowbird.  The Rocky Basin Settlement number though is pretty close to the 25 inches at Thaynes Canyon (9250 ft) in the upper reaches of Park City Mountain Resort.  However, the Oquirrhs also get about as much lake-effect as the Cottonwoods.  Below is the water equivalent snowfall (left panel) produced in lake-effect storms showing that the SNOTELs in the Oquirrhs are on par with Mill D North and Snowbird.  

Is there a powder Shangri-La as I suggest in the post?  Probably not.  I haven't been touring in the Oquirrhs this winter as suggested by the post, but I have in the past.  My guess is that there is no magic microclimate like Little Cottonwood in the Oquirrhs, although there are more mountain lions and fewer people.

Is snow farming from season to season a real thing?  Yes it is.  That article from Levi was real.  They are piling up snow, preserving it beneath geotextile blankets, and using it to open the following season.  Could such a thing happen in Utah?  I don't know, but there is the expertise at the U to figure it out and it strikes me as potentially being worth looking into as it preserves water, energy, and money.  Perhaps it would be most feasible at a place like Alta which typically closes when the snowpack close to its deepest so there's no impact on their skiing business to pile up the snow at the time of peak snowpack.  Maybe they could preserve enough to have cover for Mambo->Corkscrew come the next November.  Or Main Street where there's no snowmaking but maybe they could preserve snow near the base of Mt. Baldy which has less total incoming solar due to topographic shading.  

And finally, we have the extension of the red-line Trax into the Oquirrhs.  That was pure fiction designed to give away the April 1st foolery.  There are no such plans.  We can't even get rail to our current ski areas.  

New Ski Resort to Open in Oquirrhs

Over the past several months I have been working with a group of investors developing a new ski resort for the Oquirrh Mountains west of Salt Lake City.  My non-disclosure agreement expired today, so I thought I would take the opportunity to talk about their plans.  

The Oquirrh Mountains have extensive amounts of private land, mainly owned by Rio Tinto/Kennecott.  This investment group, however, owns approximately 8000 acres of land near the ghost-town of Ophir at elevations between 7000 and 10000 feet elevation.  You haven't seen me much in the Wasatch this winter because I've been doing a lot of ski touring on the property, avoiding crowds and getting to know the dry powder of the Oquirrhs on an intimate basis.  

I thought snowfall wouldn't be as plentiful as in Little Cottonwood, but after skiing a season there, I'm pretty certain the resort gets more.  It's simply an incredible microclimate, fueled by lake effect funneled into a terrain concavity.  If you think Alta gets a lot of snow in northwest flow, wait until you see this place in northerly flow.  I've toured in five storms with snowfall rates of more than 4" an hour.  There's little doubt that this is the future of lift-served skiing in northern Utah.  

In addition, to provide insurance against climate change, the investors have secured substantial water rights for snowmaking and are planning on developing Utah's first extensive use of snow farming in order to recycle snow from season to season.  They have hired an expert from Levi, Finland, where this is now being done to preserve snow from one season to the next.  

In fact, they are planning a trial run as they build out the resort over the next 18 months.  Next season, while they will still be under development, they will start making snow on what will be their signature run, Showcase. Comparable in length and pitch to famed upper, mid- and lower warm springs run at Sun Valley, the plan is to blow snow into deep piles next winter and then preserve those piles through the summer by covering them with white, geotexttile blankets to reflect sunlight and insulate the snow piles, allowing as much as 70% of the snow to survive through the warm season.  

They then plan to open the 2026/27 season in mid September with 3000 vertical feet of skiing on Showcase.  They expect to do this each season moving forward, pipping Snowbird for Utah's longest season.  

The main challenge at this stage is figuring out how to get people to the base of the resort.  The investors are currently working with UTA on plans for an extension of the Trax Red Line through an old mining tunnel in the eastern Oquirrhs.  Incredibly, this tunnel is built at grade, allowing light rail to deposit skiers at the base of the resort without having to use an expensive cog-railway design.  

I anticipate that this development will completely transform skiing in northern Utah.  Once skiers get an appetite for the dry powder of the Oquirrhs, Little Cottonwood will be an afterthought and the red snake will be dead.  

What Causes "Flat" Light

The quality of light strongly affects one's confidence as a skier.  On a sunny day, most of the solar energy (about 85% of it when the sun is high in the sky) is direct, meaning that it is traveling along a straight line from the sun to the Earth's surface.  This results in large brightness contrasts between directly illuminated surfaces and those in shadows.  Even subtle textures in the surface of the snow are easily seen.  

Descending a groomed run at Ischgl, Austria on a sunny day dominated by "direct" solar energy.

All else being equal, it is much easier to ski on such days.  It is easier to see the steepness of the terrain, variations in the snow surface, and subtle changes in the snow conditions.  

However, on day with thick overcast, even high overcast, the light can be "flat." Flat light is a colloquialism for situation in which there are no shadows, as was was the case at times at Alta today (Sunday).  

Flat light at Alta on Sunday, March 30th

Flat light is produced when most of the light from the sun isn't direct but is instead diffuse.  When skies are covered by thick clouds, the sunlight is not direct because it has been scattered by the liquid water and ice particles in the clouds.  As a result, the Earth's surface receives sunlight from all parts of the sky, resulting in a lack of shadows.  This is particularly problematic for seeing contrasts in a white surface like snow.  

High clouds sometimes vary in depth, so on a day like today, there can be variations in the flatness of the light.  Below is a photo I took looking up upper Sleepy Hollow off the Supreme Chair at 11:07 AM.  At that time, the sun was only partially obscured and there was enough direct radiation for the trees to cast shadows and for one to see some texture in the snow, as evident in the foreground of the picture.  

On our next run, at 11:33 AM, the sun was nearly obscured and there was considerably less direct radiation.  Shadows were less apparent and the texture and variability of the snow were harder to see.  

Such conditions though are not anywhere near as bad as they can get.  Add fog and one can have a hard time telling up from down.  Balance becomes difficult and one can even suffer from vertigo.  

Goggles can help some.  Color tints with higher light transmission that enhance contrast are usually best on flat light days.  Skiing closer to trees and other darker objects can also help.  

One of Bruce Tremper's ten commandments of avalanche safety is "thou shalt never go first."  This commandment is especially important on powder days with flat light, as a set of tracks helps to provide some contrast in an otherwise featureless landscape.  Be generous to your friends on these days and let them have first tracks.

Dusty Spring Morning

With yesterday's strong south winds, dust moved into Salt Lake Valley and environs yesterday and lingers this morning with visibility somewhat reduced as one looks to the central Wasatch from the University of Utah.

Source: https://home.chpc.utah.edu/~u0790486/wxinfo/cgi-bin/uunet_camera_explorer.cgi

Observations from the University of Utah show strong south winds yesterday afternoon and evening with PM2.5 levels increasing after about 1600 MDT from 3 to 8 ug/m3.  After settling some through 0200 MDT, they then climbed again to 13 before settling some this morning.  

These numbers are not high and are at good to low-moderate air quality levels, but nevertheless, there's plenty of dust in the air.  

Give the overall flow yesterday, the dust in the Salt Lake Valley was likely from sources to the south and the southwest given the flow direction and not from the exposed Great Salt Lake bed.  The flow direction was such that if there was dust emitted from the Great Salt Lake bed, especially Farmington Bay, it would have been transported to the northern Wasatch Front.  I'm not sure if there could be a some dust from the lake bed mixed in now given the shift to northwest flow last night but I suspect most of this is still from origins to the south and southwest. 

Dry Spring -> More Typical Spring

The work week has proven spectacular with mostly sunny skies and mild temperatures.  "Anything season" is here, meaning that you can do pretty much any type of recreation you desire, sometimes on the same day.  The lower-elevation trails have dried out now in many areas and are quite passable for hikers and bikers. Meanwhile, we still have a deep snowpack at upper elevations.  

Today's sunrise from the Avenues foothills.

We will, however, be transitioning from the dry spring pattern we've experienced the last few days to a pattern that will bring the occasional mild spring storm system to northern Utah.  This transition won't necessarily be abrupt as a couple of weak systems move through Friday and Saturday.  After that, there's a series of troughs that move through.  It's not easy for me to summarize my expectations for the timing and strength of these storms as there's a good deal of variability in the Utah snow ensemble.  Just look at the spread at the end of the 10-day period.  

The best way to summarize this is perhaps simply to say changeable, with the occasional mountain snowstorm interspersed with breaks.  It's a time for adaptation to what Mother Nature brings rather than having strong expectations for what's to come.  Fortunately, spring in northern Utah means a plethora of options.  

Downtime for weather.utah.edu

Due to utility work at the University of Utah data center, weather.utah.edu is expected to be down for approximately two days.  Don't shoot the messenger. 

In the meantime, enjoy the sunny, warm March weather and stop worrying about the next storm.  The forecasts looked like spaghetti anyway, so who the hell knows what's going to happen.  

In the meantime, for your entertainment purposes, there were some beautiful lenticular clouds over the central Wasatch yesterday.  

Such clouds are produced by mountain waves, up and down motions produced by the interaction of the atmospheric flow with the topography.  The clouds form where the flow is forced up and dissipate where the flow is forced down.  The layering is produced by vertical variations in relative humidity, which causes saturation in areas of ascent to be reached at different levels of vertical displacement.  

Lenticular clouds have been confused with flying saucers.  Let's not let this start a social media conspiracy theory.  

Avenues Avalanche!

Let's have some fun with a post based on what I'll call "sidewalk science."  

The Hall of Fame Baseball catcher Yogi Berra once said "you can observe a lot just by watching."  This applies for sure to both meteorology and snow science.

This afternoon, right near my house, I encountered an avalanche in the front yard of a house on 16th Avenue in the upper Avenues.

Oh, I didn't actually see the avalanche happen, but I did see the aftermath, which I've summarized in the below.  It contains many of the hallmark characteristics of an avalanche.  The first is the starting zone, which in this case I measured to be 35°.  Most avalanches start on slopes between 30 and 45 degrees, so the slope here was right in that range.

There is also the crown or crown line, which is the upper fracture line of the slab that broke away to create the avalanche, as well as the flank or fracture line along the side of the avalanche.  There is one on the other side, although I have not labeled it.   

Most avalanches that kill people are slab avalanches in which a cohesive layer of snow slides down the slope.  Indeed that was the case here with the slab sliding about 20 inches or 50 cm down the slope as a cohesive layer.  In this case, the length and speed of the avalanche was such that the slab remained intact.  However, eventually it encountered the lower angle sidewalk where the slab decelerated, debris built up, and the toe of the avalanche was found.  

The bed surface for this avalanche was the grass rather than a weak layer in the snow.  The trigger was likely solar heating, which led to a lubrication of the grass by liquid water as the snow melted, weakening the bond between the snow and the grass.  Such an avalanche is referred to as a glide avalanche, which is defined as a release of snowcover as a result of gliding over the ground.  

So much snow science in such a small space!  However, there are real-world applications.  Glide avalanches can be deadly and destructive.  In the Wasatch Range, they are probably most common in Broads Fork, a subdrainage of Big Cottonwood Canyon where glide avalanches commonly occur on steeply sloped rock slabs.  Here's a link to a report about one in Broads Fork: https://utahavalanchecenter.org/avalanche/69596. Note the fully exposed rocks in the starting zone of the avalanche.  

Glide avalanches are hard to predict, but can have deadly consequences.  They are also common in the Alps where the mountains are not only steep, but there are a lot of grassy, high altitude meadows that are grazed in the summer.  The resulting steep, grass slopes can make the perfect bed for an avalanche.  I saw the aftermath of many glide avalanches when I lived in Innsbruck, Austria.  Below is a photo looking down at one during a ski tour on the Gammerspitz, a 2500 meter peak in the Alps south of Innsbruck.  It was on a south facing slope and likely was triggered by solar heating like the avalanche I saw today in the Avenues.  We avoided slopes like this on our tour! 

I found the number of glide avalanches and the expansive coverage of glide cracks in the Alps to be very disconcerting and often made conservative terrain choices because of them.  Fortunately it was easy to avoid avalanche terrain today in the Avenues.

It Shall Get Warm

It's sort of weird after a winter with little valley snow to look out the window and see what might be the deepest snowpack of the season in my yard.  Admittedly, "deepest" in the valley this year is a low bar, but it will be good for the gardens eventually.  It was a wonderful storm for skiing too.

I can summarize the forecast for the next several days in two words: warming trend. Oh, today will be a cloudy "meh" day and Friday night and Saturday we'll have a trough passage that could bring a bit of snow, but after that a big ridge builds into the west.  Below is the GFS forecast valid 0000 UTC 27 March (6 PM Wednesday) showing the high-amplitude ridge parked just upstream of Utah.  

The NWS National Blend of Models maximum temperature forecast for Salt Lake City shows the warming trend nicely after Saturday, with temps climbing through Thursday.  

The median maximum temperature forecast for Thursday afternoon is 81°F, which would be the earliest 81 on record.  I thought this NBM forecast seemed a little jacked, but mid 70s seems doable.  

Bottom line: It shall get warm.

Another Spring Cold Front

The weather yo-yo of spring continues today and tonight with the approach and passage of a spring cold front.  

The 6Z GFS shows the cold front moving across northern Utah tonight at 0600 UTC 18 March (0000 MST Tuesday). 

Ahead of the front, today will be mild and windy with the possibility of blowing dust.  The prefrontal environment today looks to be mainly dry, although I wouldn't be surprised to see a spritz or two of valley rain or mountain snow.  Precipitation is not expected for the evening commute.  

The front drags through tonight.  Onset time, duration, and accumulations for the valley vary some from model to model.  For brevity, I'll focus here on the 1200 UTC initialized HRRR.  The time-series below is for the Salt Lake City International Airport.  Precipitation onset is just before 11 PM local time, but the turnover to snow doesn't occur until after 2 AM.  I'm not sure if I'd count on that timing given the uncertainties at play, but expect precipitation to move in tonight and at some point to turn over to snow at all elevations.  

The HRRR produces a bit under 3 inches of snow at the Airport by 8 AM tomorrow morning.  Bench accumulations would likely be greater.  Below is the NWS infographic for this event showing the expectation for 1-3" for the Salt Lake Valley floor and 2-6" for the benches.  

NWS infographic downloaded 9:30 MDT 17 March

For the mountains, I suspect Tuesday will be a pretty good powder day, but you didn't hear that for me.  

Winter Begins (LOL)

We really haven't had much of a winter this year in the Salt Lake Valley.  Snow has been scant (only 12.7" through yesterday, the 3rd lowest on record through that date) and temperatures mild (7th warmest mean temperature on record through yesterday). 

It's been sunny and mild the past few days, so it definitely feels like spring.  Things will change late tomorrow with the passage of a cold front.  A couple of days ago I was going to call it the Start of Winter for the valley, but the storm is splitting and right now not looking as large as I thought a couple of days ago, although it could still have impacts. 

The GFS forecast valid 0000 UTC 14 March (6 PM Thursday) has the front and frontal band moving through northern Utah.  

Frontal passage at the airport in the GFS occurs at about 2200 UTC (4 PM Thursday) and the HRRR about 000 UTC (6 PM Thursday) so expect it to get here in the late afternoon to around dinner time.  Pre-frontal conditions will be windy and mild, with perhaps a few valley showers or maybe a thunderstorm.  Temperatures and snow levels will drop rapidly behind the front, with what looks to be a brief period of heavy snow to the valley floor.

Below is a time series for the Salt Lake City International Airport from the HRRR that includes our machine learned snow-to-liquid ratio and snowfall amount forecast.  Yes, it is possible to produce these for anywhere in the continental US on weather.utah.edu if you know where to look.  One can see the frontal passage at around 6 PM with the drop in the height of the 0.5C wet-bulb level.  Precipitation also picks up at that time.  Precipitaiton with the front persists through about 9 or 10 PM (although it's light( and then there is another brief period in the early morning hours associated with some HRRR lake effect.   

Snowfall with the front adds up to a bit more than an inch and then the lake band adds another inch or so, although that's a relatively low probability possibility.  

I think it's worth keeping an eye on official forecasts tomorrow for the evening commute.  A brief period of heavy snow with post-frontal wind can make things nasty quickly, although perhaps the warmth of the roads will help stave that off some.  A lot will depend on intensity and duration and perhaps elevation.  I took a look and the official NWS forecast is for 1-2" at the airport and a bit more on the bench, which makes sense to me.  Their forecast discussion expresses similar concerns to mine above: "A rapid changeover to snow could bring some impacts to the Thursday afternoon commute."

Good News for Skiers and Water Managers

A spectacular early March weekend is now in the books.  For skiers and water managers, March is an important month.  The spigot can stay open, extending the powder season while there is a deep snowpack and delaying the spring runoff (often leading to a more efficient runoff), or we can transition to spring conditions with the snowpack suffering a slow death.  

If you are a skier or a water manager, I think you will like the extended forecasts.  For the early part of this week we'll have our share of warmth, sunshine, and spring fever, but the models are calling for a major shift later this week to a more active pattern with a parade of troughs moving across the western United States thereafter.  

It's a long-enough range forecast that I don't want to talk about details, so I'll summarize with the Utah Snow Ensemble plume for Alta. It's flatlined for water equivalent and snow until late on the 13th when thigns really start to pick up.  There are some big numbers thereafter.  The driest member pumps out 1.63" of wter and 23" of snow through 0000 UTC 20 March (6 PM MDT 19 March), which is more than what we would expect over a 6 day period based on climatology (or average).  

Most of the members are between 2.25 and 4.5 inches of water and 30 and 60 inches of snow.  That would be an impressive storm cycle for mid-to-late March.  There will be breaks in the action over that stretch, so don't assume it will snow continuously, but right now it looks like a series of storms will bring the goods.  

Adios X

After more than a decade of engagement on Twitter/X, I departed from that platform yesterday.  

The post prompted people to ask if I was Ok or if I was leaving the U, but I was merely leaving X.  It is a wonderful platform for sharing weather and science information but for some time the meanness and cruelty there have worn on me.  The nonstop bombardment of click bait was affecting my mental health. Enough was enough.  You can find me on Bluesky for now (@professorpowder.bsky.social), although I'm not going to be on it as frequently.  

Thank you for keeping this blog a place where science, weather, and snow are discussed respectfully.

Problems I Want to Work On

Last night's storm period was one that I really enjoyed simply because it shows how remarkably variable snowfall can be in the central Wasatch.  As suggested in the prior post, the low-level ESE flow favored Deer Valley, which received more snow than Alta-Collins.  

Deer Valley/Ontario: 1.41"/14+" (snow depth sensor got flaky at 7:35 AM; 14 was the total at that time)
Alta-Collins: 1.02"/10"

It seems that a lot of data is not flowing into MesoWest from there resorts right now so I couldn't dig much deeper than that, but the Utah Avalanche Center report included storm totals as of 5 am also showing the decrease in snowfall from east to west. 

Source: https://utahavalanchecenter.org/

This is not something that is unusual for such a flow pattern or that experienced meteorologists wouldn't anticipate, as illustrated by yesterday's blog post.  However, we don't have a good understanding of the physical processes operating during such storm periods, nor do we have models that reliably anticipate such fine-scale snowfall variations.  By reliable, I mean not getting it right every now and then but instead being able to do it consistently. 

This has been the most challenging academic year of my career.  I'm not getting any younger and it has me thinking about what I want to do in my last few years as a researcher.  During my career, I have always been interested in snow and I'm especially interested in understanding and predicting microscale variations of snowfall in areas of complex terrain.  

Last night's case is a good example.  What I wouldn't give to have had a portable radar in the Heber Valley to see what is happening to the cloud and precipitation system on the south side of the Deer Valley Ridgeline and over the Snake Creek area southeast of Brighton.  It would have been so exciting.  More importantly, I'd like to compare this storms to others, as we all like to generalize, but the reality is that there are a lot of variations that we can't anticipate.  

Additionally, what I wouldn't give to have the time and horsepower to improve fine-scale forecasts of these storms using traditional numerical weather prediction or newer Artificial Intelligence/Machine Learning prediction systems.  

The reality is that we do not currently have a high-resolution ensemble that can reliably predict these fine-scale snowfall variations.  If you think the HRRR can do it, think again.  Here's the forecast from yesterday morning's HRR through 1200 UTC 6 March (5 AM today).  The numbers for Deer Valley and Alta-Collins aren't bad, but note how the snowfall maximizes not on the Deer Valley ridgeline but instead on the Alpine Ridgeline near Lone Peak.  We don't have observations up there, but I think that's an unrealistic spatial pattern.  

In part, this may be due to the resolution, or the grid spacing of the HRRR, which is about 3 km, or possibly due to how it deals with cloud processes, including the growth and transport of snow in storms.  The swiss aren't running models at 3 km grid spacing.  They are running them at 1 km grid spacing and trying to get even finer.  That might help.  However, one also has to be able to handle the snow growth, transport, and fallout processes right and this is where observations and clever minds are needed to incorporate such effects into our modeling systems.  

AI/ML is pretty exciting and is going to become increasingly important moving forward, but it's unclear how to do it for such fine scale precipitation patterns in which training datasets are limited.  There are some proposed approaches, but it's going to take careful testing and evaluation to advance AI/ML capabilities for situations like this.  

I guess in the end things haven't changed much throughout my career.  I love snow and winter storms in complex terrain and these are the kinds of problems that I want to work on.  Beyond my own personal interests, advances in these areas would likely help with forecasting for the 2034 Olympic Winter Games, road weather and avalanche mitigation in Little Cottonwood Canyon, and other weather and climate applicatios over northenr Utah.  

A Big Spring Storm

The model runs are pretty spicy and exciting this morning with a significant storm moving later today.  The latest GFS suggest precipitation beginning in the central Wasatch this afternoon or evening and continuing into Friday.  Let's have a look.  

The GFS forecast valid 0600 UTC 6 March (11 PM Wednesday) shows that the early part of the storm is characterized by an deep upper-level trough (upper left panel below) with a lower-tropospheric cyclone (red L in images below) over Nevada.  Associated with this system is an inland penetrating atmospheric river  atmospheric river that approaches northern Utah via the lower Colorado River Basin.  The crest-level 700-mb flow (lower left panel) is southerly or even south-southeasterly in the vicinity of the central Wasatch with 700-mb temperatures near -5°C.

Thus, the overnight storm period has all the hallmarks of a warm, high-density storm period.  Through 9 AM tomorrow morning, the GFS generates 0.77" of water and 8.2" of 10% water content snow. The HRRR is even more excited with water with 1.65" of water and 13.8" of 12% water content snow.  And that's just the start.  For the Wasatch Back crowd, that little bit of easterly flow gets my attention and suggests this could be a period where  you do pretty well.  This could be a period where snowfall is greater at Deer Valley and less at Snowbird, but we'll see if that pans out.  

The GFS drags the main cyclone very slowly across our area, resulting in a prolongued period of moist southerly to southwesterly flow that continues through the day tomorrow,  By 0000 UTC 7 March (5 PM Thursday), the storm is so wrapped up that we actually have cooler air moving into northern Utah from the southwest, as illustrated by the 700-mb (crest-level forecast at lower left).  By this time, the AR has moved downstream, but there's fairly deep instability and it's March, so things might get a bit convective tomorrow afternoon.  

I'm not sure if we might get a break in there for a bit as sometimes there can be a dry slot that sets up in a pattern like this, but by and large I suspect we'll see periods of snow tomorrow that will continue to stack up.  

Eventually the system moves through and we get into colder, unstable, northwest flow for Friday.

I've summarized the storm phases in the time-height section below.  Time increases to the left.  First there is the warm and juicy AR period Thursday night.  Note the low-level southeast flow that might favor the eastern side of the central Wasatch.  Then on Thursday, colder air wraps around the system and moves in aloft from the south and southwest.  Call it an upper cold front if you like, but it will destabilize things.  Temperatures drop further on Friday evening with a transition to colder, post-frontal flow.  

For Alta-Collins, storm totals in the GFS through 7 PM Friday are 1.77" of water and 24.1" of snow.  Expect that to be high density snow to start and lower density Thursday night and Friday.  The HRRR only runs through 5 AM Friday but it has 2.22" of water and 20.2" of snow.  It's less bullish on the post-frontal period whereas the GFS produces snow more continuously through the period.  Such model inconsistencies are one of the joys of being a meteorologist!

I'm inclined to go for a storm total of 1.5-3" of water and 20-36" of snow for Alta-Collins through Friday evening.  There are uncertainties in timing and details, but this looks like a pretty good storm.  The early part could be a big producer on the Wasatch Back.  It's a warm storm and perhaps snow levels will flirt with the lower Jordanelle/Mayflower area during the atmospheric river phase tonight and early tomorrow.  At upper elevations, I suspect the snow will be fairly dense everywhere before trending drier late Thursday and Friday. 

Buckle up and monitor forecasts.  There's a lot of moving parts in this storm. 

Meteorological Winter Ends

Meteorological winter, which includes December, January, and February, ended yesterday.  In the Salt Lake Valley, it wasn't much of a winter.  The average temperature at the Salt Lake City International Airport was about 36.4°F, making it about the 7th warmest on record*.  The asterisk is because there appear to be two days missing from the records I can access, so those numbers could change slightly.  

Snowfall was scant with only 10.4".  A day is missing for the snowfall records too, although I believe the total is probably right (the missing day was likely snow free).  This is the third lowest on record behind 2014/15 (6.0") and 1962/63 (8.8").  

Source: https://xmacis.rcc-acis.org/

The SNOTEL data in the central Wasatch has been a bit squirrelly this year, but for end of meteorological winter snowpack, most sites are a bit below average including Snowbird (90%), Brighton (88%), Thaynes (73%), and Parley's Summit (95%).  Mill D North sits at 118% and is the high outlier.  

All of the data discussed above is collected by the NOAA/National Weather Service and National Resources Conservation Service.  Media reports suggest firings occurred in both of these agencies prior to and during this past week.  Additional employees in both agencies opted for the "Fork in the Road" buyout.  I haven't seen official numbers, but I've seen some estimates suggesting that the NWS lost about 10% of their workforce.  It is my impression that nearly everyone in the weather, water, and climate enterprise, which spans the government, private, and academic sectors, believes this will reduce the Nation's capacity to anticipate, prepare, and respond to weather and related hazards as we enter severe weather, hurricane, and wildfire season.  It will also affect the Nation's ability to advance the prediction of weather and water related hazards in the long term. 

Slim Pickings

It's looking like the last week of February and the first two days of March will largely be snowfree in the central Wasatch.  We have a weak cold front moving through today, but it's a dry one.  If it wasn't for the wind and the lower temperatures this afternoon, you probably wouldn't have noticed it.

The Utah Snow Ensemble is flatlined until 2 March when a couple plucky members produce a little snow.  Most hold off until 0000 UTC 3 March or later.  After that, the various ensemble members are throwing spaghetti on the wall.  Some do nothing, others up to 40" of snow.  The spread in the runs is pretty consistent from 0 to 40.  Your guess is as good as mine for what's going to happen.  

The reason for that spread is the highly split flow forecast to pervade over the west.  The latest Euro forecast valid 0000 UTC Monday 3 March shows the pronounced split over the eastern Pacific with one branch of the jet moving northward through Canada and the other southward across Baja, northern Mexico, and the southwest US. 

There is a closed low embedded in the southern branch of the jet, and that's the main hope for later in the forecast period.  

These are the times that try water manager's souls.  We had just gotten the mid- and upper-elevation snowpack up near average after the last storm cycle, but a long dry patch now would not be helpful.  It will make it more difficult to get to a near average snowpack at the end of the snow accumulation season and could spread out the runoff period.  It's perhaps a little early to worry about that, but now that March is a approaching, it's something to ponder.  Perhaps the wetter forecasts will verify and we'll get a more active mid March to prevent that from happening.

A Scrappy Pattern

Our recent storm cycle has pushed the mountain snowpack to about as close to median as you can get in northern Utah.  Per the latest SNOTEL observations, northern Utah basins are sitting anywhere from 90 to 107% of median snowpack water equivalent. 

Source: NRCS

The situation becomes grimmer over central and southern Utah, but I have cut that off in the plot above because who wants bad news these days. 

We should add a bit more to the northern Utah mountain snowpack over the next few days as we're in what I'll call a "scrappy" west to northwest flow through Tuesday, meaning the mountains will squeeze out a bit of snow from time to time, and then another storm system rolls in later in the week.  As is often the case, the US GEFS ensemble is a bit wetter than the European Ensemble (ENS), but through 0000 UTC 22 February (5 PM Friday), the middle 50% of the Utah Snow Ensemble is generating 1.04–1.66" of water and 14-24" of snow for Alta-Collins.  Spread is greatest for the storm later in the week when the US GEFS ensemble is more bullish than the European.  Take a wait and see attitude for that one. 

Challenges of Snow Level and Precipitation-Type Forecasting

Forecasts remain very much on track with the expectations from a couple of days ago (see An Exciting Forecast).  For the central Wasatch and Alta-Collins specifically, the models are calling for snowfall to being today in southwesterly flow ahead of the approaching trough and continuing for the most part through Saturday afternoon when we are in the colder, post-frontal storm phase.  

I wouldn't be surprised to see a break or two in the snowfall at times in there, but totals look to be impressive.  Through 11 PM MST Saturday, the GFS is coming in with 1.90" of water and 24" of snow.  The HRRR doesn't go out that far, but just through 11 PM MST Friday it's at 2.4" of water and 24" of snow, so it's quite excited about the warmer part of the storm.   Most members of the 82-member Utah Snow Ensemble are in the 1.5–3" of water and 20–40" of snow range through 0600 UTC 16 Feb (11 PM MST Saturday).  

This is also a statewide storm, not some localized miracle for Alta (at least until we're in the post frontal stage), so everyone should get some.  Good news for all. 

But the fly in the ointment for the forecast is what is going to happen in the Salt Lake Valley later today and tonight, with the forecasts providing a "teaching" opportunity for me with regards to how we forecast precipitation type using model soudings. 

We will begin with this morning's observed sounding from the Salt Lake City International Airport as it sets the stage for the changes that are coming over the next 24 hours or so.  Thanks to the cold surge earlier this week, the temperatures in this sounding (red line) are below 0ºC everywhere. It is also dry in the lower to mid levels.  Between the surface and 700-mb, the dewpoint depression (the difference between the temperature and the dewpoint) is more than 10ºC except right at the surface.  

As a result, the wet-bulb temperature, indicated by the thin blue line, is generally 2-3°C colder than the actual temperature.  Meteorologists use wet-bulb temperature to forecast precipitation type because it the temperature the air will cool to if you evaporate water into the atmosphere, as happens for example when precipitation begins to fall.  

So, this mornings sounding is cold and could get colder with precipitation.

However, that assumes that the sounding doesn't change and in reality it is going to change and change a lot today and tonight due to the transport of warm air and moisture in advance of the approaching system.  So the next step for the meteorologist is to use the computer models to try to get a handle on what those changes will be.  Typically this is done by looking initially at maps like the ones below which are from the GFS and provide the large-scale context for tonight's forecast.  Valid at 0600 UTC 14 February (11 PM MST Thursday), they snow northern Utah in warm, moist southerly flow with widespread precipitation over northern Utah except in the lower elevations of western Utah.  

After that, a meteorologist might look at forecast soundings from the model.  This is typically done using what are known as "BUFR" soundings.  BUFR is short for Binary Universal Form for the Representation of meteorological data.  That's a mouthful, but the long and short of it is that BUFR is a binary data format maintained by the World Meteorological Organization and widely used to store high-resolution forecast profiles from computer models, including those for the Salt Lake City International Airport.  

The GFS BUFR sounding for the Salt Lake City International Airport valid at 0600 UTC 14 February (11 PM MST Thursday) shows considerable warming at all levels as we would expect based on the maps above, but if you look at the temperature trace (red line), it is very different from the surface to about 800 mb (7000 ft) than aloft.  In that low-level layer, temperatures stay very close to 0°C (I've indicated the 0°C line with a dark grey line; it is skewed because of the design of this plot which is called a "Skew-T"...that's a story for another day).  

We sometimes call such a layer "isothermal" because the temperature is nearly constant with height.  An isothermal layer near 0°C is not unusual to find in winter storms because in heavy snow, the melting of the snow tends to lower the temperature to 0°C, but no lower, similar to adding ice to a cold drink.  

In that sounding, there is only a very shallow layer near 800 mb that is above 0°C, not enough to fully melt falling snow, so we would expect the precipitation to fall as snow all the way down to the valley floor if it were to verify.  

However, other models have different ideas.  The HRRR for example also warms things up aloft, but it has a very different profile below about 700-mb.  There is a larger dewpoint depression (meaning the relative humidity is lower) and it is much warmer near and below 800 mb.  In fact the surface temperature in this sounding is about 5°C (41°F) and even the dewpoint is about +1°C.  

In a sounding like that, we would expect rain or maybe mixed rain and snow.  At the airport, the snow would turn to rain much earlier in the HRRR (and precipitation would probably be lighter too). 

These differences reflect differences in the resolution and parameterization of physical processes in the two models.  The GFS is lower resolution and doesn't resolve the terrain as well as the HRRR.  It also deals with mixing due to the friction experienced by the atmosphere near the Earth's surface different than the HRRR.  As a result, during this period, the GFS produces more precipitation at the airport and the cooling effects of that precipitation combined with the difference in mixing leads to a stable layer over the valley and snow persisiting longer.  In contrast, the HRRR produces less precipitation at the airport and mixes out the atmosphere more readily resulting in warmer low-level temperatures and an earlier transition to rain. 

Neither the GFS or the HRRR is perfect, so the devil is in the details concerning what unfolds tonight on the valley floor and the amount of snow that falls before things change to rain could vary a lot across the valley simply because precipitation rates tend to vary a lot across the valley in patterns like this (as do elevations).    

For the airport, the National Weather Service Forecast calls for snow today, but "rain, possibly mixed with snow becoming all rain after 2 am" tonight.  

Screenshot taken at 8:03 AM MST Thursday 13 February

So, their expection is that we will see the snow turn into rain overnight.  

This is a time worth monitoring the forecasts, especially if you have to get around the valley tonight or in the early morning tomorrow as I do.  I have to get my wife to the airport at o'dark 30 and am keeping a close eye on this and planning an earlier awakening to see what happens and be prepared for winter driving conditions if the snow hangs overnight, especially on the east bench. 

An Exciting Forecast

I'm pretty excited about the latest forecasts as it looks like we are going to see a bonafide frontal system moving through Utah, spreading the goods around statewide.  

But before jumping ahead to the storm, it's worth talking about what is going to be happening today and tomorrow.  During the day today, a cold trough will be dropping down out of the northwest and through Utah.  It will generate a little snow, mainly in the mountains and drop temperatures at all elevations.  Below is the GFS forecast valid 0000 UTC 12 February (5 PM MST Tuesday) showing the cold northwesterly flow with 700-mb temperatures of as low as -20ºC grazing the Utah-Idaho border. 

My 20/20 rule is something I use to remember what the outlier 700-mb temperatures are in Salt Lake City and the northern Wasatch.  Anything above 20°C is unusually warm or below -20°C is unusually cold.  As shown in the sounding climatology below, 20ºC has historically been an upper-limit for 700-mb temperatures in the summer.  Below -20C is less rare, but the spike like nature of the periods below that temperature tell you that they are episodic and extreme cold surges.  Thus, when we start getting below -20°C we are getting into rare (but not extreme) air.  

Source: SPC

The GFS forecasts bottom out the 700-mb temperatures at -21°C on Wednesday morning.  If you are going to be skiing tomorrow morning, be prepared.  Our GFS-derived, machine-learned forecast product for Little Cottonwood Canyon forecasts 8 AM temperatures of -11°F on Mt. Baldy (11,000 ft) and -6ºF at Alta-Collins (9600 ft).  

After that cold incursion, the south winds return ahead of the frontal system that will affect us later in the week.  Temperatures climb on Thursday and by 0000 UTC 14 February (5 PM Thursday) we are at the tip of an atmospheric river that penetrates to northern Utah via the lower Colorado River Basin.  700-mb temperatures have climbed back to -7°C at that time, so crest-level temperatures will rebound some on Thursday, with some periods of snow developing in the afternoon.  

The GFS then goes hog wild overnight as the meat of the system moves in, with heavy snowfall developing in the mountains overnight.  Below is the forecast valid 1200 UTC 14 Feb (5 AM Friday).  Give your valentine a powder day this year instead of roses.  

Then, we get what is currently advertised as a bonafide post-frontal northwesterly flow period on Friday and Saturday.  Wouldn't that be sweet! Better than chocolate, although watch the calories as the added weight is surely going to ramp up the backcountry avalanche hazard and challenge patrollers at the resorts.  

If you are wondering, storm total at Alta-Collins in the GFS for the period from 11 AM Thursday through through 11 PM Saturday is 2.11" of water and 27.1" of snow.  

The Utah Snow Ensemble died a temporary death yesterday.  I'm going to try and resuscitate it this morning.  It's not an easy thing to keep alive.  Thus, we're missing the all too critical ensemble to give us an adea of the range of possibilities, but the Euro looks to be on track with the GFS and I'm inclined to think that this is a situation where I would be using the GFS as near the lower limit of the storm total range given the potential for snow in the northwesterly flow, which the models really struggle with.  

My view is that right now this storm looks pretty good, but details are difficult to forecast at longer range, especially the post-frontal crap shoot.  A small change in flow direction can make a big difference.  The GFS and Euro have me thinking 1.5-3" of water and 25-40" of snow for Alta-Collins for the period from 11 AM Thursday through through 11 PM Saturday, but let's see how the models evolve over the next couple of days.

Editors note: This post was edited to correct a incorrect date and phrase.  

Close, But Not Close Enough

Yesterday evening's weak disturbance tried to get us to Steenburgh Winter this season, but alas, like the Rams in Super Bowl XXXIV, we came up just short.  The obs from Alta-Collins show 5 quick inches falling, bringing the total snow depth to 99" for three hours.  

Close, but not close enough. The rules of Steenburgh Winter are quite clear and strictly enforced.  If the plane of the goal line is not broken, there is no touchdown.  If there is no 100" by Feb 10, there is no Steenburgh Winter.  

Yesterday's Graupel Fest

Hell of a storm yesterday in Little Cottonwood Canyon with copious amounts of graupel.  Observations from the Alta-Collins site showed a storm total of 19" of snow and 2.70" of water which fell over a period of 18 hours.  Water equivalent rates remained above 0.2" per hour from 9 AM to 3 PM, above 0.37" per hour from 11 AM to 2 PM, and peaked at 0.41" from 12 to 1 PM.  The frontal structure during this event was complex, but I'd say the heaviest precipitation period was prior to and possibly during the frontal passage.  

Watching the radar, it appeared that the Alpine Ridge between Little Cottwonwood Canyon was the locus for heavy precipitation development and that is confirmed by the MRMS Radar only precipitation estimate below which showed an elongated area of more than 1.25" of water and a maximum of up to 1.50" from just west of the American Fork Twin above Snowbird to Sunset Peak just east of Alta. 

Source: https://mrms.nssl.noaa.gov/qvs/product_viewer/

We have been looking at extreme events like this in Utah and around the world in collaboration with several scientists including Baker Perry, a geographer and mountaineer who has installed high-altitude weather stations on Mt. Everest and in the tropical Andes.  In the case of the latter, they have observed hourly maximum water equivalent of snowfall rates of 20.3 mm (0.80") at Laguna Sibinacocha, Peru (4,895 m), 16.6 mm (0.65") at Chacaltaya, Bolivia (5,160 m), and 14.5 mm (0.57") at Quelccaya Ice Cap, Peru (5,650 m).  We've been investigating how extreme these are.

Peter Veals at the U combed through the Alta-Collins data we have stored in MesoWest (special thanks to the Alta Ski Patrol) and it appears that the "mother of all Alta graupel fests" occurred on 5 January 2008 when an incredible 3.99" of water equivalent fell in 21 hours.  Hourly water equivalent rates were at or above 0.35" per hour for 5 consecutive hours, totaling 2.18" during the heart of the storm.  Hourly water equivalent rates reached 0.54", which is the record at Alta-Collins.  

Source: MesoWest/Alta Ski Patrol

Radar imagery at the time of maximum water equivalent snowfall rate was very similar to yesterday with a reflectivity maximum parked right over the Alpine Ridge, again ahead of the approaching cold front which in the image below is producing precipitation to the northwest over the Great Salt Lake. 

Source: NCEI

So, yesterday's graupel fest was impressive, but there have been bigger.  At issue is what are the processes that contribute to such extremes?  It does appear that the interaction of the SW-WSW flow with the Alpine Ridge is probably critical as that ridge appears to serve as a locus for localized storm and graupel development in both cases, but why doesn't this happen more frequently?  I don't have answers to these questions, but it would be fun to investigate.