Top 2023 Weather Story

There's no doubt that the top 2023 weather story in Utah was the record setting snowfall and snowpack.  

Technically, this is a story that began with the first mountain snowstorms in October 2022.  Below was the scene on the "Collins Glacier" at Alta on October 29th, 2022, when the snow depth at the Collins observing site was already a solid 23 inches.

Early season powder skiing was then had into early November.  By November 11, when the photo below was taken, we had already crested a 60 inch snow depth at Collins, my mark for the start of "good early season conditions." 

It's hard to believe now, but the spigot actually shut off after that and we went a couple of weeks without any major storms.  You may have forgotten about this brief lull in the action, but the dark purple line in the graphic below shows how the snowpack water equivalent at Snowbird flatlined from November 11 to November 28. 

After that, it started snowing, with only some short breaks until early April.  Collins reached the coveted 100" mark, officially beginning Steenburgh winter, on December 15.  It settled back below that for a time, but powered by for the final time just before the New Year's ball drop during the epic late December/early January storm cycle which added more than 10" of water and 60" of total snow depth to the snowpack.  

After that storm cycle, the snowpack was eating 3.2 meter probes for lunch.   

A little fun for Monday morning for those wondering how deep the snowpack is in Little Cottonwood Canyon, UT. This probe is 3.2 meters (125") long and doesn't reach the ground. Credit: E. Steenburgh. pic.twitter.com/Ul6xIz8L4h

— Jim Steenburgh (@ProfessorPowder) January 23, 2023

But Mother Nature wasn't finished.  Take a look at that dark purple line in the Snowbird graph above and how it starts to climb at a steep rate around February 15 and doesn't letup until early April.  The snow kept coming.  And coming.  And coming.  A sampling of the titles from my posts in March:

• Mother Nature Can't Be Stopped
• You Ain't Seen Nothin' Yet
• It May Never Stop
• Deep Trouble
• I'm Exhausted But Thankful
• We Need This to Stop

The snow depth at Collins peaked on April 4th at a record 248 inches.  This measurement was only possible because Alta Ski Patrol was able to find a way to extend the measurement pole for their automatic snow depth sensor, which was about to be buried. The period with no data in early April was probably when they were extending it.  

It was during this period in March and early April that it became apparent that we were well past a Goldilocks season in Little Cottonwood Canyon.  On April 5th, the Alta Town Marshall issued the ominous update below.

Indeed, if we learned anything last year, it's that there can be too much of a good thing in Little Cottonwood.  I now comment that perhaps 750 inches is the upper limit of a Goldilocks season.  Although avalanche risk in the canyon is a function of many factors besides total snowfall, and major cycles can occur in lower snow years, last season pushed us too deep into the extreme, resulting in prolonged canyon closures and extended periods of stress and risk for essential workers in the canyon.  

On April 25th, Alta eclipsed the 900" mark for the season at the Collins observing site.  Below is a photo of the measurement that put them over the top.  

Courtesy Alta Ski Patrol

When all was said and done,  Alta recorded 903" for the season at Alta Collins.  

Statewide, the average snowpack water equivalent reached 30", which was also a record.  

The winter of 2022/23 left many lasting scars in Little Cottonwood that will take a long time to heal.  The tree damage from the Coalpit #4, Lisa Falls, Maybird, and Tanners slide paths is extensive and will be reminders of just how dangerous Little Cottonwood Canyon can be for years to come.  The video below from Tony Korologos shows some of the damage. 

I conclude, by thanking all of the essential workers in Little Cottonwood for keeping us safe.  Your efforts are greatly appreciated.  

About That Alpine Snow

The northern and western Alps are generally off to a good start to the ski season, but there are some caveats.  Given our current snow drought, I thought we would have a look, focusing on the eastern Alps of western Austria and northeast Italy.  

Below is an analysis of total snow depth from the avalanche.report.  This is based on computer modeling and includes observations in colored circles.  Due to frequent northerly and northwesterly flow storms, snow depths are greatest (and impressive) at upper elevations along the northern Alpine Rim, especially near the Arlberg Pass (resort areas of Zurs, Lech, and St. Anton) and in the Karwendal Alps immediately north of Innsbruck.  Snow depth is lower, but still very healthy for this time of year, in the inner Tyrol (resort areas of Ischgl, Obergurgl, Sölden).   On the other hand, the Dolomites of Northeast Italy are really hurting.  The highest snow depth I could find there was 53 cm (about 21 inches).  This is because northerly and northwesterly flow puts them in the precipitation shadow of the Alps and often results in a warm downslope wind known as the Foehn (Föhn in German).   

Source: https://avalanche.report/weather/map/snow-height

On the other hand, there have also been some exceptional warm spells in the northern Alps and a quick look at the map above reveals that there is no snow at valley level in the Inn Valley and relatively little snow at lower elevations near the Arlberg.  For instance, there is only 67 cm reported in Warth (1490 m/4888 ft) and 40 cm (16 inches) in St. Anton (1285 m/4215 ft).  So, there is an incredible gradient of snowfall with elevation in that area from low amounts in the valleys to exceptional amounts at mid- and upper-elevations.  In St. Anton, for example, one goes from 40 cm/16 inches in town to 216 cm/85 inches at the Galzig Schneeestation (2025 m/6643 ft) and 352 cm/139 inches) at the Ulmerhütte (which is also right on the divide).  

A couple photos show the situation quite well.  The first is from just south of Obertsdorf, which is in the Bavarian Alps of Germany just north of the Arlberg, near an elevation of 800 m/2624 feet.  There is no snow at valley level.

Source: https://www.foto-webcam.eu/webcam/scheibenhaus/2023/12/26/0940

Similarly, the view looking south across the Inn Valley above Innsbruck shows no snow on the valley floor and in fact no natural snow cover even on north facing aspects up to about 1300 m/4265 ft. If you take a close look, you can see the artificial snow covering the lowest ski runs of the Patscherkofel just to the left of center. 

And it is still exceptionally warm.  In fact, at the time the photo above was taken, it was 4.5˚C/40F at the camera location, which is at 1921 m/6300 ft.  

The combination of heavy snowfall interspersed with warmth resulted in snow creep issues, such as this fissure that opened up on a ski run above St. Anton about a week ago.

Source: https://www.tt.com/artikel/30871819/pisten-brachen-weg-nasser-schnee-macht-tirols-skigebieten-zu-schaffen

The steep, grassy slopes of the Alps are also prone to glide avalanches, and they have been a big concern in the recent avalanche reports.  Glide avalanches typically occur on smooth surfaces, with the entire snowpack releasing down to the ground.  Melt water often lubricates the ground to enable this to occur, with glide cracks opening up over time as the snowpack begins to creek down the slope before catastrophically releasing. A friend sent me the photo below a few days ago of some impressive glide avalanches. I think these are from near Samnaun, just south of the Austrian border in Switzerland.

So, lots of snow at upper elevations in some portions of the Alps.  The analysis above shows quite nicely how variable snowpack and snow conditions can be with region and elevation in this part of the world. I suspect there are many people traveling over the Brenner pass from Italy to Austria in search of snow this holiday season.

Low Cloud Magic

'Twas a cold but beautiful Christmas Eve in upper Little Cottonwood Canyon.  Alta was solidly above the clouds through our departure at 2 PM.  The photo below taken at about 11:30 AM from Gunsight shows the low clouds lingering over the Salt Lake Valley and in portions of the lower canyon.  

By about 2:15 as we were headed down canyon, patch low clouds had reached the Snowbird base area and producing just enough ice crystals to produce a halo.  Very pretty.  

Photo: Libby Snethen

The clouds were likely forming just beneath an elevated inversion that had its base near 700 mb (10,000 feet), as suggested by the HRRR analysis for 1800 UTC (11 AM MST).  

We normally think of inversions as being near the valley floor, but they are typically elevated when we are downstream of an upper-level ridge.  They descend with time as the ridge moves overhead.  That will happen over the next couple of days.  In fact, if you look at the HRRR forecast for 1800 UTC (11 AM) tomorrow, we don't have true inversion conditions, but there is a strong stable layer with a base near 850 mb.  Essentially, Mother Nature is warming the air aloft while cutting off an increasingly shallow cold pool near the valley floor.  

Pollution though will probably be slow to build as traffic tends to be lighter on Christmas, although I suspect you will see a bit of haze tomorrow, in contrast to todays pristine airmass. 

Show Your Christmas Spirit

It's been some time since we had a significant storm.  The Alta Snowfall History pretty much shows a flatline since December 8th, other than a slight blip for the 1.5 angry inches we got yesterday.  

Source: Alta Ski Area

That snow was mainly at high elevations.  Lower elevations saw rain.  I miss winter storms.

The models suggest scant chances for precipitation until the 23rd, so basically nearly 2 weeks without a major storm.  For Alta, most of the downscaled NAEFS ensemble members are producing less than 0.75" of water and 7.5" of snow, but perhaps 25% of the members are more excited.  

Overall, the odds of us getting a minimal Goldilocks storm with 10" or more are fairly low.  I try to be religiously neutral for this blog, but desperate times call for desperate measures.  It could be that all that is needed is a bit more Christmas spirit.  If it worked for Santa in Elf, it can work for Wasatch skiers.  

Inversion Intricacies

Last night, the cold pool beneath the inversion got shallower and the upper Avenues emerged from the smog.  I took the photo below from near 13th Avenue on my way into work.  It shows the clear skies at upper bench level, but also the thin lens of smog enveloping downtown and the lower valley.  

Upper air soundings collected at the airport yesterday morning and this morning show very subtle but important changes.  Yesterday morning, the inversion base was elevated and near 860 mb (about 400 to 500 feet above the valley floor.  Beneath the inversion was the valley cold pool that contained the fog and low-level stratus clouds.  The flow was weak in and above this layer and didn't reach 20 knots until about 775 mb (about 3300 feet above the valley floor).  

Source: SPC

This morning, however, the inversion is based right on the valley floor and temperatures below about 800 mb (about 2500 feet above the valley floor) have warmed, whereas they have cooled aloft.  20 knot winds are now evident at around 800 mb.  

Source: SPC

So, we have scoured out some of the cold air at the lowest levels (resulting in the low-level warming), leaving the fog and pollution to being confined to near the valley floor.  

A look at the PM2.5 concentrations from PurpleAir shows we are still in the unhealthy for sensitive group levels across much of the valley floor, but along the east bench, there is a rim where concentrations are either good or moderate.  

Source: PurpleAir

This is an example of what meteorologists call top-down cold pool erosion.  The flow aloft is increasing and beginning to scour out the coldest air.  It's a tough job though.  It takes a lot of energy to scour out the remarkably cold and dense air that is currently at the lowest elevations.  It would be easier to scour that air out if we had a bonafide trough with cold air moving in at upper levels, but that's not on the docket for the next few days.

The models, due to lack of resolution, an inability to properly include the presence of such shallow features, and the challenges of incorporating the complicated nature of near-surface physical processes, are not particularly good at dealing with these top-down mixout events.  I'm not exactly sure how things will play out today, but my best guess is that a shallow layer of cold air and pollution will remain resident over the Great Salt Lake and the central and northern Salt Lake Valley.  It might retreat for a short time, only to return, including to the University of Utah and some bench areas, as the flow becomes upvalley and upslope with surface heating during the day.  

I'm also not sure about how things will play out over the next few days, but at the moment, I don't think we will fully crack the inversion and get a total mixout everywhere.  I think we will see a persistent cold air pool at low levels near the lake and in many low elevation areas in the Salt Lake Valley and along the Wasatch Front.  The southern Salt Lake Valley and upper bench areas may luck out at times with clearer air.  

Hopefully I'm wrong and we crack this thing.  

Inversion Hell

Salt Lake City descended into Inversion Hell with the development of low clouds and increasing PM2.5 levels.

Today's MODIS imagery, taken around 1:30 PM local time, showed the coverage quite well with widespread low clouds (in some inastances fog) over much of the central Great Salt Lake, Skull Valley, Tooele Valley, northern Salt Lake Valley, and Davis County.  

During the satellite overpass above, the avenues foothills were cloud free, but that changed at around 4:30 when the clouds pushed into the upper Avenues.

The fog and low clouds would only be depressing if they weren't also accompanied by poor air quality.  Observations from Hawthorne Elementary along 700 East in Salt Lake City show a major increase in PM2.5 concentrations today to 45-49 ug/m3, solidly at Unhealthy for Sensitive Groups levels.  

The high temperature today at the Salt Lake City International Airport was only 32˚F.  For comparison, the maximum temperature on Mt. Baldy (11,000 ft) above Alta reached 33˚F.  

Given the meteorology of the day, I was interested in having a look at things from Ben Lomond Peak above north Ogden.  Being able to enjoy that summit in the wintertime on a clear day with light winds is extremely rare.  It is one of the most wind-raked places in the Wasatch.  Today, however, although the snow was wind jacked, the view was spectacular.  One could really see the majesty of the Wasatch Range from Willard Peak (to the north), then southward where the western face of the Wasatch rises abruptly from Ogden.  One could see pollution around North Ogden and Ogden, but then the northern edge of the low clouds, also evident in the MODIS image above, near border of Weber and Davis County.  

Here's a broader perspective.  

I'll be taking the day off tomorrow to recover and avoid breathing the outside air.  

Snowpack Status

It's early in the season, but I thought we could take a quick peak at the western US snowpack .  Technically, this is through 11 December so that I could use many of the State of California snow measurement systems to get a more complete picture of the Sierra snowpack.

Then again, maybe I shouldn't have done that since things are fairly dismal in the Sierra Nevada.  The snowpack in the Sierra sits at less than 50% of median.  It gets worse the farther south you go.      

Source: NRCS

Most of the Cascade Mountain basins are below median.  If you look carefully, there are a small number of stations that are well above median.  Curiously, these are low elevation sites that don't always have much of a snowpack.  For example, the Pepper Creek SNOTEL in southwest Washington was at 300% of median on December 11th, which sounds fat, but in reality, they had only 1.4 inches of snowpack water equivalent.  Thus, the percentage relative to median at these sites doesn't really tell you much.  You wouldn't ski there anyway.  

Below median snowpacks also extend inland across the Pacific Northwest and across Montana.  

The healthiest snowpacks relative to median are in the Great Basin, including portions of Nevada and eastern Oregon and the eastern Great Basin in...wait for it...northern Utah!  The Lower Bear Basin sits at 151% of median and the Weber Basin at 130% of median.  Hooray.  There a a few stations in this area over the coveted 10" snowpack water equivalent mark indicative of what I consider to be a good early season snowpack for skiing including Tony Grove Lake and USU Doc Daniel in the Bear River Range and Ben Lomond Peak, Farmington, Snowbird, and Atwater in the Wasatch Range.  

The fattest snowpack at any SNOTEL site in the interior west appears to be Grand Targhee with 13.1 inches.  This is, however, a bit below median, so it doesn't jump out at you in the map above.  

High altitude areas of central and northern Colorado are running a bit behind median.  The fattest snowpack there is at the Tower SNOTEL in the Park Range north of Steamboat with 11.3 inches.  My friends in Colorado like to tell me how that site is often ahead of Snowbird.  #SkiColorado please.

Utah resorts and skiers should feel fortunate about our current situation.  If there is a negative, it's probably that everyone with an IKON or EPIC pass is rebooking their holiday vacations to come here.  To anyone thinking about doing this, my advice is to go to the northern Alps as they are off to a good start and you can ski bigger mountains and enjoy better food and drink there. 

A Cold Storm Is Coming

If you are into cold, northwesterly flow snow, I have a treat for you. 

Today we are getting an appetizer of mountain snow showers and valley rain showers that won't add up to much, but it gets more exciting late tonight and early Friday morning when a cold trough drops into northern Utah.  The latest GFS puts the primary trough axis at 500 mb and associated unstable northwesterly flow at 700-mb over Utah by 1200 UTC 8 December (5 AM MST Friday).  

I hate to say this as I think it could be a jinx, but the pattern looks really great through the day on Friday with the northwesterly flow locked in as another embedded trough with a reinforcing shot of cold air moves into Utah by 0000 UTC 9 Dec (5 PM MST Friday).  

Through 11 PM MST Friday, the GFS is generating 18.9" of snow (about 2" of this is from today) with snow-to-liquid ratios generally near or above 15:1.  

The HRRR is a bit drier and has slightly lower snow-to-liquid ratios, but is still cranking out about 14".  

These northwesterly flow storms can be a bit of a crap shoot, but this one looks pretty likely.  The uncertainty here is with regards to whether or not we get some embedded lake effect and/or graupel, the latter which might lower snow-to-liquid ratios some and can happen if things get really unstable. That said, I like this one putting out 1–1.5 inches of water and 12-24" of snow for Alta Collins through Friday night.  

In the wake of the trough, Saturday is going to dawn quite cold.  Our GFS-derived forecast product for Little Cottonwood is calling for morning temps near zero at 11,000 ft and in the low single digits at 9700 feet. Bundle up.  

Looking Back

Mother Nature gave us the storm we needed.  Water and snow totals per this morning's Utah Avalanche Center are 3.38-4.66"/40-50" in the upper Cottonwoods and 1.80-2.58"/20-30" on the Park City Ridgeline.  Big totals as well in the northern and southern Wasatch.  All northern and central Wasatch SNOTEL sites are at or above the 1991-2020 median for the day except Thaynes Canyon which sits at 86%.  Sorry Wasatch Backers.  

I thought we would take a look back at a few forecasts for the event, starting with the ensembles.  I'll just use the ones for Alta Collins that I included in prior blog posts and will use the range of water equivalent and snow reported for the upper Cottonwoods for verification.  The Alta-Collins numbers would be at the top end of this range.  

The 3.38-4.66" of water was in the upper 12-22% of water equivalent forecasts produced by the downscaled NAEFS from 00Z 29 November.  For snow, however, we were much closer to the mean. 

For the SREF, the 3.38-4.66" of water was in the upper 15-40% of members (and roughly at the mean for the ARW members...this is the wetter of the two forecast models used for the SREF).  Similar to the NAEFS, for snow, we were near or just below the mean. 

So, feel fortunate that for water, which is most critical for building base, we came in on the high end of model forecasts.  It won't always work out like this.  

For snow, it's clear that the simple scheme that we use for the snow-to-liquid ratio in the ensembles was off. It had the right idea for trend (see lower right panels below), but was consistently too high on Saturday night and Sunday (roughly from 00Z 3 Dec to 00Z 4 Dec).  A big reason for this is we are still using a somewhat ancient snow-to-liquid ratio algorithm that does not consider wind.  We haven't had the time for an upgrade unfortunately, which is not quite a simple as it sounds.  

My impression is that the HRRR forecasts were generally quite good for this storm.  The forecast from 12Z 1 Dec (5 AM MST Friday) called for the passage of three troughs (which happened, modulating precipitation rates) and a total of just over 2" of water and almost 30" of snow.  The Utah avalanche center report from Sunday morning had storm totals to that point of 2.43" of water and 36" of snow, so this is only slightly underdone.  

In this case, the snowfall differential is due to a slight under prediction of water equivalent rather than snow-to-liquid ratio.  The Little Cottonwood products use a newer snow-to-liquid ratio algorithm. I was generally happy with its performance through this storm, although those who were in the field can perhaps quibble some more.  

The HRRR initialized at 12Z 3 December (5 AM Sunday) also seemed to verify quite well.  It generated just over 1.2" of water and 10" of snow.  This is consistent with a mean snow-to-liquid ratio of 8.3:1.   The numbers from Alta-Collins show 1.25" of water and 9" of snow for this period, yielding a snow-to-liquid ratio of 7.2:1. 

In the weather forecasting business, you can't do much better than that.  

If one were to grumble about the forecasts for this period, it would perhaps be the lack of confidence or agreement in the ensembles when the storm was still a few days out.  I don't see that necessarily as a negative.  The reality is, as discussed in the post Active Pattern with a Lot of Possibilities, that it was a difficult pattern to nail down.  The European model, which is the best performing individual model in the world, probably produced the worst forecast of this event.  If you had bought into that hook-line-and-sinker, you would have had a major underforecast.  If anything, this event shows the value in using all the available models and ensembles to anticipate the full range of possibilities. 

And So It Continues

The skiing and snowpack situation yesterday was somewhat better than expected, making for a decent day with some deep powder.  Many people were out and tracks were laid all over upper Little Cottonwood and environs.  

A noticeable density inversion lied beneath a couple of inches of low density powder that fell just before and while we were touring.  Call me a snob, but I felt that made for "hard rock" powder skiing rather than "easy listening."  One definitely had to think about keeping the skis from diving.  

The storm continues today and I've elected to work rather than fight Mother Nature in the backcountry or  lift lines at the resorts.  I suspect for the former, the snow is too deep for good skiing on the sub 30 degree slopes I'd be inclined to ski today and for the latter it may take some time until terrain is opened and then it will be limited.  

As of 8 AM, the Collins stake at Alta has recorded 9 inches of snow since midnight with 1.01" of water.  If these measurements are accurate (and they could be off some due to the wind), that's a water content of 12.1% and in the Sierra Cement territory.  So, the new snow is upside down and we've now added weight rapidly to a weak snowpack with more on the way.  

The latest radar mosaic shows precipitation echoes extending northwest from northern Utah all the way up into the Pacific Northwest.  Echo free areas reflect poor radar coverage rather than precipitation gaps.  

Source: College of DuPage

Thus, we will keep the precipitation train rolling in the mountains today, with some modulations in snowfall intensity as weak waves in the flow move through. 

The 12Z HRRR is putting out another 1.2" of water and 10" of high density snow from 5 AM this morning to 5 AM tomorrow morning.  About 0.2"/2" of that was through 8 AM when I am writing this, so that would translate to about another 1" of water and 10" of snow.  

For the same period, the 6Z GFS (the 12Z isn't in yet) is around 1.15" of water and 10.2" of snow, so a tad wetter.  Seems like another 1-1.25" and 8-12" of snow is a reasonable expectation.  Expect snow to liquid ratios to generally be 10:1 or lower.  

The mountains will look and ski very different come tomorrow.

Stacking Up But Getting Messy

The first two waves of our storm cycle delivered in spades yesterday and last night with 23" of snow on the Collins interval stake and 0.78" of water equivalent as of 6 AM this morning.  The .78" includes .03" from just before the 24-hour period listed below. 

The snowfall numbers above are higher than being reported by the resort.  I'm not sure if that means the interval stake measurement was affected by wind transport.  Automated snow measurement is hard!

The storm started yesterday with very low density snow and overnight the winds blew like hell with peak gusts of 100 mph at 11,000 feet.  The weather and snow section of this morning's Utah Avalanche Center report suggests that many courageous dendrites died last night before they could be skied. 

I guess we can take solace in the fact that their remnants will help build the early season snowpack that we so badly need.  

Looking at the latest models, I can't help but think, avalanche conditions aside, that this is the storm cycle we've been looking for.  Most models have come around to the "wetter" solution.  Even the Euro is now putting out about 1.22" of water for Alta from 1200 UTC 2 Dec (5 AM MST this morning) through 1200 UTC 4 Dec (5 AM MST Monday).  For that same period, the GFS is dumping 2.03" of water and 23" of snow.  The HRRR, shown below, is even juicier with almost 3" of water and 30" of snow.  

If you do the math on the HRRR run, you'll find that 30" of snow and 3" of water yield a water content of 10%.  The combination of wind and warmth means we will see a lot of high-density snow out of this storm.  Although the wind is not really what we want, the water numbers are, so I say bring it on and build up the base.  

Buckle Up

The much anticipated storm cycle is upon us.  It is already snowing what appears to be pixie dust up at Alta.  Automated observations show 2" of new snow from .03" of water, which would be less than 2% water content if accurate.  The latter may be underdone, but it certainly looks light and fluffy on the snow-stake cam and pictures never lie, right? 

The models are now in pretty good agreement on the general characteristics of the storm through 0000 UTC 3 December (5 PM MST Saturday), so I'll start with that period.  There will be a crest-level (10,000 ft) trough passage at around 2 PM MST this afternoon and then a second one at 11 PM MST tonight.  I've identified each of these in the HRRR-derived guidance for Little Cottonwood Canyon below.  Then there's a third trough passage at around 5 PM MST Saturday, and this is where we start to see a good deal of model divergence. 

Through 5 PM MST Saturday, the models are calling for persistent, moist, westerly to northwesterly flow at crest level.  From 7 AM MST this morning through 5 PM MST Saturday, the HRRR generates 1.06" of water and 16.4" of snow at Alta Collins.  The GFS is a bit behind with 0.78" and 13.6" of snow.  A look at the downscaled SREF shows most members through 5 PM MST Saturday (03/00Z) are tightly clustered around 0.6 to 1.4" of water and 12 to 24" of snow (I'm eyeballing).   

Thus, I'm feeling pretty good about totals through 5 PM MST Saturday at Alta Collins 0.75-1.5" of water and 12-20 inches of snow.  

After that, I get an ulcer.  As can be seen in the SREF plume above, there is enormous spread in what happend after that.  Some members produce little precipitation, some a ton.  This is what I'll call the Atmospheric River stage of the storm. A look at the GFS forecast at 1500 UTC 3 Dec (8 AM MST Sunday) shows the situation with a broad, low-amplitude ridge over the Pacific States and moist, northwesterly flow extending into northern Utah.  True atmospheric river conditions (integrated vapor transport greater than 250 kg/m/s) never quite make it to the Wasatch, but we're just a bit below that.  We also see warm air advection (i.e., the transport of warmer air) at crest-level.  Much of the precipitation being produced by the GFS in our area is orographic and due to mountain uplift.  

Atmospheric rivers are very fickle in Utah.  Some generate heavy precipitation.  Some little.  As noted above, the precipitation amounts being produced by the various models and ensembles vary quite a bit after 0000 UTC 3 December.  One thing they agree on is that it is going to get warmer.  A look at the GFS-derived guidance for Little Cottonwood shows the wet-bulb-zero level rising from the valley floor on Saturday morning to a bit above 7000 feet by Sunday night.  This would mean snow levels to perhaps 6500 feet by Sunday night.  Temperatures climb to the the mid 20s at Alta Collins by Sunday afternoon and consistent with this is a decrease in snow-to-liquid ratios to values around 10 to 1.  

So, expect an upside down snow on Sunday assuming the storm delivers.  It looks like the Utah Avalanche Center has started issuing forecasts again and just in time as if we start seeing substantial water totals and upside down snow on the mid and upper elevation facets that exists in many areas, things are going to get interesting real quick.  

Active Pattern with a Lot of Possibilities

There are a lot of ways to look at the downscaled NAEFS plume for Alta-Collins below. 

One could focus on the upper-end forecasts.  What could be better than 5+ inches of water and 75+ inches of snow?  

Or one could look at the low end forecasts.  Those aren't so bad, but give a pretty different result with less than 20 inches of snow and 1.5 inches of water.  

Or one could see the period beginning on Friday morning (about 12Z 1 Dec) through Monday (about 18Z 4 December) as a stormy period in which there are a wide range of possibilities depending on details that cannot be confidently predicted at this stage.  

Let's take a look at what some of the models are saying. 

The GFS is pretty much right in the mean of the NAEFS forecast and puts out a total of 2.75" of water and 41.8" of snow for Alta-Collins.  A close look at the accumulated precipitation and snowfall traces shows a series of three waves moving through, with precipitation and snowfall waxing and waning with the passage of these systems.  

In addition, one can see that this is a period in which temperatures initially fall through late Friday, but then climb again into Sunday, when the winds also increase.  As a result, snow-to-liquid ratios maximize late on Friday and then decline on Saturday and Sunday.  Thus, should this forecast pan out, we will probably be dealing with upside down snow on Sunday.  

The Euro, on the other hand, is quite a bit drier.  I don't have an equivalent plot for the Euro, but totals for water equivalent in the Alta area are around 1.56".  It's not unusual for the Euro to be drier than the GFS, but there's actually a fairly large difference between the two models in terms of the storm track and strength of embedded features with the GFS favoring a somewhat more southward track with stronger troughs.  

For example, the GFS forecast for 0000 UTC 3 December (5 PM MST Saturday) has a strong, sharp, short-wave trough at upper levels over Idaho.  The 700-mb (crest-level) along the Idaho–Nevada border is 50 knots.  

In contrast, that same trough and flow in the Euro are weaker and precipitation, especially in northern Utah, is also weaker.  

Later in the storm cycle, at 0600 UTC 4 December (11 PM MST Sunday), the GFS brings another strong trough through.

But the Euro has a very different forecast with a trough up in the Pacific Northwest.  

These are very different forecasts and they reflect the challenges of forecasting the evolution of features (e.g., troughs and frontal systems) developing in and emerging from an intense jet over the western and eastern North Pacific and ridge off the coast of California and Baja California.  

I can't pick one of these forecasts over the others.  The troughs above are currently weak features embedded in the Pacific jet.  The models producing different forecasts tell us that their evolution over the next few days is someone chaotic and uncertain.  A stormy period beginning Friday looks likely, but just how big is hard to say and will depend a lot on how things come together over the weekend.  A GFS-like solution might give us a major storm cycle with 30-45" of snow through Monday.  A Euro-like solution is more subdued and perhaps in the 10-20" range.  Stuff at the upper end of the NAEFS is probably unlikely.  

Breath.  Keep expectations low and hopes high.