Monday, December 4, 2023

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. 

Sunday, December 3, 2023

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.

Saturday, December 2, 2023

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.  

Friday, December 1, 2023

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.  

Wednesday, November 29, 2023

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. 

Sunday, November 26, 2023

A Sea of Stratus

It's pretty rare that stratus clouds, which otherwise are a zero on a meteorological scale of 1 to 10, can make for a great weather day, but they did today.  

Morning dawned in the Salt Lake Valley with dreary, grey overcast in the form of stratus clouds covering the entire sky (the image below taken looking south from the U at 8:12 AM).  However, a look toward the Cottonwoods showed a brighter spot, suggesting that perhaps there was hope for sun at high elevations.  

Source: University of Utah Department of Atmospheric Sciences

Sure enough, the Alta webcam showed clear skies above a Sea of Stratus over the Salt Lake Valley.  

Source: Alta Ski Area

And the morning satellite loop was really quite incredible with snow capped mountains, especially the central Wasatch, Southern Wasatch, and Uinta Mountains, sticking up above the Stratus Sea.  

Source: College of DuPage

And, as we started out ski tour from Alta at 9:52 AM, skies were clear, although stratus clouds hung in the central canyon.  

It's not uncommon for such clouds to work their way up the canyon during the day.  This is a result of thermally forced circulations.  Nocturnal cooling favors down canyon at night and in the early morning, which sometimes keeps the upper and middle canyon cloud free.  Then daytime heating results in up canyon flow with the stratus pushing up the canyon.  Indeed, when we returned to the car at 12:56 PM, shallow but thin clouds had made it to the Albion base area.

The cool thing about this was that there were just enough ice crystals around (some natural, but perhaps some artificial) for a nice atmospheric optics display with a 22 degree halo, sun dogs, and, if you squint, evidence of what is known as a parhelic circle cutting across the sun and through the sun dogs.  

When I lived in Innsbruck, such weather was common.  The meteorologists called the low clouds valley stratus.  Such clouds often filled the deeply incised valleys of the Alps, but spectacular weather prevailed above them at mid and high elevations.  I learned quickly to check the web cams to see if you could get above the clouds.  

That was definitely the case today, and we enjoyed spectacular views from the Supreme Area.  

We need more snow.  Think, pray, whatever.