About Yesterday's Snow Bomb

Impressive snow totals and snowfall rates occurred yesterday in Little Cottonwood Canyon.  The big winner was Alta.  As readers of this blog are well aware, the Alta Ski Patrol maintains a great snow-study plot in the upper elevations of Collins Gulch.  The hourly measurements from this site are a treasure trove to meteorologists like me who are starved for observations from higher-altitude locations.  The "snow interval" data below is collected by an ultrasonic snow-depth sensor that is mounted on a pole above a white snowboard that is wiped every 12-hours.  It recorded 20" of snow over and 11 hour period prior to being wiped just after 1600 MST.  

Really, the 20" mark was attained in only 9 hours, from 0400 to 1300 MST, yielding a mean snowfall rate of more than 2 inches and hour for that period, with a peak snowfall rate of 5 inches from 0600 to 0700 MST.  Due to roundoff of the measurement, there is a little uncertainty in that estimate, but it's safe to say it was snowing very hard at that time.  A bit more snow fell 1300 MST, but its rate of accumulation was roughly balanced by new-snow settlement, so the final tally remained 20 inches.  

Observations from Mt. Baldy show that the first 6 inches fell as the flow switched from WSW to WNW and the temperature dropped about 4F from 0400 to 0600 MST.  That indicates a frontal passage, but even during this period, there wasn't a strongly organized frontal band, although there were scattered showers and clear evidence of orographic modulation of the precipitation, meaning related to flow interaction with the topography.  Radar imagery at 0425 MST (1125 UTC) when snow was picking up at Alta showed strong modulation of radar echoes by the Oquirrhs and the Wasatch with echoes strongest over and/or windward of those features and strong precipitation shadowing in their lees, including over the western Salt Lake Valley.  So, this was very much an orographic storm right from the beginning. 

During the period of heaviest snowfall from 0600 to 0700 MST, the flow on Mt. Baldy was WNW and radar coverage became more extensive.  Still, echoes were strongest over and windward of the Oquirrhs and Wasatch Range, including the northern and central Wasatch and weakest over the western Salt Lae Valley.  

Finally, by 0848 MST (1548 UTC), the influence of the Oquirrhs and central Wasatch remain apparent, but there is also an elongated band of higher reflectivity extending from the Great Salt Lake to the central Wasatch.  

We have done computer model simulations of similar storm periods in the past in which we were unable to reproduce such a precipitation pattern unless we included both the lake and the topography.  Below is an example from one northwesterly flow storm in which we ran with the best representation of the lake and terrain possible (CTL), removed the lake and the topography (FLAT-NL), removed only the topography (FLAT), included the lake and the Wasatch (WAS), included the lake and the downstream terrain but no upstream terrain (DT), and removed the lake (NL).  There's a lot to digest there, but if there's no topography, the event only produces some light downstream snowshowers.  If there is no lake, only light precipitation occurs over the higher terrain including the Oquirrhs and central Wasatch.  However, when you run with them both (CTL), you get a solid storm.  Thus, both lake- and terrain-driven processes contribute. 

Source: Alcott and Steenburgh (2013)

I suspect this may have been the case for this later stage of the storm yesterday, although that's just a hypothesis at this point.  Overall though, I see yesterdays storm as one that was strongly driven by flow interaction with topography, with perhaps some lake influences thrown in later in the event.  

A few more thoughts

Yesterday's storm was impressive for snowfall rate as measured by depth, but less of an outlier from a water-equivalent perspective.  The 20" of snow that fell had a water content around 5%.  During the period when 5" fell, only 0.15" of water equivalent was observed.  That would be around 3% (although that estimate may be a little low due to gauge undercatch of snowfall).  Peak 1-h water equivalent rates were around 0.17".  That's not bad, but it's also not exceptional.  If you are wondering, the highest 1-h water equivalent was 0.17".  That's not bad, but it's also not exceptional.  The record hourly water equivalent snowfall rate at Alta-Collins is 0.54", which occurred from 0300 to 0400 MST 5 Jan 2008 in southwesterly flow accompanying a "warm and juicy" atmospheric river event just ahead of an approaching trough.  

I share these observations to highlight to different ways that one might measure and evaluate extreme snow rates.  One is based on snowfall amount.  The other is based on water equivalent amount.  Yesterday's snowfall extreme occurred due to the high snow-to-liquid ratios (i.e., low water content).  From a water perspective, it all that impressive.  Storms that produce high water equivalent rates are often warmer, with lower snow-to-liquid ratios, yielding lower snowfall amount rates.  For these storms, my eyebrows pick up when we start approaching 0.3" per hour.  

Thus, much depends on the metric that you use, although none of these scientific semantics take away from what I hear was an outstanding day of skiing. 

Yes, 2024 Was Warm

Final numbers will become available in a few days, but it is likely that 2024 will be the warmest year on record globally.  I'm not sure if the difference from 2023 will be statistically significant, but that doesn't really change the story of long-term warming.  

With an average temperature of 57.4F, it was also the warmest year on record in the Salt Lake City area based on observations collected by the National Weather Service, and by a pretty wide margin.

Source: https://xmacis.rcc-acis.org/

If fact, it beat the previous record holder, 2012, by 0.8F.  Given the inevitable grousing about the representativeness of the airport site, I'll also add that this was also the warmest year on record at the Bountiful Bench site, which has continuous records back to 1975.

That site observed an average temperature of 55.0F, topping the previous record holder, 2012, by 0.6F.  

Precipitation for the year at the airport was 14.78" which was just a skiff below normal (15.52").  Snowfall for the year was 26.2" which was about half of average (51.9").  I have not had a chance to carefully break that down to see if that was due to dry cool months or a greater fraction of cool-month precipitation falling as rain, but a quick look at the graphs suggests the latter dominated.  

As an anecdote, we have now made it roughly through my 2024/25 "snow blowing season", that period during which I normally blow out my south-facing driveway after storms since the sun is too low to melt it out in a short period of time.  I haven't run the snow blower once.  It's sitting in the garage collecting dust.  Nothing that has fallen on our driveway or sidewalk has survived for more than a few hours.  

Update on the "Best Forecast (So Far) This Season"

With about 2.5" of water and 23" of snow, the Best Forecast (So Far) This Season, which we discussed on Christmas Eve, is verifying well so far at Alta Collins.  The yellow dots are my best guess of where things stand as of 8 AM/15Z this morning showing us in the upper end of water equivalent and about in the middle of the Utah Snow Ensemble forecasts.

For today, the 12Z HRRR is generating 1.25" of water and 10" high-density of snow from 8AM through 11 PM.  This is on top of the couple of inches that it was producing from 5-8 AM earlier this morning, which appears to have verified nicely.  I like another 6-12" of high-density, upside-down snow before this storm winds down this evening. Note that with the KMTX radar out, this is even more of a guess than usual.

Did I say high density and upside down?  It's a warm storm though, and getting warmer.  I suspect snow levels this afternoon will be pushing 7500 feet. They may go higher than that this evening if the precipitation hangs on. 

The Best Forecast (So Far) This Season

'Twas the night before Christmas when all through the house

Meteorologists were stirring, along with the mouse

The forecasts were hung by the chimney with care

In hope that deep powder, soon would be there

Now is not the time to settle your brains for a long winters nap.  The latest forecasts from the Utah Snow Ensemble are the most optimistic I've seen all season for the central Wasatch, although that admittedly is a low bar. 

Ignoring the outlier minimum and maximum forecasts, there is a tight clustering of forecasts producing  2.04 to 4.73 inches of water and 25 to 58 inches of snow for Alta-Collins by New Year's Day.  

The first storm comes in with Santa's sleigh tonight, but excitement should be tempered with the fact that it's a splitter.  The GFS shows it to be a "full latitude trough" on the west coast this morning with precipitation extending from off the SoCal coast all the way up into British Columbia.

But by Christmas morning it's split, although the GFS still gives some love to portions of Utah including 0.59" of water and 6.4" of snow for Alta-Collins.  

The HRRR though is less optimistic.  Let's call it 4-8" and if we get more you can thank Santa for the delivery.  

After that, the storms come in frequency if not size.  The GFS forecast for Alta-Collins shows both the water equivalent and snowfall (bottom two graphs below) "baby stepping" upward to 7-day totals of 3.32" of water and 35" of snow.  

These aren't huge numbers, but change sometimes comes incrementally.  I think we'll see better skiing with the approach of the New Year.  

For the valley, this first storm is not only splitting, but offering up marginal temperatures for snow.  The NWS forecast is as wishy washy as mine, calling for rain possibly mixed with snow for tonight and Christmas.  

Source: NWS, grabbed 8:35 AM 24 December

Will we have a white Christmas in the valley?  

Only Santa knows.  

Snowpack Update

A rather unscientific polling of locals on a relatively small number of chairlift rides yesterday suggests that this year is off to a slow start.  Is that really the case?

Yes.

First we can look at the snowpack water equivalent.  Three of the SNOTEL sites with long-term records in the central Wasatch, Snowbird, Brighton, and Thaynes Canyon sit at their lowest or 2nd lowest water equivalents on record.  

Source: NRCS

The Snowbird site is near the Gad Restaurant, which is currently under construction.  Site notes indicate that the pillow was moved about a hundred feet on 20 December, and indeed there is a discontinuity in the record around that time with a drop in water equivalent from 4.2", which was near the lowest on record (since 1990), to 3.2 inches, which would be the lowest on record.  

How much faith to put into the representativeness of the Snowbird measurements is unclear, but they don't paint a rosy picture.  

Brighton is the 2nd lowest on record (since 1987), although for all intents and purposes, it's pretty much at the lowest.

And of course Thaynes, which is below the prior minimum

So as not to be accused of cherry picking, the "rosiest" central Wasatch SNOTEL with long-term records is Mill D North, which actually sits a shade below median.  

Is there any silver lining in this story?

No.

But of course, there's always more to talk about.  

The sites above only have records back to at best the late 1980s.  The gold standard for central Wasatch drought remains the winter of 76/77.  In November and December of 1976, Alta-Guard recorded only 30.5" of snow and 2.04" of water.  I don't have access to Alta Guard data yet, but so far this year Alta has recorded 107" of snow and 8.6" of water at Alta Collins.  That site is higher than Alta Guard, but even with adjustments, we are still well ahead of the dreaded 76/77 drought year.  

And if misery loves company, there are any number of years in the recent past that haven't been that far ahead of this year as the holidays approached.  The current snowpack water equivalent at Brighton, for example, is very close to that observed during the 2018, 2021, and 2022 water years.  

Let's hope the snow season picks up momentum during the holidays.

Another New Product from the Utah Snow Ensemble

By popular demand, we now have a tabular output product from the Utah Snow Ensemble for Alta-Collins available at https://weather.utah.edu/text/ensgefsdslccforecast.html.  This should provide access to the most recently processed run and provides tables of total water equivalent precipitation, total snowfall, 6-h water equivalent precipitation, 6-h snowfall, snow-to-liquid ratio, and wet-bulb 0.5C height.  Times are local (mountain) time.  Cells are color filled by magnitude.  An example of the first two tables is below.

The row labeling for each table goes from the minimum value of the 82 ensemble members to the maximum value.  The P stand for "Percentile" so that P10 means the 10th percentile.  That means that 10% of the ensemble members are below that amount and 90% are above it.  

P50 thus is right in the middle, or what statistical types call the median.  50% of the members lie between P25 and P75 (this was corrected from a typo that said P50 incorrectly), or what statistical types call the interquartile range.  If you want to get an idea of what the range of the middle of the forecasts is, these are the rows to use.  

This is a lot to stick on a web page, so you may need to get used to zooming in and out and scrolling up and down.  I don't have time to program a proper web site so you get what you get and don't throw a fit.  At least that's what my mother told me.  

The table above is from last night's 0000 UTC forecast cycle.  We are finally starting to see some hope of a bit of storminess during the holiday period, with median snowfall at 22.4" by 5 PM Saturday 28 December.  That sets the over-under.  Are you betting over or under?