Recipe for a Surprise Storm

Nine inches of pixie dust fell at Alta-Collins last night.  At the same time, precipitation gauge observations measured only 0.20" of water.

That equates to a snow-to-liquid ratio of 45:1, or a water content of a paltry 2.2%.  

Precipitation gauges often do not collect as much snow as is falling, yielding higher-than-actual snow-to-liquid ratios.  Still, pretty likely that the snow was at least 35:1 or possibly 40:1, the latter a water content of 2.5%.  That's pixie dust by any measure.  

As discussed in my prior post, I expected "dribs and drabs" at times through today (Wednesday).  From a water perspective, 0.2" of water is a "drib."  At average snow-to-liquid ratios for Alta (12:1), that would translate to 2.4" of snow.  Yawn.

However, at 40:1, its 8" of snow and at 45:1 you get 9".  Voila!

Some meteorologists call snow with snow-to-liquid ratios of 25:1 or greater wild snow.  At Alta, about 6% of days with at least 2" of snow and 0.11" of water equivalent feature wild snow.  Scattergrams show that nearly all wild-snow events (above the dashed lines) happen at Alta when the 650-hPa (about 11,500 feet or just above mountaintop level) temperature is between -12 and -19˚C, the wind speed is between about 7.5 and 12.5 meters per second (about 15 to 25 knots), and the water equivalent is less than 20 mm (0.8 inches).  

Source: Alcott and Steenburgh (2010)

Temperatures between -12 and -19˚C are favorable for the formation of dendrites, snowflakes with tree-like arms (think Alta logo) that tend to stack in ways that leave lots of cavities for air rather than ice.  You really can't get wild snow without dendrites (at least I don't think you can).  Wind speeds also need to be light, so you don't see much strong flow wild-snow events, in part because winds break up and destroy dendrites, densifying the snow.  I'm not sure why there is a low wind-speed cutoff. Maybe it is a statistical oddity given the limited number of events.  Feel free to hypothesize.

Wild snow doesn't happen when the water equivalent is high because high water equivalent storms either occur with higher temperatures, which are less favorable for pristine dendrites, and tend to produce more compaction of the snow on the ground, making it harder to maintain large snow-to-liquid ratios.

Last night the winds on Mt. Baldy were actually moderately high, reaching as high as a sustained 35 mph with gusts as high as 54 mph.  These equate to 30 and 47 knots and seem to be outside the range above. Wind speeds at 650 hPa this morning were also above 25 knots.  Perhaps a larger sample size than we used above would broaden the range of winds for wild snow.  Additionally, the winds last night dropped off quickly below 650 hPa so just below crest level the flow was weaker.  

Forecasting these events is also exceptionally difficult.  They are statistical outliers.  Simple techniques for predicting snow-to-liquid ratio based on linear techniques don't even like to go much above 20:1.   Other approaches, including human interpretation, might get them from time to time, but with a lot of false alarms to erode utility.  It's a good problem for us to work on moving forward. 

The Week Ahead

The anticipated transition to a somewhat drier pattern appears to be underway, although the models are still advertising some snow showers this week.  I'll summarize with the 7-day GFS-derived forecast guidance for Little Cottonwood which shows a few dribs and drabs at times Monday night through Wednesday, then a break until the next system comes in on Saturday night.  Total water 0.6" and total snow 10", which would be about half of average for this time of year.  

Those numbers are in line with a super majority of the downscaled NAEFS ensemble members which are heavily clustered between about 0.25 and 0.6" of water and 5 to 12" of snow.  

There are, however, a few members going bigger.  There's always hope for more.  

The good news is it looks cool so ski conditions should remain generally good, even if a big refresh doesn't happen.  I confess that I feel very spoiled by the last several weeks and feeling a little disappointed.  Lol.

Saturdays Weather and Snow Obs

It was a beautiful day today if you were above the stratus clouds that were hanging around in some areas and tonguing up Little Cottonwood Canyon.  Upper White Pine on a sunny day with a deep snowpack is as pretty as it gets.  

However, in the lower canyon and in Little Cottonwood, stratus lurked.  

Such clouds can develop in mountain valleys and canyons when a ridge builds in and produces an elevated inversion or stable layer that caps moisture at low levels.  Indeed that was the case today with a stable layer evident in the morning (5 AM) Salt lake City sounding between about 725 and 675 mb, which is just below crest level.  

This stable layer likely lowered some by mid morning.  In fact, during our tour we ate lunch right at the top of the cloud layer.  You could feel the temperature cool when the clouds moved in and then warm when they retreated.  

In addition, there was just enough humidity in the air to enable ice crystal growth.  Look carefully in the photo below and you can see the sparkling ice crystals. 

Here are a few of my dendritic friends on my glove. 

Meanwhile, on the Earth's surface, we have the best snowpack in a decade.  I have a friend visiting from Innsbruck and pulled out my 3-meter probe to impress him.  The bottom was not hit.  That's 120 inches in US units and of course the snow depth was deeper than that.  

In the short-range forecast, a cold trough is forecast to move southward right along the Nevada–Utah border tomorrow and tomorrow night.  It will bring a few snow showers to the Wasatch tomorrow, although accumulations are likely to be modest.  The 18Z GFS is putting out just under 4" of low density snow for Alta-Collins by Sunday evening and the normally exuberant HRRR a miserly inch.  Call it 2 to 4, hope for more, and pray we don't settle for less.  

There is, however, some potential of easterly flow including downslope winds on Sunday night and Monday morning.  Note the northeasterly winds in the GFS forecast over northern Utah at 0900 UTC Monday morning.  

Right now this looks like an enhanced but not severe easterly wind scenario, but keep an eye on forecasts if you live in canyon or downslope wind prone areas.  Napoleon and meteorologists know all too well that nothing good comes from the east.

Pattern Change

What an amazing winter that we are having.  Per the Alta Snow history, seasonal snowfall through yesterday was 426".  Daily totals over the four days prior to that were 11", 16", 12", and 15".  Looks like another six inches or so overnight too.  All of this on an incredibly fat snowpack.  

A change in the pattern is, however, underway.  The parade of storms slamming into California and moving into Utah is about to come to an end.  That parade of storms was associated with an extension of the Pacific Jet across the eastern Pacific and into California.  An example of this pattern from 0000 UTC 15 January (5 PM MST Saturday 14 January) is shown below.  

The GFS forecast for 1800 UTC (11 AM MST) Saturday, however, shows the development of a high-amplitude ridge off the Pacific Coast.  Instead of extending to California, the Pacific jet moves poleward over this ridge and into British Columbia.   

Utah remains in the northerly or northwesterly flow downstream of this ridge for the next several days.  Cooler weather will prevail, with weaker storms sliding down the downstream side of the ridge into the western U.S.  The first of these storms looks to affect northern Utah Thursday and Thursday night, although it's a weak system and we are on the northern periphery.  

The second comes in slides by to our northeast over the weekend.  It could be more of a Steamboat Special, giving more to the Park Range of Northern Colorado than to us.  

Summarizing using the GFS-derived forecast guidance for Little Cottonwood, temperatures remain cool (near or below 17˚F at 11,000 ft) and dribs and drabs of snow occur over the next week (totaling just over 0.6" of water and 10" of snow).  

Most NAEFS members are at or below 1.25" of water and 20" of snow, and I usually cut these a bit given the tendency for this product to be somewhat jacked at Alta.  

Although true deep powder days may be limited at the resorts, this pattern should otherwise maintain good ski conditions over the next week.  Backcountry snow conditions should remain outstanding, although clear skies might lead to some sun crusting on south aspects and there could be some wind damage in places with the trough passage over the weekend.  

Without a doubt, these are minor complaints after the season so far.  There are some suggestions that the ridge could be around for a while and, gulp, shift inland, but I'm not going to think such negative thoughts right now.  

And Another Storm is Coming

Forecasts look good for another storm Saturday night and Sunday.  The latest GFS forecasts a healthy trough to be over Utah at 1800 UTC (11 AM MST) Sunday with precipitation across much of the state.  

The corresponding GFS time-height section for Salt Lake City shows a relatively dry low-level airmass this morning, with moisture moving ain at low levels after 0000 UTC (5 PM this afternoon).  Through about 1500 UTC, moist SSE to SSW flow prevails near and below crest level.  This is a good setup for precipitation not only in the Cottonwoods, but also on the Wasatch Back.  

The GFS forecasts the trough to move through between 1500 UTC and 1800 UTC (8 AM and 11 AM) Sunday, after which moist northwesterly flow prevails.  If you wanted to complain about this storm (and you probably shouldn't), the post-frontal northwesterly flow doesn't look especially unstable, although I'm not going to lose any sleep over that because this still loops like a decent storm. 

Let's look at some numbers.  The GFS is going for a Goldilocks storm with 0.95" of water and 13" of snow at Alta through Sunday evening.  Snow-to-liquid ratios increase with time, favoring a right-side up snowfall.  Wet bulb zero levels max out tonight at about 6200 feet, so expect snow levels to start out near or just above the benches and lower with time.  It's a tough call whether or not they will reach the valley floor (they get close) so if you need to drive Sunday morning or Sunday, suggest keeping an eye on the forecast.  

As has been the case the last few storms, the HRRR is a bit warmer, with the wet bulb zero perhaps 500 ft higher, and wetter, producing 1.75" of water and 20" of snow for Alta by Sunday evening.  That's more at the upper-end of the Goldilocks range.  

It seems like 0.9 to 1.8" of water and 10-20" of snow is a decent bet for accumulations through Sunday evening.  Sunday looks like a storm-skiing day with free refills at the resorts with the GFS and HRRR forecasting the trough moving through upper Little Cottonwood from 11 AM to 2 PM (note how the flow in the center left plots above shift from southerly to WNW.  Late-day red-snake potential looks high.  

Our Cup Runneth Over

Another day, another storm delivering in spades.  What a season!

The last 24 hours at Alta Collins is summarized below.  From 7 am yesterday to 7 am this morning a total of 22" (there are wipes of the interval board before 1700 and 0500 that reset the snow interval measurement) and 21" since noon yesterday, so we are quickly approaching the upper-end of my forecast range from yesterday (which was 14–22" through 5 PM today).  

Current radar imagery shows the northwesterly flow orographic precipitation machine is operating at full capacity as I write this at about 7:20 AM.  

The 12Z HRRR calls for this to taper off this morning, with little to no precipitation at Alta after 11 AM.

Do you think Mother Nature got that message?  Maybe not this year.  I'll call for another 2-4" this morning just be safe. 

Meanwhile, it's worth a look at just how fat the snowpack is currently across California, Nevada, and Utah.  Below is the current snowpack water equivalent percent of median *water year peak*.  Basically, how does the current snowpack compare to the peak, which typically occurs around April 1.  Most sites near or south of Lake Tahoe are at at least 90% of median peak, with some more than 130%.  Similarly, sites in the Ruby Mountains are at or above 90%.  In Utah, there are some sites now running at 90% or greater.  Some are in the 50-69% color range, but I looked at the raw numbers and most of those are at or above 60%.  

The State of California snowpack obs are late to come in, so there's no data from the south Sierra, but I took a quick look at the numbers yesterday and the snowpack is quite fat as well.  

At SNOTEL sites in the Jordan River basin, which covers the Cottonwoods through City Creek Canyon west of the Wasatch Crest and the eastern Oquirrh Mountains, the average snowpack water equivalent is 19.2".  This is about the equivalent of median snowpack in mid to late March.  

If Little Cottonwood Canyon opens this morning and you are able to ski steep terrain at Alta or Snowbird, you should hug each and every ski patroller, plow driver, UDOT snow safety specialist, and any public safety person that you see.