Snowbird Magic

In the previous post (Real Winter), I commented that the storm for Friday Night and Saturday was "a crap shoot situation given the limited moisture" and that "much will depend on instability, lake influences, and, for Little Cottonwood Canyon which might have the best chance of something, maybe some Alta magic."

In hindsight, I really didn't have any idea how complicated and productive the storm was going to be and that it was really "Snowbird Magic" that we were going to get.  

What Actually Happened

Automated data from Alta-Collins showed that at upper-elevations in Collins Gulch (9962 ft), about 11 inches fell with only 0.5" of water, yielding a 22:1 snow to liquid ratio or 4.5% water content. In other words, very low density snow, even if the use of gauge water equivalent measurements, which often don't catch all the snowfall, might result in a low estimate of water content.  However, remarkable contrasts of snowfall occurred in Little Cottonwood and Alta was not the big winner.  

As noted in the Utah Avalanche Center report "some areas of Upper Little Cottonwood received 20.5 inches of snow (0.97 inches of water)." That 20.5 inches appears to be from Snowbird, which reported a 48-hour total of 20 inches on their mountain report this morning. I wasn't skiing yesterday, but from what I've heard, snowfall appeared to increase with decreasing elevation at the Little Cottonwood resorts, with perhaps the base of Snowbird serving as the snow capital of Utah for a day.  

How Could This Happen?

I can only put forth some hypotheses that might be worthy of greater investigation.  These are based in part of my analysis of the event and other storms that have done some oddball things like produce more snow on the east bench than in upper Little Cottonwood Canyon. These are sometimes called "upside down storms" because snowfall decreases with elevation (see Classic "Upside Down" Storm), not to be confused with upside down snowfall in which higher density snow sits on top of low density snow. 

First, let's establish the time period that the snow fell.  To do this I'm going to use data from Alta-Collins since I don't have access to observations from the Snowbird Base.  Measurable water-equivalent precipitation was recorded at Alta-Collins in all but two hours from 0600 UTC 18 January (11 PM MST Friday) to 0100 UTC 19 January (6 PM Saturday). The break was from 1800 to 2000 UTC (11 AM to 1 PM Saturday).  Water-equivalent rates were unimpressive throughout this event and less than or equal to 0.07" for the entire period and automated measurements suggested that at no time did snowfall rates exceed 1" an hour.  This doesn't mean that there couldn't have been some short bursts (less than an hour) that were heavy, but they never produced more than an inch increase in interval-board snow at this location.

Snowbird, however, had nearly double the snowfall.  Peak hourly water equivalent and snowfall rates there were certainly higher. 

Large-Scale Environment

The large-scale environment in which that snow fell featured the passage of an upper-level trough in large-scale north-northwesterly flow.  This was a cold system and somewhat moisture starved compared to troughs that approach us from the west or northwest (even at Snowbird where snowfall was heaviest, the low-density nature of the snowfall in this event was certainly an inch or less).  Below is the GFS analysis for 1200 UTC 18 Jan (5 AM MST Saturday) showing the trough just upstream of northern Utah.  

The sounding from the Salt Lake City Airport at 1200 UTC 18 January (5 AM MST Saturday) is below.  This is upstream and not necessarily representative of what was going on in Little Cottonwood at that or other times, but there are some things that capture my attention. First, the atmosphere below 700-mb (roughly 10,000 ft) was consistent with the shallow convective clouds that I observed during the event.  It was well mixed at low levels with the dewpoint depression (the difference between the temperature and dewpoint decreasing with height to an apparent shallow cloud layer between 750 and 700 mb (for whatever reason, the NWS soundings rarely show the same dewpoint and temperature even in cloudy air). I've identified this layer with a purple box.  

That layer also had temperatures between -12 and -18C, or what is called the dendritic growth zone or DGZ.  Those are the temperatures that given suitable relative humidities, favor the development of dendritic snow crystals (the six armed beauties that we have all come to know and love).  Given the low-density snow in this event, I'd expect a lot of snow growth in this layer.  

Above the apparent snow layer, the dewpoint and temperature diverge again with height.  In addition, that layer is weakly stable, which would make it difficult for convective clouds to penetrate to higher altitudes.  

So, this one sounding suggests this was a shallow system and that the layer for dendritic growth was confined to below 700 mb or 10,000 feet.  

Radar Analysis

Finally, we have the radar.  There are a lot of problems using radar to estimate precipitation in a situation like this.  First the beam is partially blocked by the topography, and the influence of that blockage varies depending on location.  Second, this is a very shallow storm, so the storm clouds may only partially fill the beam.  Finally, this was lower density snow, which is notoriously problematic for radar precipitation estimates.  With those caveats in mind, below is the estimated water equivalent precipitation for the 24-hour period ending at 0300 UTC 19 January (8 PM Saturday).  The highest amounts are on the Cottonwood ridge (green squares indicate 0.5 to 0.75" of water) from about Mt. Superior westward. Amounts decrease as one moves eastward and northeastward from Mt. Superior into upper Big Cottonwood Canyon. 

That estimate is roughly consistent with what I observed on radar during the storm which was that echoes tended to be stronger and more frequent not in the highest reaches of Little Cottonwood, but west of Alta. Because of terrain blockage, however, the radar cannot tell us much about the local snowfall variations within the canyon itself, especially near the canyon floor and the base of Snowbird.   

An Initial Hypothesis

My best guess of what was happening in this event is as follows.  It was a shallow storm in flow that was generally from the northwest (on Hidden Peak and Mt. Baldy the flow was WNW or NW for the entire storm period).  The latter is consistent with enhancement in Little Cottonwood, but the shallow nature of the cloud layer and the weakly stable and dry nature of the airmass above crest level may have limited storm penetration into the interior of the central Wasatch and certainly into the lee.  This, or maybe just dumb luck, led to greater precipitation frequency and intensity west of Alta. 

In addition, this was a cold storm.  Temperatures on Hidden Peak and Mt. Baldy were at or below -18C for most of the storm period.  As a result, the dendritic growth zone was below crest level, potentially resulting in greater crystal growth within the canyon volume (i.e., below the ridge tops), leading to greater accumulations not at the highest elevations, but instead at mid elevations.  

There could be other factors at play.  Feel free to share your observations of snowfall or your ideas for mechanisms in the comments.  

Summary

What a storm!  Snowbird Magic 1, Alta Magic 0.