Thursday, October 4, 2018

Potential "Purgatory" Week Ahead

Fall Break for the University of Utah is this coming week and this year there won't be much time spent lazing by the pool if you are recreating in Utah.  Unseasonably cool weather is coming and will predominate for most of the break period.

Forecasts from the GEFS show the situation fairly well and illustrate some of the uncertainties at longer forecast lead times.  The forecast for 1200 UTC (0600 MDT) Friday 5 October shows an upper-level trough moving across the western United States with the axis over Utah. All members are in strong agreement of the passage of this system, which will likely give us our first coating of white in the upper elevations (more on this in a minute).

Source: Penn State E-Wall
However, that is just the first of a series of troughs that will affect the western U.S. and the beginning of a pattern in which large-scale troughing predominates in our region.  The next trough moves into the western U.S. and is centered over the Great Basin at 1200 UTC (0600 MDT) Sunday 7 October.  Note that while there is general agreement on the large-scale pattern, there are differences in the strength of the trough. 

Source: Penn State E-Wall
Following that, a third trough drops into the long-wave trough.  However, although all GEFS ensemble members call for a trough over the western U.S. at 0000 UTC 10 October (1800 MDT Tuesday 9 October), there are major differences in the pattern over the western U.S.  Thus, we can expect it to remain unseasonably cool through at least the middle of next week (possibly longer), but the timing and intensity of precipitation has a wide range of possibilities. 

Source: Penn State E-Wall
Focusing initially on potential for snow tonight, the models are in good agreement with regards to the passage of an upper-level trough and surface front tonight.  This is a colder system than the one that came in Tuesday night.  The NAM forecast below shows 700-mb temperatures (lower left-hand panel, roughly 10,000 ft elevation) at 0ÂșC over the central Wasatch by 0900 UTC (0300 MDT) Friday morning. That would probably equate to snow down to about 8000 or 8500 feet, lower if the intensity is high. 

Thus, the expectation for today will be for showers and thunderstorms, with more widespread precipitation and thunderstorms tonight during the frontal passage.  Additionally, snow levels will be falling.  For Alta Collins (9700 feet), the NAM puts out about 0.8 inches of precipitation during the frontal passage and about 6 inches of snow based on our snow density algorithm. 

This is a situation where the temperature contrast across the front is not huge, so much is going to depend on how quickly snow levels drop with the front.  In the central Wasatch I expect snow below 7000 feet to be pretty scant and probably less than an inch.   Snow totals will increase above that elevation, especially from 8000 to 10000 feet, with perhaps 4-8 inches of creamy snow at upper elevations by tomorrow morning when this event should be ending.  This looks like an event that will shut down quickly, but there are low lake effect probabilities (about a 1 in 6 chance) tomorrow morning. 

While waking up to snow covered peaks will probably result in pandemonium, the reality is that we may be stuck in Purgatory after this storm.  The forecasts above all favor a relatively deep trough over the western U.S.  That will give us colder weather and some periods of mountains snow, but Utah really needs a more progressive pattern for things to go big. 

The downscaled NAEFS forecast plume for Alta-Collins shows dribs-and-drabs of precipitation and snowfall in the coming week.  It slowly adds up, but most members end up with 20 inches of total snowfall or less (note, this is total snowfall, not the depth of snow on the ground).  A few members of the Canadian ensemble go for large totals, but each of those members produces storms at different times. 

Thus, the odds favor Purgatory, with some snow on the ground in the upper elevations, but not enough snow to ski.  There are lower (but non-zero) odds of something delivering more, so keep an eye on forecasts as they evolve over the next few days.  

I'll be honest, I don't know what to root for at this stage.  It is very early in October.  It would be better if we can hold this stuff back until late October or early November and then open up the spigot.  


  1. That's the dilemma I always feel. In a perfect world, I want significant snowfall to wait until the end of the month or early November. But at the same time, I fear that if it that spigot doesn't open, then we'll end up with terrible early-season snowpack numbers heading toward the holidays. If we could only guarantee a 2004 October every year...

  2. RIP PC mtb'ing above the mid moutain trail....:(

  3. Hey Jim. Things are getting so much more interesting. I thought that was a pretty extraordinary lightning storm we had last night. It was incredibly rapid fire as it rolled up over the front. I don't think I have ever seen so much continuous flickering. Would love any insights and commentary you might have on this.

    1. Sadly, I slept through the whole thing! Plus, electrification is outside my wheel house.