A Storm System with Lots of Moving Parts

Enjoy the last day of summer today.  Big changes are coming in the form of a storm system with a lot of moving parts.  

Let's have a look at some of the details as predicted by the GFS and HRRR.  For this 2100 UTC 16 Oct (3 PM MDT this afternoon), the GFS calls for dry southwesterly flow at 700-mb (lower left) and by extension, the surface.  I don't expect strong winds today, so let's just call it breezy.  

For the same time, the more detailed HRRR is producing some scattered showers over central Utah and the western Uintas.  This could produce some strong gusty winds for the Yellow Lake Fire. I am hoping for the best there during this windier pre-frontal period. 

Summer ends sometime tonight with the arrival of the surface cold front.  The latest HRRR has the cold front entering the northern Salt Lake Valley around 0500 UTC 17 Oct (11 PM MDT this evening).  Unlike some cold fronts which have an intense line of precipitation, this one looks to come in mainly dry initially with just some scattered showers. 

The GFS forecast valid 0600 UTC 17 Oct shows that this front and it's accompanying upper level trough are relatively weak and it is really the stronger trough over the Pacific Northwest that is going to bring the bulk of the precipitation to northern Utah.  

Indeed, by 0000 UTC 18 Oct (6 PM MDT Thursday), that trough has dug into Nevada with widespread precipitation over northern Nevada and northern Utah. 

All of this suggests some periods of precipitation on Thursday, becoming more widespread later in the day and into Thursday night.  

Let's have a look at the forecasts for upper Little Cottonwood.  The latest HRRR produces two periods of precipitation, one early Thursday morning just after the initial frontal passage, the other late Thursday and Thursday night with the second (and deeper) trough.  The wet-bulb zero level is very high initially (near 11,000 feet) and even after the initial frontal passage is between 9500 and 10000 feet.  The snow level is typically a bit below this, so some of the early precipitation could be rain even up to 10,000 feet.  By Thursday morning though, snow levels may be down to about 9000 feet or so.  It is during the approach of the second trough that temperatures and snow levels finally come down significantly Thursday night.   

In total, through 6 AM MDT Friday, the HRRR puts out a bit over an inch of water and 9 inches of snow for Alta-Collins (~9600 ft). 

The GFS covers a longer period so the plot below is a 7-day forecast.  In total, through Friday afternoon, it generates about 1.3" of water and 12" of snow for Alta-Collins.

Finally, we have the Utah Snow Ensemble.  The GEFS Mean through 00z 19 Oct (6 PM Friday) is around  1.75" of water and 18" of snow and the ENS Mean is around 1.5" of water and 15" of snow for Alta-Collins.

My view is that something in the 8-16" range for a storm total through Friday afternoon at elevations near and above 9500 feet is probably reasonable.  If the trough digs a bit further to the west, we may come in lower than that.  In addition, the trough is expected to close off to our south, so instead of getting a prolonged period of unstable northwesterly flow as sometimes occurs with mobile troughs, we will instead see a transition to northerly, then northeasterly, and possibly easterly flow from late Friday through Saturday.  The Wasatch could see a little snow during that period, but probably not a lot.  Accumulations at mid-elevations will be lower.

Thus, I'll call it a purgatory storm.  Somewhere between heaven (enough snow to ski) and hell (no snow to ski). If this forecast verifies, we could descend into a faceted hell shallow snowpack on shady aspects if we don't stack up more snow on top of it in the near future.