As November as It Gets

If you are into grey, today is your day.

Morning dawned with a very November scene.  Valley smog trapped beneath and inversion with cirrostratus clouds aloft. 

Radar shows a harbinger of things to come with echoes moving into the western part of the state at 1440 UTC (0742 MST).

Its pretty dry at low levels, so I suspect that's virga or light rainfall on the desert floor, but precipitation is coming with rain developing in the Salt Lake Valley today, and snow at upper elevations.

A look at the latest (0600 UTC) GFS-derived forecast for Little Cottonwood shows not much has changed from yesterday, although it has gotten a bit wetter and snowier compared to yesterday's run discussed in the prior post.  Winds on Mt. Blady begin to shift beginning around 11 AM today, veering from southerly to westerly through evening.  Temperatures are mild this morning and sitting at 39˚F at the base of Alta, but the wet-bulb temperature is around 30˚F and the GFS wet-bulb zero level forecast for today has it around 8000 ft.  Thus, although we might see a spot or rain at 8000-9000 ft to start, snow levels should fall quickly to about 7500 ft once precipitation picks up.  For Alta-Collins, up at 9600 ft, the GFS puts out 1.33" of water and 13" of snow through 4 AM Wednesday morning, which would be a very healthy and needed snowfall with a mean water content of about 10%.  

Numbers from the Euro, as is often the case, are a bit lower and around 0.82" for Alta, which would be about 8" of snow.  Additionally, the downscaled SREF mean sits at around 0.6".  I'm inclined to stick with the 6-12" forecast for Alta Collins from yesterday, but hope the GFS verifies.  

Wednesday looks to be a break day, but Wednesday night and Thursday the GFS is advertising strong northwesterly flow with warm air advection and high crest-level relative humidities. 

Moist, unstable northwesterly flow is often good for the Cottonwoods, but this is stable northwesterly flow.  The GFS forecast sounding valid 1800 UTC (1100 MST) Thursday shows stable, saturated conditions from just below 700 mb (10,000 ft) to 600 mb (13,750 ft).  Above that, the air is subsaturated in the middle troposphere.  This would result in a stratus deck that envelops the upper elevations of the central Wasatch with cloud top temperatures at or above -10˚C.  Those are marginal temperatures for generating snowfall due to a lack of ice nuclei.  

Thus, Thursday could be a riming event for the upper elevations, with perhaps some fits and starts of snow at times.