50% Chance of Skiing by Monday

Forecasts for the weekend storm are pretty locked in and I think there's about a 50% chance that there will be enough snow to ski at Alta on Monday.  

For that forecast, I'm assuming that 2" of snowfall water equivalent (SWE) is the minimum needed for skiing the main run down Collins Gulch.  Some might go with less than that, but historically that's my bare minimum. 

As discussed previously, the remainder of the work week looks dry and pleasant.  The 0600 UTC initialized GFS brings the much anticipated cold front into Utah during the day on Saturday and at 0000 UTC 23 October (6 PM MDT Saturday) widespread frontal precipitation covers the Salt Lake Valley and central and northern Wasatch.  

It's a slow moving frontal system and by 1200 UTC 23 October (6 AM MDT Sunday), the 700-mb (roughly 10,000 ft) trough axis has only progressed into central Utah. 

If we are fortunate, this will give us several hours of frontal precipitation.

During the day on Sunday we have a prolonged period of cold, unstable, northwesterly, post-frontal flow with the 500-mb trough and coldest temperatures aloft passing through overhead as illustrated by the GFS forecast valid 0000 UTC 24 October (6 PM MDT Sunday). 

The GFS time height section for Alta shows higher humidity air moving in aloft in advance of the front late Friday night and early Saturday.  The low-level front moves across Alta late Saturday afternoon, after which there is a prolonged period of moist WNW to NW flow through Monday morning.  

Our Little Cottonwood forecast derived from the GFS run presented above shows a total of just over 2 inches of water equivalent by Monday with 25" of right-side up powder.  Note that the wet-bulb zero level early Saturday afternoon is near 8500 feet, but crashes quickly to 6000 feet.  The snow level is typically about 1000 feet below this level.  So, perhaps we'll see some rain at mid elevations right at the start, but that should turn quickly to snow.  

The forecast above is pretty close to the mean of the downscaled NAEFS forecast, which has a total storm total accumulation of about 2.5 inches (and often runs a little high).  There is pretty strong clustering of the NAEFS forecasts between about 1.75" and 3.75" of SWE.  Making some adjustments for this tending to run a little high, mixed with what past events like this have done, yields my 50% chance of skiing by Monday.  

It's worth noting that there are no ensemble members producing less than an inch of SWE.  This is a lead-pipe cinch for a significant October storm with the potential for it to be a big one.  

Ultimately much will depend on what happens in the post-frontal crapshoot.