Deja Vu All over Again

The forecast has largely been locked in now for a couple of days so there are few changes to report this morning.  Thus, this post is deja vu all over again.  

Today brings the warm before the storm, as well as wind.  At 7 AM this morning it was a balmy 60˚F at the University of Utah and 35˚F on Mt. Baldy (11,000 ft).  I bet you forgot it's only late October.  Overnight wind gusts through 8 am include 87 mph on Ogden Peak, 77 on Mt. Baldy, 73 at the top of Jupiter Bowl (PCMR), and 73 near Point of the Mountain, the latter a lowland site.  

Expect more of the same today as we are in the warm and windy southerly flow ahead of cold front.  Below is the HRRR forecast valid 0000 UTC 26 November (1800 MDT Monday) showing what has been expected for some time with strong southerly pre-frontal fro, a very well defined cold front, and heavy precipitation near and behind the front.  In the Salt Lake City area, pre-frontal precipitation is limited, although there could be a shower here or there mainly in the upper elevations.  

Our latest GFS-derived forecast guidance for Little Cottonwood Canyon is remarkably similar to what we've seen the past two days with temperatures and snow levels dropping with the arrival of the cold front, heavy precipitation immediately behind the front, and then a period of post-frontal instability showers through Tuesday night.  Total storm water of about 2" and snowfall of about 20" at Alta-Collins.  

Thus, windy but mainly dry today with perhaps a shower or two (snow levels near 10,000 ft), but then as the front approaches this evening, perhaps a brief period of rain to 10,000 ft, but then very quickly snow levels drop to 7000 feet with heavy snow at upper elevations.  After midnight, snow levels will gradually lower to near 5500 feet by morning.  

Below is some tabular output.  I've highlighted the critical period from about 6 PM MDT this evening through 5 AM MDT tomorrow morning.  At Alta, some light precipitation falls from 6 PM to 9 PM (totalling 0.04" water equivalent) and the wet-bulb zero level drops during this period from 10,500 to 8300 ft.  Snow levels are usually a bit below this level, although the drop in snow level might be a bit delayed compared to this for reasons noted below.  After that, the front arrives with the heavy precipitation and high-density snow overnight.  After that is an extended period of post-frontal snow showers.   

A quick comment that the drop in snow levels might be a bit slower than indicated by that table for this reason.   We can't estimate snow levels at lower elevations using the modeled atmospheric profile at Alta because the bottom of the profile is too high and starts at about 8200 feet.  As a result, we need to use an upstream profile from over Salt Lake City.  In a situation like this, with a rapid drop in snow level, there can be a difference in timing between Salt Lake and Little Cottonwood Canyon. 

The latest GFS also is forecasting fairly moist, unstable, northwesterly to develop behind the cold front for Tuesday and Tuesday night.  

It should be noted that the GFS is one of the wetter models right now.  The ECMWF model has about 1.28" of storm total water for Alta and a maximum of 1.63" just upstream.  Our downscaled SREF has a mean of 1.5" of water and about 17 inches of snow with a somewhat skewed distribution in which most members are between about 1.1 and 1.75" of water.

The NWS forecast below is for 12-18" at Alta.  I'm inclined to go a bit higher than that with a storm total of 10-20", although that might reflect my focus on Alta-Collins (9600 feet) whereas they may be thinking about the town of Alta, which is about 1000 feet lower and that would yield somewhat lower numbers in a situation like this.  

I suspect the skiing on Tuesday afternoon and Wednesday will be good as this will be a well-stacked storm with low density on top of high density, but avalanche danger is rising, the green light is not on, and the resorts are de facto backcountry.  Be careful out there.