A Look at the Models

Forecasts continue to be on track for a decent storm tonight and tomorrow.  The GFS (below) and other models have the frontal precipitation band pretty much over us tomorrow morning.  


The GFS for this one is the driest model, generating about 0.6" of water and 6.5" of snow for Alta-Collins through 0000 UTC 15 November (5 PM Saturday).  The HRRR and RRFS ensemble members are much more optimistic with water totals of 0.87 to 1.86" and snow totals of 10 to 21 inches depending on the member.  One reason for this is that these higher-resolution models produce more precipitation tonight ahead of the front, as does the European.  Thus, the GFS appears to be an outlier for this one.  

The HRRR-derived Little Cottonwood Forecast for Alta-Collins, for example, has on and off showers starting around 5 PM this afternoon.  Then things ramp up just ahead of the front from about 2-4 AM.  The wind shift accompanying the front happens gradually on Mt. Baldy from about 4 to 8 AM as winds shift from southerly to west-northwesterly.  

It's a warm storm to start, but not as warm as our last one.  The wet-bulb zero level (the level where the wet-bulb temperature is 0°C is currently forecast to be somewhere between Alta Basin and the Collins observing site at 9662 feet until 6 AM, after which it drops to about 7000 feet.  This event will likely be all snow at Collins and probably at the base, although I wouldn't be surprised if there was some a touch of rain this evening to start. 

Snow densities will be high, with snow-to-liquid ratios < 10:1 to start and perhaps finishing off close to 12:1.  Average for Alta is 13:1.  Dense snow is preferred to build a base, so this is a good thing.  

I like 8–16" at Alta-Collins for this one.  Numbers could be higher if the early precipitation happens and the snow comes in lower density.

A reminder that there are many hazards are out there (stumps, logs, rocks, etc.) and early season avalanches happen.  As Drew Hardesty of the Utah Avalanche Center commented in his Thursday avalanche forecast, "a pattern change is afoot and winter storms are on the way...this will require a no-nonsense change in your mindset as avalanches will start to be on the menu."

Alta is planning to close to uphill travel at 5 PM today (Friday 13 November).  I do not know if this is temporary or permanent for the season.

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