An Exciting Forecast

I'm pretty excited about the latest forecasts as it looks like we are going to see a bonafide frontal system moving through Utah, spreading the goods around statewide.  

But before jumping ahead to the storm, it's worth talking about what is going to be happening today and tomorrow.  During the day today, a cold trough will be dropping down out of the northwest and through Utah.  It will generate a little snow, mainly in the mountains and drop temperatures at all elevations.  Below is the GFS forecast valid 0000 UTC 11 February (5 PM MST Tuesday) showing the cold northwesterly flow with 700-mb temperatures of as low as -20ºC grazing the Utah-Idaho border. 

My 20/20 rule is something I use to remember what the outlier 700-mb temperatures are in Salt Lake City and the northern Wasatch.  Anything above 20°C or below -20°C is unusually cold.  As shown in the sounding climatology below, 20ºC has historically been an upper-limit for 700-mb temperatures in the summer.  Below -20C is less rare, but the spike like nature of the periods below that temperature tell you that they are episodic and extreme cold surges.  Thus, when we start getting below -20°C we are getting into rare (but not extreme) air.  

Source: SPC

The GFS forecasts bottom out the 700-mb temperatures at -21°C on Wednesday morning.  If you are going to be skiing tomorrow morning, be prepared.  Our GFS-derived, machine-learned forecast product for Little Cottonwood Canyon forecasts 8 AM temperatures of -11°F on Mt. Baldy (11,000 ft) and -6ºF at Alta-Collins (9600 ft).  

After that cold incursion, the south winds return ahead of the frontal system that will affect us later in the week.  Temperatures climb on Thursday and by 0000 UTC 14 February (5 PM Thursday) we are at the tip of an atmospheric river that penetrates to northern Utah via the lower Colorado River Basin.  700-mb temperatures have climbed back to -7°C at that time, so crest-level temperatures will rebound some on Thursday, with some periods of snow developing in the afternoon.  

The GFS then goes hog wild overnight as the meat of the system moves in, with heavy snowfall developing in the mountains overnight.  Below is the forecast valid 1200 UTC 14 Feb (5 AM Friday).  Give your valentine a powder day this year instead of roses.  

Then, we get what is currently advertised as a bonafide post-frontal northwesterly flow period on Friday and Saturday.  Wouldn't that be sweet! Better than chocolate, although watch the calories as the added weight is surely going to ramp up the backcountry avalanche hazard and challenge patrollers at the resorts.  

If you are wondering, storm total at Alta-Collins in the GFS for the period from 11 AM Thursday through through 11 PM Saturday is 2.11" of water and 27.1" of snow.  

The Utah Snow Ensemble died a temporary death yesterday.  I'm going to try and resuscitate it this morning.  It's not an easy thing to keep alive.  Thus, we're missing the all too critical ensemble to give us an adea of the range of possibilities, but the Euro looks to be on track with the GFS and I'm inclined to think that this is a situation where I would be using the GFS as near the lower limit of the storm total range given the potential for snow in the northwesterly flow, which the models really struggle with.  

My view is that right now this storm looks pretty good, but details are difficult to forecast at longer range, especially the post-frontal crap shoot.  A small change in flow direction can make a big difference.  The GFS and Euro have me thinking 1.5-3" of water and 25-40" of snow for Alta-Collins for the period from 11 AM Thursday through through 11 PM Saturday, but let's see how the models evolve over the next couple of days.