Base and Blow Ahead

Things are looking pretty exciting for the next few days, with a moisture laden storm for Friday night with strong winds, a decent frontal passage Friday night, post-frontal instability showers Saturday, and then another system Sunday.  Did I mention it's going to be windy?

Let's talk large-scale setup for Friday night first.  At 0000 UTC 14 November (5 PM MST Friday), the GFS forecast shows a strong upper-level trough and low-level cold front approaching Utah.  Moisture is spreading into northern Utah things beginning to get going over the northern Wasatch in the pre-frontal southwesterly flow.  It's a warm storm to start, but I'm hoping we'll see snow levels at the start of the storm late Friday afternoon near or just below the base of Snowbasin (6500 feet).  

The airmass ahead of and with the front is a juicy one.  The GFS integrated vapor transport forecast shows atmospheric river conditions penetrating across California and Nevada into northern Utah.  

Source: CW3E

Such conditions, with strong cross-barrier moisture transport, should persist over the northern Wasatch for about 6 hours as the front approaches. This is a situation ripe for heavy pre-frontal and frontal snowfall at Snowbasin and on Ben Lomond Peak.  I would not be surprised to see water-equivalent precipitation rates of 0.3 inches per hour at the Ben Lomond Peak SNOTEL.  

The GFS has the surface and 700-mb front passing through northern Utah at 0900 UTC 14 November (2 AM Friday).  Note the transition from southwest to northwest flow across the northern half of the state at that time (lower right), and west-southwest to west-northwest at 700 mb (lower left).  

The time-height section below for salt Lake City summarizes the situation,  Tomorrow (red box) we'll see strengthening southerly flow and relatively mild conditions compared to the week so far.  During the afternoon, high clouds will thicken and lower.  

Things get rolling late in the afternoon or in the early evening as strong, moisture-laden southwesterly flow impinges on the Wasatch range (green box).  Snow levels should lower fairly quickly with the onset of precipitation to about 6500 or 6000 feet.  The front passes salt Lake City at around midnight to 2 AM, with heavy precipitation and snow levels falling to the valley floor.  

Post-frontal snow showers will continue in the wake of the front (blue box), especially in the mountains.  Some lake effect could occur late Friday night and Saturday morning.  After a break late Saturday/Saturday evening, we reload for another storm Saturday night and Sunday (purple box). In the interest of time, I'm going to skip discussing that storm.  You'll just have to look it up yourself. 

For the Friday night/Saturday storm, the NAM is gong for 1.19" of water and 13.5" of snow at Alta through 11 AM Saturday.  The GFS calls for .81"/10.5".  For those of you keeping score at home, this is a rare time when the NAM is more excited than the GFS.  The SREF mean for water through 18Z 14 Nov (11 AM Sat) is about .75"/14", with a range of .4" to 1.35"/6" to 22"

I'm pretty encouraged by the storm and would lean toward .6 to 1.25" of water and 8-16" of snow by 11 AM Saturday at Alta Collins.  I expect it to be a windy affair as well.  I'll be hoping that once again I underpromise and the storm overproduces.

The Utah Avalanche Center is not yet issuing avalanche ratings, but I would be on full alert if I was heading out touring this weekend, remembering that the resorts, if they continue to allow uphill skiing, are de facto backcountry.  There are lots of hazards lurking beneath the snow surface.  Our medical community is beyond stressed.  Let's not add to their burden.