Let It Snow!

The long wait is nearly over, with an approaching storm moving in tonight and affecting all elevations with snowfall tomorrow.

It's a three part storm.  The first part features moist westerly flow with precipitation developing this evening primarily over the mountains.  

Towards morning, an approaching upper-level trough initiates frontal development over the Great Basin, leading to more widespread precipitation in the vicinity of the front.  By 2100 UTC (2 PM) tomorrow (Thursday) afternoon, the front at the surface (lower-right panel) and 10,000 ft (lower-left panel) is forecast by the GFS to be near or just south of Provo.  

Eventually, the frontal system moves out Thursday evening and we transition into post-frontal snow showers that could include lake effect Thursday night and Friday.

These periods are summarized in the GFS time-height section for Salt Lake City below (recall time increases to the left).  

Our GFS-derived forecast guidance product for Little Cottonwood Canyon is below.  Total water equivalent at Alta-Collins from 7 PM this (Wednesday) evening through 4 PM Friday is 1.18 inches.  Unlike many of the fall storms we've had, the wet-bulb zero level at the start of the storm is about 6500 feet.  Typically snow level is a bit below this.  Maybe a few sprinkles to start at mid elevations if it were to start a bit early, but otherwise this will be all snow even for Park City.  

Note that temperatures on Mt. Baldy and the wet-bulb zero level drop significantly during the period, including the frontal period tomorrow.   Thus, precipitation will likely change to snow on the benches late tonight or early Thursday morning and eventually reach the valley floor sometime during the day tomorrow.  This is an evolving situation, so monitor forecasts and be prepared for winter driving conditions.  Below is the latest infographic (downloaded 8:11 AM Wednesday) from the National Weather Service.

Given the lack of lowland snowfall so far this season, below is an actual computer model simulation of societal response assuming we have accumulating road snow.  

Getting back to the mountains, our Alta-Collins GFS-derived snow ratio starts at about 13:1, which is average for Alta and where it remains around through early Thursday afternoon, when it increases to 20:1 by Friday morning.  In other words, a right-side up snow with cold smoke.  The one possible fly in the ointment for that forecast is if the lake really lights up and we get highly convective and see graupel development, which would result in higher density snow.  If that happens, I won't mind.  We need the base and all the water for the snowpack possible.  

Total snowfall for Alta-Collins from our GFS-derived product is 17" through Friday afternoon.  This is very close to the downscaled SREF mean (see below).  Sixteen of the 26 SREF members (60%) produce between 10 and 24 inches.  A few are less excited, and four are above 24".  

In an unusual development, the Euro is even a bit wetter than the GFS.  

The numbers above suggest 12-24" to me for upper Little Cottonwood.  The NWS is going for 14-28" of snow with 1.5-2.25" of water, which is a bit higher and reflects the potential for lake-effect Thursday night and Friday morning, which none of the models or downscaling techniques above properly account for.  

The current average lake-surface temperature for the Great Salt Lake is 5.35˚C.  By Friday morning, the 700-mb (10,000 ft) temperatures are forecast to be -16 to -17˚C with relatively high low-level relative humidities.  This provides a pretty high likelihood of lake-effect, although much will depend on whether or not the lake can get going and organized, the flow direction, and other nuances that cannot be predicted this far in advance.  For instance the NAM has NW flow forecast for Friday morning, whereas the GFS is WNW.  That doesn't sound like much of a difference, but it could be the difference between the Cottonwoods being in the crosshairs or the Bountiful area mountains.  

Let's hope this all comes together.  We deserve it.