Winter to Return

After about a week of splendid spring weather, winter will be returning to the Wasatch Range today with the arrival of a cold front this evening.  

Although there will be some periods of high-elevation snow and low elevation rain ahead of the front, the main action will be associated with the frontal passage which will bring a few hours of steady snowfall overnight.  

The cold-front is very apparent in the latest HRRR run with an abrupt drop in the wet-bulb zero height at between 7 and 8 PM.  

We base the wet-bulb zero level estimate on the model forecast from the airport (if we used data from Alta we would not be able to estimate snow level most of the winter since there's no data below ground), so expect that drop to be more like from 8-9 PM in Little Cottonwood.  Snow levels are usually a bit below the wet-bulb zero, so expect them to be in the area of 7000-7500 feet today, possibly higher in the afternoon with heating, and then drop to near bench levels after the frontal passage.  

The HRRR is calling for about 0.85" of water and 8.6" of snow at Alta Collins through 9 AM Sunday.  A bit less than half of this is high density snow that falls in fits and starts ahead of the front, including this afternoon, and the remainder is with the frontal passage.  Densities will be decreasing behind the front.  

The HRRR then shuts things down late tonight through tomorrow afternoon when things begin to pick up again.

The GFS is less enthused about the pre-frontal precipitation today but is in rough agreement on the frontal passage.  It's also a bit more active during the day tomorrow.  Through 9 AM Sunday it's at .55" of water and 7.6" of snow.  

So, expect some fits and starts of wet snow at upper elevations today and then steady snow with the frontal passage this evening and tonight.  I'm inclined to go close to the model water numbers on this with 0.5 to 1" of water and 6-10 inches of snow at upper elevations through 9 AM tomorrow.  

Given the warmth of the past week, I'm not overly optimistic that the frozen coral reef will be buried by tomorrow morning.  Things will need to go above those numbers probably to prevent a lot of bottom feeding.  If that doesn't happen, the better skiing tomorrow morning will probably be in lower angle terrain where the snow surface is currently fairly smooth.

That said, behind the front, unstable northwesterly flow looks to predominate through at least Tuesday and possibly Tuesday night.  The GFS time-height section shows this well.

Thus, expect more periods of snow Sunday through Tuesday, with the coral reef becoming more and more submerged and a return to more winter-like ski conditions in the upper Cottonwoods.