President's Day Powder Potential

As I write this at 9 am on Sunday morning, it's been a beautiful holiday weekend so far.  Alta's Superior web cam shows sunny skies currently, but some high clouds to the west that will eventually spread over the area during the day in advance of an approaching upper level trough and cold front.  

Source: Alta

As discussed previously, the models continue to favor a solution with the cold front weakening near the Utah-Idaho border and then redeveloping over central Utah.  

Below are GFS forecasts to illustrate this point.  Each includes color fill of 3-hour accumulated precipitation in inches based on the scale at the right.

For the 3-hour period ending at 0300 UTC 21 February (8 PM MST Sunday), the front produces a continuous band of prcipitaiton from northern Nevada into southeast Idaho.  It certainly looks good.  

However, the front falls apart as it enters northern Utah.  For the 3-hour period ending at 0900 UTC 21 February (2 AM MST Monday), the frontal band has fallen apart and there's virtually no lowland precipitation.  Instead the precipitation is confined to higher elevation areas.  

Eventually, the frontal band redevelops again over central Utah on Monday, as shown by the 3-hour accumulated precipitation ending at 2100 UTC 21 February (2 PM MST Monday). 

The good news in that forecast is that the northern edge of the redeveloped frontal band is far enough north to give the central Wasatch some snowfall.  It's a sharp northern edge and not very far north of the central Wasatch, which gives me some heartburn.

The GFS-derived time series for Alta generates about 9 inches of snow from Sunday night through late in the day on Monday.  Dribs and drabs add another seven inches through Thursday.  

The downscaled SREF, which has 26 members, produces a mean of about 7 inches through 0000 UTC 22 February (5 PM MST Monday).  However, if you look carefully, you can see that the forecasts are skewed, with a bunch just below 7 inches and a few well above 7 inches. 

This is consistent with the central Wasatch being very near the north edge of the redeveloping frontal band.  More members call for that band to be a bit to the south, so that the central Wasatch gets less snow.  Those that have it to the north produce a lot of snow.  

If one were to pull numbers out of the downscaled SREF, 6/26 members (23%) generate 10" or more of snow for Alta-Collins through 5 PM MST Monday, 9/26 members (35%) 5-10" and 11/26 members (42%) less than 5 inches.  These aren't the kinds of odd that I like as a forecaster.  I like it better when there's more clustering within a snowfall range.  

If one wants to see such clustering, one need only look farther south in the heart of the redeveloping frontal band.  On the Manti Skyline, for example, the spread of the forecast is smaller and more tightly clustered around the mean.  This is why forecast confidence for central Utah is higher.

Getting back to the central Wasatch, if I had to pick numbers, they would be for 5-10" at Alta Collins, but that's not a confident forecast.  Much is going to depend on the position of the frontal band tomorrow.  If that band develops too far south, the forecast will similarly "go south."  If the band develops a bit to the north, then it could be pretty good storm.  I'm splitting the difference going for 5-10".