Gut-Check Forecast

Wednesday's expectation exceeding storm notwithstanding, we desperately need snow.  The Jordan and Weber River Basins now sit at about 75% of median snowpack. This is no longer a matter of skiing quality, but becoming deeply concerning for water-resource availability this summer and fall.  

Our next shot at snow is Sunday night through Tuesday.  This is a gut-check forecast if ever there was one.  There has been a lot of talk about pattern changes for the past few weeks.  The storm for Sunday night through Tuesday does not reflect a major pattern change, but a slight shift upstream (westward) shift in the position of the upper-level ridge that has produced our dry spell.  This has shifted the track of troughs in the northwesterly flow east of the ridge a bit westward.  While that gives us better shot at snow, these troughs tend to be splitters, greatly increasing the forecast complexity and range of possible outcomes. 

Here's the forecast from the 0600 UTC GFS for 0000 UTC  22 February (5 PM MST Monday).  There's sa ridge over the eastern Pacific.  That ridge has been a persistent feature the past six weeks, but it is a bit farther west than it has been, enabling a couple of short-wave troughs to drop into the western United States, which at that time are over Nevada and the Pacific Northwest.  

That's all fine and dandy, but the position and track of those troughs is really critical for snow in the central Wasatch.  That run of the GFS is actually very pessimistic about snowfall and decays the frontal band associated with the upper-level trough to our north and redevelops it to the south.  For Alta, it gives a couple of inches of snow on Sunday night and then an inch and a half mid-day Tuesday with the second trough and that's it.  

On the other hand, if you looked at the run before that, initialized at 0000, you'd be pretty excited about a dump with 9 inches Sunday night and Monday and another 6.5 inches Tuesday.  

That sort of model-to-model whiplash reflects the uncertainty in the precipitation forecast for the central Wasatch.  Start the model with different initial conditions, and you get a significantly different result.  This is why one should be cautious in using one model for forecasting.  

We can look at the ensembles and see similar uncertainty.  The SREF leans toward a low-end storm.  There are 26 members in this ensemble and for Alta the average snowfall through 0000 UTC 22 February (5 PM Monday) is about 5 inches, with a range of 1 to 9 inches.  

The NAEFS is typically wetter, with an average of about 15 inches by that time.  The NAEFS extends farther out, so it also produces a bit of snow on Tuesday or later in the week by some members. 

Although we can expect a mild weekend and a shift to colder weather on Monday, there's still a wide range of possible outcomes for snowfall amounts in the central Wasatch.  Let's hope the higher forecasts come through and this isn't another case of Mother Nature snatching defeat from the jaws of victory.