Thursday, March 12, 2020

Spring Break and a Bit Thereafter

What do University faculty do during spring break?  For me it depends on the weather.   This year I've had to do a number of software upgrades for my forecasting class, prep for moving online, and put out a few fires related to COVID-19, so I've been working.  I've also been alternating between biking and skate skiing for my daily workouts.  It's a nice mix and good social distancing. 

Today we face an uncertain future, and I'm not talking about COVID-19.  Instead, I'm talking about the medium range snow forecast.

There are two challenges.  First, there is a major transition in the large-scale flow pattern over the north Pacific basin forecast to occur over the next several days.  As depicted by the GFS forecast below, we have a major omega-block developing, characterized by high-amplitude ridging over the Gulf of Alaska, flanked by troughs to the west and east, the latter near the US west coast. 

We can have confidence in the omega block developing, but in such a pattern, the local conditions in the Wasatch Range are very dependent on the details of the closed low and its position and movement across the west.  So that's one source of uncertainty.

The second is that the precipitation associated with the closed low will come in fits and starts, which are very difficult to time and anticipate intensity for at long lead time. 

Thus, if we look at the NAEFS plume for Alta Collins, we see quite a bit of spread.  Through  0000 UTC 17 March (6 PM MDT Monday), there are ensemble members generating just a couple of inches of snow for Alta, whereas there are a few others that generate 10-20 inches.  Even through 0000 UTC 19 March (6 PM MDT Wednesday), the range of all but two outlier Canadian members is 2 to 38 inches. 

One thing that is likely is whatever we get, the storm will be warm and the snow will likely be high density.  That might be a good thing if we don't see a lot of snow as it will help smooth things out.

Bottom line: Keep expectations low, hope for the best, and wash your hands. 

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