Monday, November 25, 2019

Jumpy Model Forecasts for Today and Tonight

A few of the students here have commented on how "jumpy" the model forecasts have been for the post-frontal period later today, tonight, and tomorrow morning. 

To illustrate this, the loop below shows a succession of downscaled NAM 6-hour snowfall forecasts for northern Utah beginning with the 84 hour forecast and ending with the 24 hour forecast, all valid at 1200 UTC 26 November (0500 MST Tuesday). You can see quite well that the location and intensity of the snowfall during the period is quite variable. 

In the last three forecasts, there is a bit of "convergence" toward a solution that has a band of snowfall extending from southeast Wyoming across Salt Lake County.  However, the amount of snowfall varies from run to run, with some evidence of a drying trend. 

All of this is evident of the chaotic nature of precipitation and snowfall in the post-frontal environment.  There is a lot of sensitivity to the initial conditions given the model.  An analog is to think of this situation like a random draw from a deck of cards.  You could get anything from the ace of spades to the two of hearts.  Toward the end of the run, perhaps the deck is stacked toward face cards, so the jumpiness decreases a bit, but there's still uncertainty in what you will pull.  This is one of the reasons why I favor the use of ensembles, which give you multiple "pulls" from the deck of cards to provide some idea of the range of possibilities (although current ensembles do not reliably predict lake effect). 

I have no magical insights in situations like this.  I can't tell you precisely how much snow Alta will get tonight, or where and when lake effect might form.  There is a wide range of possibilities.  This is reflected in the jumpiness above.  As noted in the post from the weekend, something in the 5-10 inch range through tomorrow is most likely for Alta, with the potential for more should Mother Nature decide to go big and put Alta in the crosshairs.  That's about the best we can do with the tools available today. 

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