Monday, February 2, 2015

Meteorological "War Games"

Sometimes I liken weather forecasting to the movie War Games.  In that movie, staring Matthew Broderick, the computer eventually learns, "The only winning move is not to play."  That's good advice for the forecast through Wednesday morning.

The basic problem is this.  We're right on the edge of the storm track and there is a remarkably rapid dropoff in accumulated precipitation through Wednesday morning as you move southward.  For example, the image below shows the accumulated precipitation (liquid equivalent) produced by the NAM through 1200 UTC (0500 MST) Wednesday.  The northern Wasatch get over 1.5" of water, the southern Wasatch about an inch, and the southern Wasatch, especially south of Provo, just a few hundredths.

Thus, a slight shift in the position of the position of the moisture plume accompanying the northwesterly flow over the next two days makes a big difference in the total precipitation.

Let me further highlight this by showing the accumulated precipitation from the downscaled North American Ensemble Forecast System (NAEFS) for Snowbasin and Alta-Collins.  At Snowbasin, members of the downscaled NAEFS produce anything from about 0.4" of precipitation to over 3" for the period ending Wedesday morning (12Z 4 Feb).  That's a huge forecast spread.  The US Global Ensemble Forecast System (GEFS) produces most of the relatively "dry" forecasts, the Canadian Ensemble (CMCE) produces most of the "wet" forecasts.  The average of these is a bit under 1.5".

Precipitation (liquide equivalent) plume for Snowbasin
If we go a bit farther south to Alta, we also find a large spread in model forecasts, but a shift to somewhat lower totals with an ensemble mean of a bit over an inch.

So, the good news in a situation like this is that we're going to get some snow, but the problem is that there is a wide range of possibilities for how much.  The only winning move is not to play.  I'm sitting this one out and will let you enjoy the wide snowfall forecasts issued by the other weather guessers.  

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