Monday, October 22, 2012

Winter Begins, but the Forecast Is Tricky

Winter arrives in northern Utah this week.  Whether or not it is here to stay is debatable, but the next few days look quite active.  As I write this on Monday morning, the snow level is sitting around 9500 feet.  As impressive as the Dodge Ram appears in TV commercials, I think this guy ought to get his pickup off of Hidden Peak really fast.

Although snow levels are currently high, they will be dropping tonight.  Through noon tomorrow, however, the short-term precipitation forecast is remarkably difficult.  For example, this morning's 1200 UTC NAM calls for only 0.31 inches of SWE to fall at Alta through tomorrow (Tuesday) at noon.  Contrast that to the 0600 UTC GFS, which is going berserk with 1.10" of SWE.  That is a huge differential, especially when one considers the poor terrain resolution of the GFS, which usually underpredicts mountain precipitation.  

Part of the reason for the difference is that we are right on the edge of the storm over the next 24 hours. As can be seen in forecasts produced by the Short Range Ensemble Forecast system, more than 90% of the members call for at least 0.25" of SWE to fall over extreme northern Utah, southeast Idaho and western Wyoming, but the fraction of forecast members that call for that much precipitation drops off rapidly as one moves southward.  The central Wasatch sit right in that dropoff area.  

Through noon tomorrow, I'm inclined to lean toward the NAM, with periods of precipitation through noon tomorrow and accumulations of up to 6 inches in the upper elevations.  Those hoping for more can always hope that the GFS turns out to be the model of choice, or that the high terrain around the Cottonwoods lights up the southwesterly flow more than either model indicates (it happens, but not all the time).  

There's also the storms that will follow during the rest of the week.  Keep your fingers crossed.

1 comment:

  1. I love this time of the year, I hope the "lake" can make its self known:)