Thursday, March 24, 2011

Frontal Friday Approaching

Friday is shaping up to be a very interesting weather day.  A cold front is presently moving into western Nevada and bringing heavy snow to the northern Sierra and southern Cascades.
The 0000 UTC 25 Mar initialized NAM brings the front through the Wasatch Front at around 1800 UTC (noon MDT).  Right now, this looks like a bench snow valley "wintery mix" event.  The 700-mb temperature during and following frontal passage is -8C to -10C, which is right on the cusp for valley snow this time of year, especially during the day.  

With a strongly forced cold front coming through, the forecast challenge for this event is related to precipitation type in the Salt Lake Valley.  A degree or two warmer or cooler can make the difference between valley rain and snow, respectively.

A useful product for these types of situations is the Short Range Ensemble Forecast system (SREF), which is an ensemble of 21 simulations by numerical forecast models.  For noon tomorrow, the average 700 mb temperature from all the SREF members is a shade colder than the NAM above (~ -10C), with a standard deviation amongst the members of ~ 1C.

This supports snow making it down to the valley floor, with perhaps an inch or two accumulating on grassy surfaces near the valley floor.

Temperature forecasts are most important when the temperature is near freezing, and tomorrow is such a day.  A degree or two is all it takes to make or break a forecast.   We'll see how things pan out tomorrow.

Of course, the easy forecast is for the mountains where it will snow yet again tomorrow!  This is one of the better patterns I've seen for Spring Break week in some time.  Hope you are enjoying it.

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