Tuesday, January 22, 2013

January 22, 2013: More Snow Than Ice This Friday

The computer models are beginning to suggest a colder solution for Friday's winter storm.  Yesterday's runs showed some mild air invading aloft, meaning the precipitation would be a wintry mix, mostly in the form of ice.   However, this extremely cold air mass has been under-forecast, and a deep wedge of cold air is expected build in east of the mountains (cold air damming a.k.a. "the wedge") Thursday into Friday...just as the area of low pressure arrives from the Ohio Valley.   The model solutions today are showing a deeper, colder wedge of air in place during the precipitation.  This means more snow, and if some milder air were to invade from the southeast aloft, then any ice would take the form of sleet, not freezing rain.

The forecast map below shows the wedge of cold air at the surface, valid Friday evening, during the hours of heaviest precipitation.  

Surface forecast map for Friday evening.
Those deep blue colors over VA-MD-PA are sub-freezing air, and the 32F isotherm is the heavy maroon line dipping down into South Carolina.   This freezing layer extends all the way through 5,000 ft (the critical snow-formation altitude) across the metro region.

However, the models remain consistent with precipitation amounts, which are on the light side, and fast movement of the storm.   The storm will deepen once over the Atlantic, but the moisture will have departed our area by the time the storm begins to bottom out.

Bottom line:  Prepare for some snow and/or a snow-sleet mix on Friday, not a major storm, but enough to accumulate. 

The next major storm upstream (Wed timeframe of next week) at this time appears to be a rain-maker.

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