Late Evening Winter Storm Update

1007 millibar low in southern AL...click for larger image

3 hour pressure falls off the NC coast will pull the storm northeast

The winter storm forecast to move into the mid Atlantic region on Wednesday continues to slowly organize over southern Alabama tonight.  The low pressure system at the surface has not intensified very much; the central pressure down to just 1007 millibars as of 10:00 p.m., but the surface pressures are falling rapidly just east of Wilmington, NC, suggesting the storm will be scooting in that direction and gaining strength overnight.

7pm upper air sounding from near Dulles airport...click for larger image

The evening balloon launch near Dulles airport revealed the expected forecast dilemma: temperatures above freezing in the lower levels of the atmosphere.  This will cause a wintry mix of precipitation to fall as the storm arrives in the early morning, most likely rain, sleet, and some snow, but watch for pockets of freezing rain as well.

Upper low tracking through eastern VA 7pm Wednesday

The early evening computer model runs have shifted the main snowband to the southeast and are pointing to the possibility of nasty weather for the evening rush hour as the upper level low tracks across east central Virginia around sunset.  On that track, the upper level low could produce strong upward lift which would rapidly cool the atmosphere and change the mixed precipitation to a period of heavy snow.  Patterns like this can also produce occasional thundersnow, which would create some intense snow showers during the early evening.

Put it all together and a general 3-6″ snowfall looks very possible for much of central and eastern Maryland from late afternoon Wednesday through about 2:00 a.m. Thursday.  Timing the changeover from mixed precipitation to all snow and trying to account for possible convective snow bursts makes this a very tricky forecast which may have to be tweaked further…so as always…stay tuned!

Tom Tasselmyer

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