Sunday, December 23, 2012

December 23, 2012: Big Storm This Wednesday

Christmas Eve Storm


This will be a relatively small nuisance event, compared to what's coming this Wednesday.  The models appear warm enough for light rain tomorrow, ending tomorrow evening, as a weak wave in the polar jet stream passes through the region from west to east.   A few pockets of sleet may mix in, but any persistent frozen precipitation will fall in colder air to our north and west...where light accumulation may occur (< 1 inch). 

Wednesday's Winter Storm

This storm, Winter Storm Euclid, will intensify over the Tennessee Valley and is coupled to a stronger wave in the southern branch of the jet stream (the subtropical jet).   This will be a more intense and larger region of low pressure, with a strong feed of Atlantic and Gulf moisture.   There will be two phases to this storm - first cold, then warm. 

As the storm approaches from the southwest, a wedge of cold air will be trapped in place at the surface, piled up against the Appalachians.   This is called cold air damming and is a common winter occurrence in the Mid Atlantic.   The flow of sub-freezing air is fed by a region of high pressure over Ontario, which funnels the cold air southward, along the eastern slopes of the Appalachians (blue arrow in the surface weather map, below):

Modified from NWS image
Cold air damming means precipitation - starting as rain several thousand feet above the surface - will freeze before reaching the surface, or right at the surface.  Thus, Winter Storm Euclid may start as a period of light sleet/freezing rain late Tuesday night.

Phase Two occurs Wednesday morning, as the low intensifies and begins to draw relatively warm air off the Atlantic Ocean into its lower circulation.   A brisk southeast wind should scour out the cold wedge and warm the lowest layers of the atmosphere above freezing.   This occurs as the slug of heavy moisture arrives...meaning a rain event for Baltimore on Wednesday.    The rain could be moderate to heavy, and 1"-2" may fall before the storm moves away Wednesday night.  This surface weather map shows the arrival of warm air (red arrow) and heavy rain (solid light blue shaded region) on Wednesday:

Modified from NWS image
Thus, Euclid should begin as a "multiple choice" phase of sleet/freezing rain/rain, turning to all rain - possibly heavy.   This will NOT be a big snowmaker storm, since the source of cold air to the north cannot be maintained.  Nor is it a classic Nor'easter or coastal storm, because it is expected to track largely inland.  But the storm will wring out a fair amount of moisture.

Caveat:  Models such as these consistently underdo the intensity and persistence of cold air damming.   Worst case scenario - the iciness hangs on through Wednesday morning, and takes longer to transition to all rain. 

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