Friday, December 21, 2012

December 21, 2012: Parade of Storms Now Through Christmas Week

Winter Storm Draco Exits The Stage


Cold air spiraling into the back side of intense winter storm Draco has filtered into our region.   Draco is the very intense, large mid-latitude cyclone that brought heavy snow and a blizzard to the Upper Midwest and Ohio Valleys.  Its tight pressure gradient is sustaining wind gusts in the 30-40 mph range across our region today.   Below is a snapshot of this impressive storm from this afternoon:

Surface Map Courtesy of WeatherBell
The storm has nearly attained its deepest minimum central pressure, and has occluded (the converging warm and cold air masses have become tightly wound-up).  When these storms occlude, they hit peak intensity, and move very slowly or even become stationary.  Thus, Draco will be slow to exit toward the northeast.  As it does, and as the pressure gradient relaxes gradually over the next 24 hours, winds will begin to abate across our region.  HOWEVER:  Winds are expected to surge once again, for several hours during the day tomorrow.   Reason:  Winds a few thousand feet above the surface, wrapping around the vortex from the west, are expected to intensify.  If there is sufficient sun and heating of the surface tomorrow, the atmosphere will become unstable and overturn.  This will mix down some of the faster-moving air aloft (70 mph at 3,000 feet) as intermittent, strong gusts.  The NWS has backed off the prospect of a High Wind Warning, and instead has issued a Wind Advisory, for winds that could gust tomorrow afternoon in the 40-50 mph range.   Below is the NWS wind gust prediction map, with maximum surface gusts (kts) shown at 1 pm tomorrow:

Max values on this map are around 50 mph.  I do not anticipate widespread power outages from this event - any outages will likely be spotty.  With sundown, the wind machine dies down and conditions turn calmer.

Next Storm:  Getting Clipped On Christmas Eve


Next up in our parade of storms comes a weak area of low pressure, passing to the south of Baltimore, on Christmas Eve.  It does not have strong upper-atmospheric support and will be relatively starved for moisture.   Look for a brief period of light rain, possibly with a little wintry mix to the west and north of Baltimore:



Post Christmas:  A More Potent Storm Sets Up


Storm #3 will originate in the Tennessee Valley and track up the Appalachians.  This track generally places our region on the warm (rainy) side.  This storm will have a much greater tap on Gulf and Atlantic moisture, so precipitation amounts will be heavier.  HOWEVER:  The computer models are suggesting that this system will develop a "secondary" low along the NC/VA coast - a classic breeding ground this time of year for Nor'easters.

When the primary low transfers its energy and moisture to a secondary, coastal system - the forecast scenario can become quite complicated, with respect to (1) amount of precipitation;  (2) type of precipitation;  and (3) timing/duration of precipitation.   Whether this means a "multiple choice" type of day (wintry mix) for our region, remains to be worked out.   However, it does NOT appear to be a major "snowmaker" storm for Baltimore, for two reasons:  (1) the coastal low will pull quickly away from the coast;  and (2) a deep, sustained feed of very cold air to the north is lacking.

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