Thursday, December 20, 2012

December 20, 2012: Draco To Blow Through With Strong Winds

Winter Storm Draco is creating a mess of hazardous weather conditions across the eastern U.S. - including an outbreak of severe thunderstorms and tornadoes in its warm, southern side...and blizzard conditions on its cold, northern half.   Strong winds are blowing around the center of the entire system.  This is both an immense, and an intense, mid-latitude cyclone.   This radar mosaic snapshot from this afternoon illustrates the warm (green colors) and cold (blue colors) side of Draco:

Courtesy of WeatherTap
The center of Draco will pass north of our region tomorrow morning.  Tonite, the storm's cold front will push through.  Ahead of the front, the warm air and Gulf moisture will fuel bands of heavy rain showers, associated with gusty winds.    There are myriad weather hazards predicted across the Mid Atlantic.  Whenever you see lots of colors on the NWS watch and warning summary, you know we are in for a high-impact weather event:

National Weaterh Service
Heavy snows and intense winds will combine to create a blizzard scenario along the high mountain elevations to our west (pink and red colors).  The copper-shaded region across DC-Baltimore is a High Wind Watch.   The watch indicates a potential for sustained winds exceeding 40 mph, and/or gusts exceeding 58 mph.   The timeframe for these winds is Friday night and Saturday, peaking Saturday morning.   This is well after the precipitation has cleared the region, and in fact sunny to partly sunny (but chilly) skies may prevail during the period of strong wind.

The strong winds will be driven by two factors:  (1) an intense pressure gradient between Draco's low pressure center, retreating to the northeast, and high pressure building in across the southeast;  and (2) an unstable air mass (cold air aloft, heating of the surface by the sun) - which creates a tendency for the lowest air layers to "stir" or overturn.  With 60 mph winds at about 5,000 feet, some of this momentum may be occasionally mixed down to the surface as strong gusts.

I see a couple factors that might mitigate high winds across the region.  First, the models are weakening the low pressure center as it moves away from our region on Saturday morning, with the pressure gradient weakening throughout the day.  Second, any cloud cover that develops as a result of the unstable air layer may help limit the amount of instability, and therefore mix-down of higher winds aloft.   As we get closer to the event, NWS will either upgrade the High Wind Watch to a Warning, or downgrade it to a Wind Advisory.   Either way, it's fair to expect gusts into the 40-50 mph range for several hours Saturday morning and into the afternoon.   Highest wind gusts will be in elevated or mountainous area (remember that wind speed increases with height above the surface) and along the eastern shore of the Bay (winds pick up speed as they blow from west to east across the water surface).  With foliage off the trees and soils that likely won't become saturated, any power outages in the Baltimore region will likely be isolated.

Here is the predicted maximum gusts (kts) from the NWS, for 10 AM Saturday morning:

Note these values are in knots.  The 40 kts in vicinity of Baltimore translates into about 45 mph.

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