December Begins with a Windstorm

The windstorm that evolved along and ahead of a cold front over the Mid-Atlantic region during the morning of 1 December 2010 culminated in the development of a squall line that produced downburst winds over the Tidal Potomac region.  Between 1400 and 1430 UTC, a line of convective storms developed along the cold front over northern Virginia and then tracked east and northeastward into central and southern Maryland between 1430 and 1500 UTC.  Between 1430 and 1445 UTC, several strong downbursts occurred over the Tidal Potomac River and were recorded by WeatherFlow observing stations.  Similar to the 17 November downburst event, the channeling of mid-tropospheric dry air into the core of convective storms provided significant downdraft energy that resulted in high winds.  

Figure 1.  GOES-13 VW-IR brightness temperature difference (BTD) image at 1445 UTC 1 December 2010.

    Figure 1 shows a GOES-13 BTD microburst risk image over Maryland and Virginia near the time of downburst occurrence over the Tidal Potomac River.  At 1430 UTC, a wind gust of 41 knots was recorded by Potomac Light 33 WeatherFlow station associated with a squall line downburst.  Five minutes later, at 1435 UTC, a stronger downburst wind gust of 43 knots was recorded by Cobb Point station.  Note that the dry-air notch identified in Figure 1 on the southwestern flank of the squall line was pointing toward Cobb Point, indicating that the injection of dry air into the convective storm was resulting in evaporational cooling and the generation of negative buoyancy that would result in the downburst winds over the Tidal Potomac.

Figure 2. Radiosonde observation (RAOB) from Dulles Airport, VA at 1200 UTC 1 December 2010.

 Figure 2 displays favorable conditions for strong convective winds over the Tidal Potomac and Chesapeake Bay region.  The sounding profile identifies a dry-air layer near the 800-mb level with winds near 45 knots, very close to the wind gust speed observed at Cobb Point.  This provides evidence that in addition to precipitation loading and evaporational cooling, the downward transfer of horizontal momentum from the dry-air layer to the surface by heavy rainfall was also an important forcing factor in the strong downburst winds.
As the squall line continued to track eastward, strong downburst winds were observed over the Chesapeake Bay between 1500 and 1530 UTC.  At 1517 UTC, Thomas Point Lighthouse Coastal-Marine Automated Network (C-MAN) station, displayed in Figure 3, recorded a wind gust of 50 knots with the passage of the squall line.  

Figure 3.  Thomas Point Lighthouse, Maryland.

By this time, the dry-air notch on the southwestern flank of the line had become more pronounced in both satellite and radar imagery with a southwest to northeast orientation toward Thomas Point Light (white cross in Figure 4).  The peak wind gust recorded at the lighthouse was from a southwesterly direction with a local maximum in radar reflectivity (> 40 dBZ) overhead.  Nearby WeatherFlow observing stations, Tolly Point and Greenbury Point, recorded wind gusts of 44 knots and 46 knots, respectively, at 1520 UTC.  The concurrent peak wind gust and passage of the heaviest rainfall core over the observing stations suggest that higher momentum from the mid-tropospheric dry air layer was being transported to the surface by heavy precipitation within the squall line.  The physical process in downburst generation described with this event exemplifies the highly dynamic environment that is typical for cold-season severe convective storms.

Figure 4.  GOES-13 WV-IR BTD image at 1515 UTC 1 December 2010 with overlying radar reflectivity from Dover, DE NEXRAD.