Late Winter Squall Line

The GOES Microburst Windspeed Potential Index (MWPI) product performed effectively during a late winter squall line event that occurred on 28 February 2011.  During the morning of 28 February, an intense squall line developed ahead of a cold front over the Appalachian Mountains and tracked through Virginia, eastern West Virginia, and central Maryland.  The squall line produced numerous downbursts over the Blue Ridge mountain region of Maryland, West Virginia, and Virginia as observed by Road Way Information System (RWIS) sensors.  Downburst wind gusts of 37 to 38 knots were recorded by RWIS sensors over western Frederick County, Maryland between 1500 and 1530 UTC.  The 1346 UTC MWPI product indicated a local maximum in values northwest of Washington, DC prior to the occurrence of the downbursts.

Figure 1 shows favorable conditions for downbursts over the greater Washington, DC area during the morning of 28 February 2011.  Locations of downburst occurrence as recorded by RWIS sensors are marked as “15” and “16”.  Sensor 15 (I-70 @ Frederick / Washington County line) recorded a wind gust of 38 knots at 1514 UTC followed by a wind gust of 37 knots recorded at sensor 16 (US-340 @ MD-180) at 1528 UTC. Downburst occurrence was confirmed in doppler radar imagery in Figure 2 by the presence of spearhead echoes near each sensor. As noted in previous studies, the spearhead echo is closely associated with the development and occurrence of downbursts.  

   

The nearby RAOB sounding from Dulles Airport indicates the presence of an unstable temperature lapse rate and well-defined dry-air layers above the 850-mb level.  These conditions foster downburst generation by the interaction of dry air with the storm precipitation core in the presence of static instability.  Strong winds observed above the 850-mb level suggest that dry air was channeled into the convective precipitation cores of the squall line, resulting in evaporational cooling, generation of negative buoyancy, and the subsequent acceleration of downdrafts toward the surface.  Accordingly, a local maximum in MWPI values (orange marker) was located in proximity to Dulles Airport shortly after the time of the RAOB and corresponded to wind gust potential near 35 knots based on the regression chart shown in Figure 3.

Figure 3.  Regression chart based on the comparison of measured downburst winds gusts to proximate index values for 208 events between 2007 and 2010.