Strong November Storms over Mid-Atlantic Region

During the late evening of 16 November and early morning of 17 November 2010, lines of strong convective storms developed and tracked eastward over Pennsylvania, Maryland and Virginia.  The line of convective storms that developed along a cold front boundary produced strong downburst winds over north-central Maryland, especially eastern Frederick County, as well as over the Tidal Potomac region.  Several downbursts occurred over eastern Frederick County between 0545 and 0625 UTC.  Sterling, Virginia NEXRAD indicated winds between 35 and 40 knots associated with the downbursts.  

      

Figure 1.  GOES-13 WV-IR BTD image at 0615 UTC with overlying radar reflectivity from Sterling, VA NEXRAD.

Figure 2.  GOES-13 WV-IR BTD image at 0615 UTC with overlying radar velocity from Sterling, Virginia NEXRAD.

Figures 1 and 2 show a downburst in progress over eastern Frederick County near the location of the white cross.  Strong winds near 39 knots were measured by Doppler radar at the time of downburst occurrence between 0605 and 0610 UTC.  The strong downburst occurred immediately downstream of a mid-tropospheric dry-air channel, marked by the white line pointing toward eastern Frederick County. The dry-air channel, pointing into the rear flank of the convective storm line, fostered downburst generation by injecting drier (unsaturated) air into the precipitation core of the convective storm.  The resulting evaporational cooling and generation of negative buoyancy accelerated the convective storm downdraft toward the surface, producing strong winds on impact.
The Tidal Potomac Region was also effected by the squall line between 0545 and 0630 UTC, as evidenced by high winds recorded by NOAA data buoys and WeatherFlow observing stations.  During this time, several dry-air channels became apparent in BTD imagery on the western flank of the convective line.  Figures 3 and 4 show that three channels over western Virginia were coming in line with rear-inflow notches as identified in radar imagery.

Figure 3.  GOES-13 WV-IR BTD image at 0532 UTC with overlying radar reflectivity from Sterling, VA NEXRAD.

Figure 4.  GOES-13 WV-IR BTD image at 0632 UTC with overlying radar reflectivity from  Sterling, VA NEXRAD.

Figure 3 shows the squall line moving into the Washington, DC metropolitan area with the apex of a bow echo moving into southern Maryland.  A severe wind gust of 50 knots was recorded by the Upper Potomac River buoy (white cross in Figure 3) between 0540 and 0550 UTC.  Note that a dry-air notch, as apparent in the GOES BTD image, was apparent on the western flank of the squall line pointing east-southeastward toward the Upper Potomac buoy.  At this time, the dry-air notch was in phase with rear-inflow notch (RIN) as evident in NEXRAD imagery.  By 0602 UTC, as shown in Figure 4, two dry-air notches were pointing toward the location of downburst occurrence at Potomac Light 33 station where a wind gust of 43 knots was recorded.  About 20  minutes later, near 0625 UTC, a stronger downburst wind gust of 45 knots was recorded  by Cobb Point station.  At this time, as displayed in Figure 5,  a dry-air notch had come in phase with a bow echo moving over the lower Potomac River.  It is evident that the injection of mid-tropospheric dry air into the heavy precipitation core within the convective storm line was providing a large amount of downdraft energy to realize strong downburst winds at the surface.  

Figure 5. GOES-13 WV-IR BTD image at 0632 UTC with overlying radar reflectivity from Sterling, VA NEXRAD.

Near this time, as a bow echo was tracking over the Chesapeake Bay, a wind gust of 46 knots was recorded at Thomas Point Lighthouse Coastal-Marine Automated Network (C-MAN) station.  The downburst wind gust was associated with the passage of the apex of the bow echo that tracked directly over Thomas Point Light.  Figure 6 shows the passage of the bow echo over Thomas Point Light.

Figure 6.  GOES-13 WV-IR BTD image at 0632 UTC with overlying radar reflectivity from Dover, DE NEXRAD.

The GOES BTD image in Figure 6 shows that a dry-channel (white line) was pointing toward Thomas Point Lighthouse (white cross) in phase with a (RIN) as apparent in radar imagery. Again, the channeling of dry air into the rear flank of the bow echo was a major forcing factor for downburst winds that were observed at the C-MAN station.  It is noteworthy that one of the strongest wind gusts of the event occurred as the dry-air notch, RIN, and bow echo apex were in phase as the convective storm line passed over the Chesapeake Bay.