An Early November Downburst on the Chesapeake Bay

During the afternoon of 5 November 2009, an upper-level disturbance triggered the development of scattered convective storms over the mid-Atlantic region. Solar heating of the lower atmosphere during the early afternoon fostered conditions for strong convective storm downdrafts that were more typical of the Great Plains region. A convective storm that developed west of Baltimore, Maryland during the late afternoon produced a strong downburst as it tracked from the Chesapeake Bay eastward into the Delmarva Peninsula. An associated wind gust of 38 knots was recorded at Tolchester Beach, Maryland at 2242 UTC. The 1800 UTC Geostationary Operational Environmental Satellite (GOES) imager microburst product, derived from brightness temperature differences obtained from the GOES-11 imager (in full disk mode), indicated high downburst risk in proximity to Tolchester Beach.

Figure 1. GOES-11 imager microburst product at 1800 UTC 5 November 2009 with radar reflectivity from Dover Air Force Base NEXRAD at 2237 UTC overlying the image (top) and RUC model analysis sounding over Tolchester Beach, Maryland at 1800 UTC (bottom).

Figure 1 compares the Geostationary Operational Environmental Satellite (GOES)-11 imager microburst product to a corresponding Rapid Update Cycle (RUC) model sounding over Tolchester Beach, Maryland at 1800 UTC, 5 November 2009. At 1800 UTC, the GOES-11 image product showed a large area of high downburst risk (red shading) over southern New Jersey and the Delmarva Peninsula where strong solar heating was destabilizing the lower atmosphere. Output brightness temperature difference (BTD) in this region was greater than 40°K, indicating wind gust potential greater than 40 knots based on a previously established statistical relationship. Overlying radar reflectivity imagery from Dover Air Force Base NEXRAD at 2237 UTC displayed the downburst-producing convective storm as a spearhead echo over Tolchester Beach. At 2242 UTC, the Tolchester Beach Physical Oceanographic Real-Time System (PORTS) station recorded a wind gust of 38 knots. The corresponding RUC sounding profile echoed favorable conditions for downbursts in the Tolchester Beach area with the presence of a 5000-foot deep mixed layer and steep temperature lapse rate in the lower atmosphere. These boundary layer conditions promoted strong downdraft generation due to the effects of precipitation loading, evaporational cooling, and subsequent generation of negative buoyancy.