December Downburst over the Chesapeake Bay

During the early morning of 3 December 2009, strong convective storms developed over the lower Chesapeake Bay region ahead of a cold front. A supercell storm produced a strong downburst over the lower Chesapeake Bay, with a wind gust of 44 knots recorded at York River East Rear Range Light PORTS station at 0636 UTC as shown in Figure 1.


Figure 1. Wind histogram from York River East Rear Range Light PORTS station.

Although radar reflectivity displayed in Figure 2 was modest with the supercell (35-40 dBZ), downward horizontal momentum transport within a shallow mixed layer still resulted in the generation of strong surface winds.


Figure 2. Radar reflectivity image from Wakefield, Virginia NEXRAD.

The 0600 UTC RUC analysis sounding near the mouth of the York River, displayed in Figure 3, indicated wind speeds between 40 and 45 knots about 1000 feet AGL. This supercell downburst was a typical cold-season low CAPE, strong shear-forced event that is not well-anticipated by the GOES microburst products. In this case, downburst generation was driven by a combination of precipitation loading and downward momentum transport processes.


Figure 3. RUC analysis sounding at 0600 UTC 3 December 2009.