New Look at the 3 December 2009 Downburst Event

During the early morning of 3 December 2009, a line of convective storms developed over Frederick County, Maryland and then tracked eastward over north-central Maryland between 0800 and 1000 UTC before dissipating over the upper Chesapeake Bay. The convective storm line produced strong winds over eastern Frederick County between 0845 and 0855 UTC. Doppler radar-derived winds between 23 and 34 knots were associated with the storm line as it tracked through eastern Frederick County. National Weather Service spotter estimated winds between 30 and 34 knots over eastern Frederick County reflected wind speeds near the top of the boundary layer at the 925-mb level (about 1500 feet above ground level). Near the time of the strong wind observation, radar reflectivity imagery indicated the presence of a well-defined weak echo channel over central Frederick County, signifying the immenent occurrence of downburst winds. RUC model-derived graphical guidance indicated high precipitable water values near 30 mm (> 1 inch) and 925 mb level wind speeds near 32 knots (16 m/s). These parameter values indicated that the combination of precipitation loading and downward horizontal momentum transfer was an important forcing factor in the strong convective winds observed over Frederick County. These conditions prompted the NWS to issue a Special Marine Warning for the Baltimore Harbor at 0915 UTC.



Figure 1. RUC model-derived graphical microburst guidance product at 0900 UTC 3 December 2009 with radar reflectivity image from BWI TDWR at 0850 UTC (top) and corresponding RUC sounding profile over eastern Frederick County.



Figure 2. RUC graphical microburst guidance product at 0900 UTC with radar reflectivity image from BWI TDWR at 0958 UTC (top) and wind histogram from FSK Bridge PORTS station (location "X" in RUC image).

A peak wind gust of 24 knots was observed at Francis Scott Key Bridge PORTS station at 0954 UTC as the southwestern end of the convective storm line tracked over the Baltimore Harbor. RUC model graphical guidance indicated 925 mb winds near 24 knots (12 m/s) in the vicinity of the Baltimore Harbor, comparable to the wind gust speed observed by the FSK Bridge PORTS station. Although a wind gust of this magnitude did not satisfy special marine warning criteria, a 24-knot wind gust would still pose a threat to small craft over the open water of the Baltimore Harbor. This event demonstrated the effectiveness of boundary layer wind speed data in the nowcasting of convective wind gust potential.