Tornadoes that cause deaths in the Maryland-northern Virginia area are very rare events. In Maryland during the period 1950-2000, only 2 deaths occurred from tornadoes, while prior to 2001 the last deaths from a tornado in the immediate vicinity of the District of Columbia (D.C.) occurred in 1979. On the afternoon of 9/24/01, two different supercells produced strong and violent tornadoes in the northern Virginia-central Maryland area, including an F3 tornado that killed 2 people in Maryland immediately northeast of Washington D.C.

This paper will look at some mesoscale aspects that were important to forecasting this event. These include the formation of a mesoscale low pressure area that tracked from western North Carolina into northern Virginia during the day. This small-scale low appeared to focus an environment more favorable for supercells and tornadoes over northern Virginia and the D.C. area as it moved northeast during the afternoon. The progression of the mesoscale low and backing of winds ahead of this feature will be shown, along with some derived parameters suggesting the more favorable wind and thermodynamic environment northeast of the low.

This event reaffirms the importance of careful mesoscale analysis of surface maps on an hourly basis in assessing severe weather potential, and how mesoscale details can play a role in significant tornado events. In this case, the mesolow needed to be identified hours ahead of time to help in the forecast process prior to a tornado watch being issued by the Storm Prediction Center (SPC).

This case also provides an example of effective merging of powerful “old school” analysis methods (e.g., mesoscale surface analysis) with assessment of wind and thermodynamic factors more recently recognized as having importance in supercell and tornado forecasting. Both are important in severe weather forecasting.