Breaking All the Rules: The Washington, D.C., Area Flash Flood of 8 July 2019

Previous research has shown several common themes amongst flash flood events in the Mid-Atlantic region. These include slow-moving and/or backbuilding convection, usually occurring in the afternoon or evening hours. On 08 July 2019, a large thunderstorm formed around sunrise in the vicinity of a mesoscale area of low pressure and then propagated along the north side of a stalled surface boundary. While the presence of the stalled surface boundary is somewhat common in flash flood events in this area, the time of day, and progressive nature of the storm in the absence of backbuilding in this event are quite unusual. Three to five inches of rain fell rapidly (~60-120 minutes) in a narrow corridor that included the heavily-populated western counties of the Washington DC metro region during the peak of a weekday morning commute.

This presentation will review the evolution of this significant flood event, and consider how an event that breaks the mold of the conceptual model was still able to produce significant flooding in the region, with millions of dollars in damages. Also discussed will be the challenges of the warning strategies required to be able to meet the lead time goal set as part of the Government Performance and Results Act (GPRA).