Hydrometeorology of Extreme Flooding in a Small Urban Watershed

A multicell thunderstorm system on 7 July 2004 produced record flooding in the 14.3 km2 Dead Run watershed, which drains an urbanized portion of the Baltimore metropolitan region. Storm total accumulations of 100 - 130 mm during a period of less than 2 hours resulted in flood peaks that exceeded those produced in 1972 by Hurricane Agnes. Analyses of the 7 July 2004 thunderstorm system focus on the spatial and temporal variability of rainfall at scales relevant to flash flooding in small urban watersheds. Spatial scales of interest extend below 1 sq. km and the time scales of interest are typically less than 60 minutes. Analyses are based on volume scan reflectivity observations from the Sterling, Virginia WSR-88D and BWI TDWR radars, a network of 19 rain gages in the Dead Run watershed and drop spectra observations from a Joss-Waldvogel disdrometer. We also use analyses of these observations to examine the capability of measuring extreme rainfall from weather radar at spatial scales relevant to flash flooding