Stream flooding occurs across a wide spectrum of watershed area and rainfall duration. The great majority of flash floods occur on watersheds less than 50 km2, with rainfall time durations of less than 6 hours and frequently less than 2 hours. At the other end of the stream flooding spectrum is the “large watershed” flood with areas of 250 to 1,300 km2 with rainfall time durations of 6 to 48 hours. Flash floods can and do occur on these larger watersheds when heavy rainfall persists for time periods of 3 to 6 hours. This paper will examine several case studies of flash floods on watersheds greater than 400 km2 including the Red Bank Creek flash flood of 1996, the eastern Ohio flash floods of 1998, and the Hurricane Ivan flood in western Pennsylvania of 2004. The case studies will demonstrate the watershed size and rainfall duration definitions needed to detect “large watershed” flooding.

The primary tool used by the National Weather Service (NWS) for the detection of stream flooding is the Flash Flood Monitoring and Prediction (FFMP) program. The initial implementation of FFMP in 2003 provided a small stream database for detecting flash flooding in small watersheds with rainfall time durations of less than 6 hours. In the spring of 2005 a database of “large watersheds” was provided to NWS forecasters to support the detection of larger scale stream flooding. The case studies presented will show FFMP type displays of these “large watersheds” to demonstrate how this new FFMP feature will support detection of flash floods in the larger streams.

While the 2006 version of FFMP has maximum time duration of 6 hours, future versions of FFMP will support time durations of 24 to 48 hours. These increased rainfall time durations will greatly aid in the detection of large stream areal flooding. The detection capability of FFMP for the widespread flooding typically caused by tropical storms will especially benefit from rainfall durations of 24 to 48 hours. The case study of Hurricane Ivan will demonstrate the improvement in flood detection resulting from changing the maximum rainfall duration of 6 hours to 24 hours.