Flash Flood Processes

This module presented an overview of how the National Oceanic and Atmospheric Service's (NOAA's) National Weather Service produces gridded flash flood guidance (FFG) from drainage basins, and uses these gridded values to produce headwater and county FFG. The gridded guidance is an estimate of the rainfall and duration over an area needed to produce a flash flood and is based on Rainfall/Runoff curves and the threshold runoff needed to produce flooding on small streams.

The flash flood guidance can underestimate flash flood potential in many locations such as urban areas, deforested areas, and basins subject to ice dams or dam failure. The county FFGs often underestimate flash flood risk by averaging out gridded values. But the model used does account for many of the important factors contributing to flash flooding such as soil characteristics, rainfall patterns, and area topology, though the basin sized used is often larger than desirable for accurate prediction.