The Flash Flood Monitoring and Prediction (FFMP) program is now operational at all National Weather Service forecast offices in the continental United States. FFMP alerts forecasters to possible flash flooding in watersheds down to 5 square kilometers in area. The potential for flash flooding is determined by comparing the observed average basin rainfall (ABR) with the flash flood guidance ( FFG) provided by the river forecast center. The most recent version of FFMP provides the capability of locally changing flash flood guidance (FFG) values for each individual watershed in the FFMP database. Local modification of FFG can be critical to timely detection of flash flood potential with FFMP.

Many factors may support a local change in the FFG values that results in a more accurate representation of flash flood potential. FFG may be reduced in highly urbanized watersheds, due to the increased runoff resulting from a high percentage of impermeable soil. FFG may be reduced in areas of fire scars, or widespread strip mining, where the removal of trees and vegetation may greatly increase runoff. Steep terrain can also act to increase runoff and stream response time, requiring a reduction in FFG. Multiple rainfall events in small watersheds may greatly increase soil moisture content, resulting in reduced infiltration of additional rainfall. None of these factors are easily accounted for by the river forecast centers in the production of FFG. The realization that some of these factors are at work in a given watershed may allow local modification of FFG. Any local improvements to FFG can result in increased flash flood detection by FFMP. This paper will examine several possible reasons for local modification of FFG. Several case studies will be examined to demonstrate increased flash flood detection capability resulting from the local modification of FFG.