The Flash Flood Guidance System (FFGS) implemented in the modernized National Weather Service provides both standard methodology and finer resolution for computing flash flood guidance (FFG). Flash flood guidance is computed for areas containing small, ungaged streams (grids, zones, and counties) and for areas containing gaged streams (primarily headwaters). FFG is computed for 1-, 3-, and 6-hour durations, and optionally for 12- and 24-hours.

Rainfall estimates from the WSR-88D radars are derived on the Hydrologic Research Analysis Project (HRAP) grid, nominally 4 km on a side. To more effectively use the radar rainfall estimates in the flash flood program, flash flood guidance is needed on the same spatial grid scale.

1. Modernized Flash Flood Guidance Document
2. Release Notes
3. Threshold Runoff