About County Flash Flood Guidance:
County flash flood guidance is a numerical estimate of the average rainfall (in inches) over a one, three, and six-hour time duration, required to initiate flooding along small streams in the indicated county. These estimates are derived from averaging the gridded flash flood guidance over each county and are based on current soil moisture conditions computed by the River Forecast Centers. In urban areas, less rainfall is required to produce flash flooding.
About Headwater Guidance:
Headwater guidance is the basin averaged rainfall required to produce flooding at an RFC basin outlet.