FLOODING/RAINFALL

Flash Flood Guidance (FFG) provides a general indication of the amount of rainfall evenly distributed over various durations (in hours), necessary to cause small streams to overflow their natural banks. During heavy rainfall, accurate interpretation of the guidance consists of the combination of rainfall that has already occurred plus any additional rainfall that can be confidently expected to fall over the same location in the immediate future.

It should be noted that FFG values represent estimates of average soil moisture and stream flow conditions. Flooding may occur with less rainfall than indicated in areas of high rainfall intensities, impervious surfaces, or steep slopes.