Gridded Flash Flood Guidance (GFFG) provides a general indication of the amount of rainfall necessary to cause small streams to overflow their natural banks. During heavy rainfall, usage of this guidance must involve combination of rainfall that has already occurred plus any additional rainfall that is expected to fall over the same location, not to exceed the guidance duration (example: 1-hr or 3-hr). It should be noted that these GFFG values are based upon estimates of average soil moisture and stream flow conditions over 4km x 4km grids. GFFG values also assume that the rainfall event will be evenly distributed over the guidance duration. Flooding may occur with less rainfall than indicated in areas of very high rainfall intensities, impervious surfaces (urban areas), or steep slopes.
