How to use the Flash Flood Guidance - National Weather Service, Southeast River Forecast Center, Peachtree City, GA

Flash Flood Guidance Example

The three hour Flash Flood Guidance is a tool used by meteorologists to predict flash flooding of
urban areas, creeks and streams. Soil type and moisture are used to predict how much rain is needed in a three hour period to produce flash flooding.

The number of inches of rain are computed for each county across the
SERFC area as shown in this graph below. The number of inches shows 
how many inches of rain in three hours is needed to produce flash
flooding. 
For example, in the graph below, Dade County, FL which is 
the southeastern most county, shows a 3 hour flash flood guidance value 
of 5 inches. This means that a thunderstorm needs to produce 5 inches 
or more of rain in Dade County before there will be Flash Flooding. 
This is very rare.

The lower the number the more likely there would be flash
flooding during a heavy rain event.

Here are the color coded values: (in case you can't read the numbers
on the map)

THREE HOUR VALUES IN INCHES

RED	LESS THAN 1 IN
YELLOW	1 TO 1.9 INCHES
BLUE	2 TO 2.9 INCHES
GREEN	3 TO 3.9 INCHES
WHITE	> THAN 4 INCHES

As you can see here, the lower values of Flash Flood Guidance (FFG) are in the mountains and the higher values of FFG are in Florida and along the coast. The soil moisture content on this day was fairly uniform. However the differences in values are due to the type of soil and the terrain.

In the mountains the soil is rocky and the terrain has steep slopes. This is prime territory for a lot of runoff and thus flash floods. (see Big Thompson Flood)

Along the coast and in Central and South Florida, the FFG values are high. This due to the sandy soils which allow rainfall to soak in rather than run off. However in metropolitan areas, where there is a lot of concrete, rainfall often runs off and produces localized urban flooding.

Across the southeast, in between the mountains and the coast the soils are generally clay.