The National Weather Service maintains a special river and rainfall reporting network, and continually analyzes river and rainfall data to provide river forecasts and flood warnings.
Floods begin when soil and vegetation cannot absorb falling rain or melting snow and when water runs off the land in such quantities that it cannot be carried in normal stream channels or retained in natural ponds and manmade reservoirs. River Forecast Centers issue flood forecasts when the rain that has fallen is enough to cause rivers to overflow their banks.
On small streams, especially near the headwaters of river basins , water levels may rise rapidly in heavy storms and flash floods can begin before the rain stops falling. There is little time between threatening flood conditions and the arrival of the flood crest. Swift actions are essential for the protection of life and property.
The meteorologist must determine the possibility of flash flood producing rains reaching his area of responsibility. When this threat exists a flash flood watch is issued to the public. The watch means check preparedness requirements, keep informed, and be ready for immediate action if a flash flood warning is issued.
When radar or observer reports indicate that flash flooding is likely or is occurring, a flash flood warning is issued. The warning requires prompt reaction if you are in an area subject to flooding. Flash flood warnings are one of the most urgent types of weather warnings issued and they are transmitted to the public by the most rapid means available.
Preparation is the key. Every resident of a community should know what a river height means in terms of his own situation. They should know how far their property is above or below anticipated flood levels and how this elevation relates to the river gauge for which forecasts are prepared. Always have a plan to move to higher , safer ground should flooding threaten your area.