Schematic of how thunderstorms can produce flash flooding.

The most common cause of flash flooding in spring and summer is a slow-moving or stationary thunderstorm that dumps heavy rain relentlessly on a small, localized region. This is especially true when the jet stream is often very weak or north of our area. The jet stream is a current of air high up in our atmosphere, which "steers" weather systems along, including thunderstorms. When the jet stream is weak or well north of the region, thunderstorms that form often have very weak steering currents and move slowly, producing flash floods. The hills and mountains of our region often are the breeding grounds for these slow-moving thunderstorms. As winds rise up the mountains, the air cools and condenses forming clouds, and, when the conditions are right, the clouds can develop into thunderstorms. With weak steering currents aloft, these thunderstorms often remain stationary on a hill or mountainside. This often has disastrous consequences since many small streams and creeks flow down our mountains and hills.

Flash Flood's kill more people than any other storm-related hazard-find out more from the NWS's flood awareness campaign "TURN AROUND, DON'T DROWN!"