Flash flooding is most often caused by heavy or excessive rainfall from thunderstorms in a short period of time. Other causes of flash flooding could be the result of dam, levee, or ice jam break.
Flash flooding can occur within minutes or hours of excessive rainfall. The intensity of the rainfall, location, and type of land use (suburban, urban, farm, etc.) all determine how quickly the flash flooding may occur. Urbans areas are prone to flooding in short time spans due to the amount of asphalt and concrete (sidewalks). These surfaces do not allow the water to go into the ground causing flash flooding more rapidly.
Flash flooding can happen so quickly that people can be caught off-guard. If you encounter high, fast moving water when you are driving, do not drive through the water. It is too dangerous and could cause your vehicle to be washed away with you in it. If you are at home or work, the water could come into the structure you are in and either trap you inside or cause property damage without you having time to protect either you or the property.
If you hear a watch or warning from the weather station, assess your situation and be prepared.