The flash floods that swept through Arkansas today, taking the lives of at least 20 people and leaving others missing, are a reminder of a little-known fact: Flash floods are the No. 1 cause of weather-related deaths in the United States, according to the National Weather Service.

Two key factors that lead to flash flooding are the intensity of the rainfall and its duration. For this reason, most flash flooding is caused by slow-moving thunderstorms, thunderstorms that move repeatedly over the same area, or heavy rains from hurricanes and tropical storms, according to the National Oceanic and Atmospheric Administration (NOAA).

Intense rainfall led to the Arkansas flash flooding, causing the Caddo and Little Missouri rivers to rise quickly overnight, sometimes faster than 8 feet (2.4 meters) per hour, according to news reports.[ LIVE SCIENCE ]