"Turn around, don't drown."

We've all seen the signs encouraging drivers to avoid certain roads during heavy rains, but flash flooding remains the leading cause of weather-related deaths in the U.S., according to the National Weather Service.

Flash floods are "the most dangerous kind of floods," according to the National Severe Storms Laboratory, and result in the highest fatalities. In densely populated urban areas like Houston, this type of flooding is most severe, the National Weather Service reports.