Heavy rainfall swept across North and Central Jersey July 14, leading to road closures, power outages and flash flooding.

Flash flooding ranks among the top weather-related killers in the U.S. But what is it exactly, and how does it happen?

The main thing about a flash flood is its speed. By definition, the rushing waters begin within six hours after an intense storm—and often within three hours, which is why it’s called a flash.

People usually only have hours of notice in advance. The disaster in Texas was a worst-case scenario.

The official definition of a flash flood warning says it is issued when “flash flooding is in progress, imminent, or highly likely.” ...

Flash floods happen when there is heavy rainfall. There isn’t a global definition of what a flash flood is. Predicting rainfall is harder than it seems, and there are other complicated factors ...

“Flooding is a longer-term event than flash flooding: it may last days or weeks. A (flash flood is) caused by heavy or excessive rainfall in a short period of time, generally less than 6 hours ...