While hail can cause lots of damage, one of the most deadly products of thunderstorms is flash flooding. In fact, flash floods are now the number one weather-related killer in the United States. Rainfall intensity and duration are the key ingredients for a flash flood, and the most intense rainfall rates on earth can come from thunderstorms.
Flash floods can form within hours or even minutes of a storm. Heavy rains will saturate soil quickly producing runoff. Streets and parking lots can become swift moving rivers. And too many people underestimate the power of moving water. Believe it or not, even six inches of fast-moving water can knock you off your feet, and only two feet of water can float most cars.
Nearly half of all flash flooding fatalities are auto related, so never try to cross a water covered roadway. Even if you've crossed that road a hundred times! Turn around! Don't drown! Don't hang around a rising creek or stream. Head for higher ground.