Flash flood warnings up here in the Northeast are usually a bit superfluous. Each time there's a slight bit of drizzle, we get either flash flood watch or warning, and I still have yet to see a torrent rushing down my Brooklyn street. But out West, where it's usually dry, flash floods are incredibly serious business.
Flash floods are especially dangerous in places like southern Nevada, where this video was shot by a man named Doug Bennett. The arid ground is usually not very absorbent, so when a passing thunderstorm dumps a large amount of rain in a short period of time, the result is that you get a swiftly moving river where previously there was just dry land.
Drivers often don't take flash flood warnings seriously, not seeing any floods immediately in front of them, and thus seriously underestimating the "flash" part of "flash flood." But it'll ruin your day, your car, and possibly your life if you get caught in one.
If you see a flash flood happening, don't try to drive through it. It's nuts.