You can always count on a few cases of localized flash flooding around here during the summer. Start with very moist air, add some sun to help clouds bubble up, then mix in weak upper-level steering winds ñ and you have the recipe for slow-moving, drenching thunderstorms.

And that's just what happened about a week ago in Bucks County, PA. Very localized, nearly-stationary thunderstorms dropped 4 to 8 inches of rain in just a few hours. This graph of the water level along Neshaminy Creek in Bucks County shows what that kind of torrential rain can do. In just a few hours, the water rose from harmless levels of a foot or so to above flood stage at nearly 12 feet. It's a classic example of flash flooding, when there's very little time to protect property or seek higher ground.

Fortunately, no lives were lost in this flash flood. But flash flooding remains one of the weather's biggest threats, accounting for more deaths, on average, than lightning, tornadoes or hurricanes.