Flash Floods
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Hi, I'm Bryan Yeaton and this is the Weather Notebook.

In most parts of the country, an inch of rain won't cause many problems beyond a mild soaking. But in the southern Utah desert, an inch of rain can cost you your life. Correspondent Jeff Rice has the story.

The hikers were caught completely by surprise...It was a sunny day . The only clouds were miles in the distance. But suddenly, a 10 foot wall of water was raging down the canyon. And 11 hikers were suddenly dead.

LD: The thunder storm was many miles to the south. and the wall of water came down this narrow canyon and there was no place to go and everyone but the guide was killed in that situation. So it's very dangerous.

National Weather Service meteorologist Larry Dunn is describing a flash flood that swept through Antelope Canyon in 1997, near the Utah/Arizona border. Flash flooding occurs after sudden downpours because the water can't seep into the hard desert slick rock.

LD: Any rain at all, it's just like falling on concrete. And it gets channeled into these extremely narrow canyons that hikers and backpackers love to frolic in during the summer.

What can fool hikers is that even a thunderstorm 20 miles away can be deadly. The water will just keep rolling. The narrower the canyon, the higher and faster the water.

LD: Relatively small thunderstorms can produce big problems.

As a result, places like Zion National Park require permits to enter narrow canyons, contingent upon favorable weather reports and a "flash flood index" prepared each day by the National Weather Service.

Correspondent Jeff Rice stays dry in Boise, Idaho. The Weather Notebook is a production of the Mount Washington Observatory. It is underwritten by Subaru and The National Science Foundation. Thanks today to executive engineer Sean Doucette.