What causes flash flooding in Southern Utah?

Weather

Courtesy of Capitol Reef National Park Service

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(ABC4) – As monsoon season has moved within the Utah state lines, with high-pressure winds bringing moisture in from the Gulf of Mexico and California, flash floods are becoming more and more common in Southern Utah.

On Tuesday, flash floods struck the Capitol Reef National Park area, with rainfall creating a dramatic waterfall effect off the Fruita Cliffs, as seen on Meteorologist Cesar Cornejo’s Twitter.

The most surprising part of the raging water ripping down the slot canyons and towards the path of least resistance? It was caused by just one inch of rainfall.

As Cornejo explains, that’s really not that much.

“If you go out and see an inch of water in a rain gauge, it’s not that high, it doesn’t create that much of an issue,” Cornejo says.

It becomes a bigger issue when all the rain over a large amount of space gathers together at a common path, he continues.

“It’s enough to create waterfalls off cliffs and also fill up dry stream beds that become basically roaring streams again.”

The red rock, desert terrain in Southern Utah, while quite picturesque and Instagrammable when dry, can serve as the perfect tinder box for a flash flood.

“The areas that have seen flash floods are mountainous and many have Utah’s famous red rock and slick rock,” says ABC4 Utah Chief Meteorologist Alana Brophy. “Water is not absorbed in these areas, it gets slapped around and funneled toward lower ground. As a result, a small amount of water can easily cause mud and debris flow in a typically dry area.”

Whereas a normal flood happens when an area takes a steady downpour of rain over a longer duration of time, a flash flood can happen much faster, usually reaching its peak of impact in less than six hours.

While it may seem like the rain that causes a flash flood may not be more than a slight inconvenience to those caught underneath it, when the water hits the dry, rocky earth in places such as a slot canyon, things can become dangerous very quickly.

“You might not think this is a lot of rain as it’s hitting you. Yes, your shirt might get soaked for a bit, or your shoes might start to get a little soggy but when there’s nowhere for it to go it really starts to accumulate in those smaller areas and becomes a danger that’s why, when they say do not go into slot canyons when there’s a threat of thunderstorms,” says Cornejo.

All that water gathering in a canyon can create a lot of momentum as it descends down the path of least resistance.

Brophy, who enjoys exploring many of Utah’s natural wonders when not leading the state’s top weather forecast, cautions that even ankle-deep water is enough to cause serious damage. She advises folks in the Southern-Central Utah area, to be aware of the weather if they’re out exploring on Wednesday, with flash flood potential at Zion, Capitol Reef, and Bryce Canyon National Parks.

“Flash flooding is about intensity, and the more water you have the more intense and dangerous it gets, but I would not want to be caught in any flash flood at any point,” she says. “It only takes 6 inches of water to sweep away a person, so it’s not a force of nature to gamble with.”

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