Rain and terrain produce deadly floods

Published 7:00 am Tuesday, September 22, 2015

Eight days ago, on Sept. 14, a few thunderstorms began to form in the mid-afternoon along the Arizona-Utah border, as they often do this time of year. The loop of weather radar images for the next couple of hours showed scattered splotches of red, yellow, and green moving north across the area, but many locations didn’t receive any rain. The radar estimated that some of the storms were producing one to two inches of rain.

Remove the map background from the radar imagery, and it could be mistaken for radar images over south Mississippi and Louisiana on a typical summer afternoon.

But, there was one huge difference: 20 people died last Monday afternoon in Utah as a result of the thunderstorms there. It was the most deadly day due to weather in Utah’s history.

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How could just a couple of inches of rain from afternoon thunderstorms, something we barely take note of when it happens here, be so devastating in Utah? The answer can be summed up in two words: steep terrain.

Pick any two points within one mile of one another in our county and the change in elevation will be less than one hundred feet. In the vast majority of cases the change will be less than 30 feet.

In southern Utah within one mile of where the people were killed, the elevation changes over one thousand feet. The rain falling on the rocky ground quickly drains downward, surging down narrow canyons and gullies. In a matter of minutes a dry canyon bed can go from being dry to being full of twenty feet of violently rushing flood waters.

Seven of the people killed were hikers in Zion National Park. According to a Reuters news story, that morning the hikers were warned by park rangers that flash flooding was “probable” that day. Cindy Purcell, Zion’s chief ranger, is quoted as saying, “The ranger who handed that permit to that man said, ‘I would not go today.’”

But, the group decided to set out anyway for Keyhole Canyon, one of the beautiful – and very steep – slot canyons within the park.

Most of the others killed were in two vehicles that happened to be traveling along a road in the steep terrain that afternoon.

Forecasters are pretty good at predicting when there will be scattered afternoon thunderstorms, whether it’s for Utah or Mississippi. What is nearly impossible to predict is exactly which locations will get storms and which will stay dry. Only after the storms begin to form and move can forecasters monitoring radar begin to issue short-term forecasts and warnings.

That was the case last Monday in Utah. That morning the National Weather Service in Salt Lake City said in a Hazardous Weather Outlook that locally heavy rain was possible, especially across southern Utah. That afternoon at 2:22 P.M., based on what they saw developing on radar, the forecasters issued a flash flood warning. It is not known exactly when the fatalities occurred, nor whether any of those killed received the warning before the flooding reached them.

Don’t underestimate the danger to life and property of flash flooding here along the Gulf Coast. But, be especially careful if you happen to be visiting a part of the country with steep terrain.

By Skip Rigney