At 8:30 p.m. on Monday, Mark Licht sat in his home outside Ames, Iowa, for a conference call with other agronomists and meteorologists from around the state. Iowa had a dry spell, the western half of the state was hit by severe drought. What farmers needed was a big storm, thought Light, a specialty in cropping systems at Iowa State University. The meteorologists on call told him that someone was just about to strike in South Dakota and Nebraska. But, they said, it did not look like it would have the energy to make it to Iowa. Everyone crossed their fingers and hung up.
At 10:15 a.m., Light received an email from the group; the storm looked like it could actually sit together.
Less than an hour later, he heard storm warning sirens sounding from the nearest crash. He went outside. It was sunny, barely a cloud in the sky. The air was still and the humidity was suppressing. “That’s weird,” he thought. But when he checked radar, he saw an enormous mass deviation in his direction at about 60 mph.
He got his family into the basement, and 10 minutes later the storm was upon them. Rain so heavy you could not see more than a few feet ahead. Winds so fierce that they could shove a tree in half. When Licht and his family came out about 45 minutes later, the steel shed where their cars were parked had completely collapsed. “We were sitting in the middle of one of the more devastating storm paths,” says Licht. “It will be a long process to deal with the damage, but we are lucky that it was not worse.”
Iowa knows summer storms. But the one that swept across the Midwest on Monday, traveling 770 miles in 14 hours, leveling 10 million acres of crops, spinning grain silos and powering out hundreds of thousands of people for days, was a rare type of storm known as a derecho.
The term means “straight ahead” in Spanish, and was coined in the late 1800s by a Danish professor of nature at the University of Iowa, who used it to describe “the right blow of the prairies”, as opposed to to the circular winds associated with tornadoes. Today, for a derecho to meet the National Oceanic and Atmospheric Association’s definition, it must travel at least 240 miles and travel at speeds of at least 58 mph. This week’s derecho hit top speeds of 112 mph in Cedar Rapids, Iowa, about two hours east of the House of Light.
“Derechos are these long-lived, fast-moving walls of super-thunderstorms,” says Paul Huttner, a meteorologist who watches the weather for Minnesota Public Radio. They are regular but not uncommon events, occurring once or twice a year in the Midwest. Derechos come in two varieties: linear and progressive. Monday’s storm was a progressive derecho, moving faster, more compact, and packing more of a punch than his more scattered sibling. And this one, says Huttner, “was a real doozy.”
Derechos are often referred to as “domestic hurricanes” because of the experience on the ground – whipping, destructive winds; side-pouring rain – is the same as in a Category 1 hurricane. But they are completely different weather phenomena. Derechos are driven by various factors and behave more like a stampede of wildebeest than a bloom of hippopotamuses. And in contrast to the path and severity of hurricanes, which scientists have been good at predicting in days or even weeks in advance, predicting where and when a derecho will form remains one of the most challenging tasks for meteorologists.
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