Rare ‘derecho’ leaves swath of damage across Iowa, Q-C region

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By Tory Brecht | Monday, July 21, 2008 9:39 PM CDT | () comments

This Hackberry tree fell on a home on Front Street in Buffalo, Iowa. (Larry Fisher/QUAD-CITY TIMES) Buy this Photo

The power-punching, blustery storm that pummeled the Quad-City region Monday was a rare severe weather event called a “derecho,” according to the National Weather Service Quad-Cities office.

Derechos — from the Spanish word for “direct” or “straight ahead” — were first categorized in 1888 by a University of Iowa physics professor. They are most common in the late spring and summer, especially in the Corn Belt running from the upper Mississippi River valley to the Ohio River valley.

The storms, which are rarer than tornadoes, are known for their longevity, incredibly high-sustained wind speeds and fast-moving nature.

“We had a 60-mile-wide path of damage from Omaha through the Quad-Cities and on toward Chicago,” said Linda Engebretson, lead forecaster for the weather service office at the Davenport Municipal Airport. “Storms don’t usually stay that strong, that long. It’s like a 60-mile-wide train barreling across the state.”

This is the first derecho to hit Iowa in two years, Engebretson said. This one packed winds ranging from 65 to 95 mph.

“We had one in the late 1990s that took a whole train off the tracks,” she said.

Conditions have to be very precise to create a derecho, Engebretson said. There must be certain amounts of dry air aloft and moisture below. When conditions are right, the storm gears up and takes off at a high rate of speed in a “bow” shape. The front edge of the bow packs the heaviest winds, with less damaging winds on the outer edges.

Most thunderstorms, even severe ones, tend to blow themselves out fairly quickly, as the air being ejected behind the storm cuts off the flow of new air and moisture from the front side, “kind of like a vacuum cleaner sucking up a piece of paper,” Engebretson said.

In a derecho, however, its rapid speed prevents the vacuum effect from stalling the storm.

“It’s like a carburetor — if you’ve got your exhaust too close to the intake, it cuts off the engine,” Engebretson said. “But if they are far enough apart that they can work together, off they go. That’s what we had here.”

That fast speed also limits the amount of rainfall in a derecho, she said. Despite heavy downpours, the quick-moving storm managed to dump only 0.59 of an inch at the Davenport airport and 0.68 of an inch at the Moline airport.

According to a derecho fact sheet from the National Oceanic and Atmospheric Administration, the storms are particularly dangerous because they appear quickly, often before sufficient warning can be given.

“Another reason that derechos can be very risky to people is their rapid movement,” the fact sheet stated. “For someone involved in outdoor activities, the rapid movement means that the darkening of the sky and other visual clues that would alert them to the approaching danger occur extremely fast. Therefore, the visual clues of the approaching derecho gust front typically don’t offer much time to take protective action.”

Tory Brecht can be contacted at (563) 383-2329 or tbrecht@qctimes.com. Comment on this story at qctimes.com.