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The perfect storm: Forecasters look back on the origins of the May 8 derecho
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The perfect storm: Forecasters look back on the origins of the May 8 derecho

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CARBONDALE — On the evening of Thursday, May 7, 2009, as a growing group of cumulus clouds produced light rain and small thunderstorms over eastern Colorado, Stephen Corfidi made a prediction that some of his colleagues thought went much too far.

“We issued a particularly severe thunderstorm watch, of a kind we rarely issue,” for all of Kansas, said Corfidi, then the head forecaster at the National Weather Service Storm Prediction Center. “I caught a lot of static for that, but I knew it was going to be a big event. There were just too many compounding factors.”

Even Corfidi could not predict the magnitude of the destruction he would see the following day, as those clouds traveled 1,000 miles to unleash an inland hurricane on Southern Illinois.

But he saw signs, some of them tens of thousands of feet above our heads, that the eastern Colorado storms could grow dangerously.

Above the dense, water-filled cumulonimbus clouds that rained on Colorado, Corfidi and his team detected changes in a jet stream — one of many rivers of air that form miles above the Midwest as the seasons change.

In the springtime, jet streams typically rush across the Midwest at high rates of speed, pulling warm air from the south towards cooler air in the north at every point along the way.

At points where there are particularly strong contrasts between warm and cool air, with great disparities in temperature or moisture content, storms are born. In other cases, existing storms are grabbed and carried along in a jet stream’s flow.

“Jet streams move constantly and our day-to-day weather is a manifestation of their presence and changes in them,” Corfidi explained. “In the springtime, the Midwest hosts a jet stream that wiggles from day to day and week to week, but whose mean position is right along I-70,” running through the middle of Kansas and Missouri, and the lower half of Illinois.

Pinwheeling along that jet stream, the storm began to self-accelerate. Falling rain created a bank of cool air at its leading edge. As the storm moved east through Kansas, that cool air rushed violently towards the ground, creating high-speed winds that pushed lower, warmer air upwards.

That warm air released new moisture into the clouds, producing new rain to fuel the storm’s cold-air motor.

“There’s a lot of factors that go into how a thunderstorm complex organizes, but the thing that separates them is the wind profiles, as you go up through the atmosphere,” explained Mike York, a meteorologist with the National Weather Service, based in Paducah. “This storm was very unusually long and unusually well organized, with strong downdrafts of cool air from the thunderstorms creating a mini-cold front to force more hot air upwards.”

By 11 p.m., western Kansas saw golfball-sized hail. By midnight, gusts up to 67 mph were recorded, according to a case report on the May 8 derecho, compiled by Stephen Corfidi and National Oceanic and Atmospheric Administration colleagues Mike Coniglio and Jack Kain.

Meanwhile, another lower level jet stream rapidly brought more warm moist air into central Kansas from the Gulf of Mexico.

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By 4 a.m., the bands of new storms produced by that unusually large inflow of warm, wet winds were swallowed into the eastward moving squall.

“A storm is nature trying to reach equilibrium. If it’s too warm at the surface and too cold aloft, it makes the air want to overturn,” Corfidi explained. “That low-level jet stream gave it lots more fuel to release.”

The storm continued to grow, beginning to take the wide, bowed shape that characterizes a derecho, what meteorologists call a “bow echo.”

Eighty mph winds hit Eureka and Buxton, Kansas, as the storm gathered force in eastern Kansas, in the early morning.

“A woman died in New Albany when high winds destroyed her mobile home. A semitrailer was overturned near Independence, and buildings in the area suffered major damage,” Corfidi and his colleagues report. “At the same time, the storms dropped up to four inches of rain in less than an hour.”

As the storm crossed Missouri it generated several spinning storm cells, some producing isolated tornadoes. At the northern end of the derecho’s bow shape, one area grew especially strong.

“It began to develop this circulation,” Corfidi said, as an area of low pressure pulled in warmer air from all sides. “You can almost see an eye on it, and it had a lot of spin to it. That’s what people have since called an inland hurricane.”

That ferocious, four-county-wide storm cell is what hit Southern Illinois, bringing sustained 70 to 90 mph winds that wreaked havoc including nearly 100 windows blown out of the residence halls at Southern Illinois University Carbondale, per the NOAA report.

The spinning vortex finally weakened upon reaching southwest Indiana, but its remnants produced a deadly tornado in Kirksville, Kentucky, and hail in eastern Kentucky and Tennessee.

Looking back, Corfidi sees an impressive confluence of “Goldilocks factors,” that allowed the May 8 storm to last so long, and become so intense.

Had temperatures been slightly warmer or cooler in weeks before, the jet stream might not have been in position to pick up the clouds created by ascending winds over the Rocky Mountains.

Had there been more moisture in the air in Colorado and Nebraska, the initial storm might have matured there, with instability culminating in Missouri and Kansas, before reaching Southern Illinois.

“We were starting with a clean slate, with no storms of consequence in Kansas, so there was nothing to upset its development,” Corfidi said.

The environment in its path was “pristine,” added his colleague Mike Coniglio, a researcher at the NOAA’s National Severe Storms lab, with plenty of warm, moist, unstable air, ready to fuel the storm.

“When storms become this big and intense for a long period of time it depends on many factors coming together just right,” Coniglio said. “That’s the difficulty of forecasting weather. Small changes somewhere can make a huge difference somewhere else.”

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