Sloppy Storm, Possible 'Derecho,' Threatens Millions

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A large, sloppy storm system brewing in the Midwest on Wednesday threatens to spawn thunderstorms with heavy rain, hail and even the chance for a rare, powerful derecho.

"We've got a couple of rough days from the Midwest over to the Mid-Atlantic area," said Tom Moore, meteorologist of The Weather Channel.

A derecho that struck last June caused 13 deaths and did $1 billion in damage. NBC's Mike Viqueira reports.

A derecho, a massive line of storms that spans 240 miles and brings straight-line winds of at least 58 mph, usually only hits the Midwest once a year. The phenomenon is difficult to forecast.

"It's like predicting a large tornado is going to happen," said MSNBC Meteorologist Bill Karins. "No one can do that. The only thing we can do is say conditions are favorable for one to happen.

"It's about as common as a really powerful tornado system, which we usually see about four or five times a year," he said.

Last June, a derecho with winds gusting close to 100 mph stretching from Illinois to Washington, D.C., killed 13 people, caused $1 billion in damage and left more than 4 million people without power, according to The Associated Press. An additional 47 people died as a result of downed trees and the ensuing heat wave that followed the massive storm.

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This storm is shaping up to be much different than last year's derecho, Moore said. "I don't know if this situation is going to match that because it's a sloppier system," he said.

Moore said it's still unclear whether this system will develop into a derecho, but he said the storm system will bring widespread wind damage and severe weather -- with tornadoes and localized flooding possible for some areas.

In eastern Iowa, scattered thunderstorms are already under way with another storm cluster developing in Minnesota.

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"The morning won't be too severe," Moore said. "In the afternoon hours, it becomes much more volatile, especially in northern Illinois into north-central Indiana."

Later tonight, the system will move through Ohio and cross the Appalachian mountains.

Karins said if a derecho occurs, it likely will form later Wednesday evening at about 9 p.m. and last until the early morning hours of Thursday

—By Sophia Rosenbaum, NBC News Staff Writer