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© Chase McNulty
Chicago, Illinois -- Schools and at least one courthouse in the Chicago area were closed Wednesday after violent summer thunderstorms knocked out power to thousands.

At Northwestern University in Evanston, Ill., 42 buildings were reported to be without electricity, the Chicago Tribune said. There was no word on when the lights would be back on.

In the north and west suburbs, crews armed with chainsaws worked to remove downed trees and branches from the streets. Delays were reported for suburban commuter trains and the city's mass transit system.

The storms moved through northern Illinois Tuesday night. Hundreds of flights were canceled at Chicago airports, subway passengers were stranded and at least 300,000 homes and businesses lost power. Commonwealth Edison said repairing the damage could take days.

The threat of severe weather shifted to the Ohio Valley Wednesday as a system carrying destructive winds, hail and isolated tornadoes crosses the region, AccuWeather.com said. A few thunderstorms could erupt over parts of the mid-Atlantic.

Officials at Chicago's O'Hare International and Midway airports said at least 330 flights were canceled and hundreds of flights delayed. Passengers were stranded on stopped commuter trains, and downed power lines and trees damaged homes, officials said.

Wind gusts as high as 81 mph were reported in the Chicago area.

"Winds that high can bring down tree lines, they can bring down power lines, they can tear roofs off buildings," said David Beachler, an NWS meteorologist. "It depends on how it's coming together; it depends on the structural integrity of buildings."

Joliet, Ill., was hit with 0.85 inch of rain in 16 minutes Tuesday evening, officials said.

Forecasters said the weather system moving in the Midwest and mid-Atlantic could bring potentially destructive storms from southern Michigan to New Jersey and southward into central Tennessee. AccuWeather.com meteorologists said Detroit, Pittsburgh, New York, Philadelphia, Cincinnati, Indianapolis, Louisville, and Nashville could experience dangerous weather.