Forecasters say hot weather could give way to violent storms across parts of Illinois and are warning residents to be prepared for strong winds, hail and even tornadoes.
The National Storm Prediction Center says a "large and organized cluster of storms" could pelt northern and central Illinois on Wednesday afternoon and evening.
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The National Weather Service in suburban Chicago says there could be baseball-sized hail and flash flooding. Meanwhile, the agency's office in Lincoln says it could be the hottest day of the year and predicted a triple-digit heat index. Quad Cities forecasters say winds could top 80 mph.
The storm system is expected to strike a region from Iowa to Maryland and could even spawn an unusual weather event with strong straight-line winds called a derecho.
For the suburban area specifically, the weather service is warning of thunderstorms likely today, with widespread severe weather a possibility.
"The main severe threat is going to be tomorrow in the afternoon and evening hours," said Ben Deubelbeiss, a meteorologist with the National Weather Service, referring to Wednesday. "Probably by mid-day we'll see some isolated strong storms developing with strong winds and large hail and potentially tornados."
The threat of tornadoes is higher for those south of Interstate 80 and in central Illinois, he said, where storms are expected to congeal in the afternoon.
The weather service earlier warned of heavy rainfall that could lead to localized flash flooding. And it said Lake Michigan could be a danger for small watercraft.
Derecho windstorms occur once every year or two across the central and northeastern U.S. in a band from Texas to New England. They pack hazardous winds of at least 75 mph or more and maintain their intensity for hours as they sweep across vast distances.
In some cases a derecho will spawn tornadoes and accompany storms that produce hail the size of golf balls.
The current pattern could affect larger metropolitan areas in Milwaukee, Chicago, Detroit, Cleveland and Pittsburgh in the next two days, said Bill Bunting, a meteorologist in the weather service's storm prediction center in Norman, Okla.
"We tend to be careful using the D word, but yes, a derecho is possible," Bunting said.
The weather service was predicting a chance of storm activity beginning in southern Montana and northeastern Wyoming on Tuesday afternoon. It was expected to sweep eastward, with a 30 percent chance of severe wind activity in a rectangle covering parts of South Dakota, Nebraska, Iowa and Minnesota.
The storms could generate straight-line wind gusts of 70 mph or more. That's enough to rip shingles off a roof, knock down trees and even tip over semitrailers trucks. They could also cause flights to be delayed or canceled, said Collar, who added that commercial airlines have onboard navigation that allows pilots to navigate around the worst weather.
The weather agency also is predicting further storm activity today. Thunderstorms, powerful wind gusts and possible tornadoes could hit parts of Wisconsin, Illinois, Indiana, Michigan, Ohio and Pennsylvania.
"It's scary because of the potential, but we don't want to over-forecast," Bunting said. He said residents in affected states should remain aware that severe weather is possible, and pay close attention for any weather warnings.
Campers or hikers in forested areas should be cautious about the potential of falling trees and boaters shouldn't venture too far from shore, the weather service said.