The area at risk for tornadoes on Wednesday includes Chicago, but the threat is most likely away from the city and east of Lake Michigan, Mr. Lyons said. St. Louis, which experienced a deadly tornado outbreak last month, is also under some risk, but the conditions are expected to be different and far less severe than what they were then, he said.

A cold front will team up with warm, humid air as it tracks eastward, setting the stage for intense storms to spread from the Plains on Tuesday to the East Coast by Thursday.