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.

Dangerous storms and extreme heat will affect millions this week from the Midwest to the Northeast, with flooding and high temperatures forecast.