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

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.

The severe weather threat is a level 3 out of 5 for places like Indianapolis, Louisville and Fort Wayne, Indiana, on Wednesday. Clusters of storms should develop Wednesday afternoon, merging into a line of storms Wednesday evening.