The National Weather Service said 14 states were under some form of heat warning with triple-digit highs likely over much of Dixie.
Highs in the metro Washington area were expected to be in the 90s Sunday, which was cooler than recent days but scant relief for thousands of people whose power remained out following Friday's wild weather, which left a dozen people dead nationwide.
Officials at Pepco, the utility that serves the capital, said it could be a week before all electricity is restored. Meanwhile, residents coped as best they could with the lack of power, refrigeration and air-conditioning.
"It's awful," Carolyn Stewart, 45, Landover, Md., told The Washington Post. "There's no light in my building, it's hot, sticky."
Another woman told the Post she figured she should fill up her gas tank but ran out of fuel before she could find a filling station that was operational.
CNN said as many as 4 million people from Ohio to Virginia had their power interrupted by Friday's fast-moving thunderstorms.
The Ohio National Guard was called up to check on disabled and elderly people stuck in darkened homes. People across the affected region were urged to seek out air-conditioned public cooling centers rather than try to ride it out.