The intense early-summer weather has baked areas from Missouri to New York to Georgia with record-breaking heat and unleashed fierce storms that knocked out power over the weekend. At least 16 people were killed from the series of storms.
Extreme heat is the leading cause of weather-related deaths in the United States.
While the worst of the heat has passed for many, cooler temperatures Monday will still reach as high as 99 degrees for Louisville, Kentucky, and 96 degrees in Columbus, Ohio.
But more triple-digit heat is expected to smother areas from Missouri to South Carolina on Monday, and at least 16 states are under heat advisories or warnings.
"Hot and hotter will continue to be the story from the Plains to the Atlantic Coast the next few days," the National Weather Service said Monday. "The widespread excessive heat warnings and heat advisories have certainly decreased in coverage, but temperatures will remain well above average across a large portion of the U.S."
In addition, severe thunderstorms will be possible across the southern Mid-Atlantic region and north-central United States, the weather service said.
Federal agencies in the Washington area will remain open Monday, but non-emergency employees have the option to work remotely.
Cities and towns in the Southeast and Midwest have already endured temperatures of more than 100 degrees for days.
Nearly 2 million customers from Indiana through Maryland were dealing without power early Monday after a massive storm late Friday and Saturday that was fueled, in part, by the extreme heat.
Each household is a customer -- so the number of people impacted is well in the millions.
While down from a peak of about 4 million customers without electricity, the number of storm-related outages was still dangerously high given that people were trying to get by without working air conditioning or ice.
"It's the combination of heat and power outages that are hitting people," said Tamara McBride of the Ohio Emergency Management Agency. She predicted some may not get electricity until "well into the week."
Another bout of storms Sunday afternoon brought some relief to the heat but danger as well.
The National Weather Service reported that a man in Calico, North Carolina, was killed when a shed fell on him as he was trying to store his golf cart.
A few minutes later, in a nearby town, storm-related winds knocked over a tree that fell on a couple in a golf cart, killing them both.
More deaths tied to the weekend storms were reported from Ohio to New Jersey.
Virginia had the highest death toll, at seven, with the state Department of Emergency Management noting the deaths were spread across four counties.
And in New Jersey, 2-year-old and 7-year-old cousins died after a fallen tree crashed on the tent they were huddling inside in Parvin State Park, said Larry Ragonese, a spokesman for the state Department of Environmental Protection.
The stubborn, oppressive heat wave can cause serious health problems, with the effects of heat cumulative. The longer you experience it, the more likely you are to have conditions such as heat stroke and dehydration.
"Current indications are that this heat wave may continue for much of the upcoming work week," the National Weather Service's St. Louis bureau warned. "Do not wait until the heat wave has lasted for several days. Take action today to protect your health and continue to do so."
Between June 24 and Saturday 1,928 record-high temperatures were broken or tied nationwide. That number doesn't include new ones expected from Sunday that aren't in the National Climatic Data Center's official count.
But some tried to make the best of the roasting temperatures outside.
Holly Coons of Nashville decided to bake cookies on the dashboard of her car when it was 107 degrees outside.
"I actually burned my hand when I grabbed the cookie sheet out of the car," she said. But the cookies were done.
CNN's Rob Marciano, Jareen Imam, Josh Levs, Greg Botelho and Maggie Schneider contributed to this report.
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