As temperatures again reached triple digits Saturday, millions of people in the Mid-Atlantic and Midwest were without power after violent storms toppled trees, cut power lines and killed at least 13 people, 6 of them in Virginia.
Ohio also saw up to 1 million homes and businesses without power due to overnight storms, and at least one person died there.
Two cousins, ages 2 and 7, were killed by a falling tree at a campsite in New Jersey's Parvin State Park. Two people were also killed in Maryland, one in Kentucky and one in Washington.
Five other deaths in recent days are thought to have been tied to the heat wave hanging over much of the nation.
100-degree plus temperatures were expected in areas across 25 states, a heat scenario impacting 47 million people, the Weather Channel's Julie Martin said on NBC's TODAY show.
By early Saturday afternoon, cities across the Southeast -- from Tennessee to the Carolinas -- saw triple digits, while Washington, D.C. was in the 90s and Baltimore at 100.
Atlanta saw 106 degrees, breaking its all-time record of 105 degrees, set in 1980.
The storms cut power more than 2 million homes and businesses across the Mid-Atlantic area -- including 1.5 million in the Washington, D.C., area, NBCWashington.com reported.
"We have more than half our system down," said Myra Oppel, a spokeswoman for Pepco, a utility serving the D.C. area that had 400,000 customers without power after 80 mph gusts knocked down trees and power lines.
"This is definitely going to be a multi-day outage," Oppel added -- not good news for those relying on air-conditioning to deal with the muggy, triple-digit temperatures this weekend.
Repairing damage "is a monumental task," added Ed McDonough, a spokesman for the Maryland Department of Emergency Management. "This is something that is going to take days, not hours."
Water restrictions were ordered in Montgomery and Prince George's counties because the storms had knocked out power to its filtration plants and other facilities.
And the high heat prompted the AT&T National golf tournament at the Congressional Country Club in Bethesda, Md., to close the competition to spectators and volunteers on Saturday. Play was delayed for hours as crews cleaned up fallen trees.
In suburban Washington, residents were told to call non-emergency phone numbers or go to fire and police stations if they needed help because even 911 emergency call centers were without electricity.
Trees were blamed for two deaths in Springfield, Va. — a 90-year-old woman in her home and a man driving a car.
Gov. Bob McDonnell in a statement said four others were killed by falling trees in Virginia, which saw its largest non-hurricane power outage in history. Some 250 roads in Virginia were blocked by trees, the state said.
In addition, a park police officer was injured by an uprooted tree in northern Virginia, and an 18-year-old man was struck by a power line. He was in stable condition after receiving CPR.
Damage in the D.C. area included a rooftop blown off a 7-story apartment building, and dozens of damaged homes.
Widespread power outages were reported from Indiana to New Jersey.
On Friday, the nation's capital reached 104 degrees — its hottest June day on record.
The heat is also suspected to have been the cause of the deaths of two young brothers in eastern Tennessee, Reuters reported. The boys, aged 3 and 5, had been playing outside Thursday. The younger boy died Thursday, and the older boy on Friday afternoon, according to Eric Blach, administrator for the Bradley County Medical Examiner's Office.Video: Storm Watch
In Kansas City, Mo., city health officials said Friday they were investigating the deaths of three area residents, including a baby boy, to determine if they were heat-related, according to Reuters.
Early Saturday, the National Weather Service issued excessive heat warnings for parts of Maryland, Delaware, Pennsylvania, Washington, D.C., Virginia, North and South Carolina, Georgia, Indiana, Ohio, Illinois, Missouri, Kansas, Kentucky, Alabama, Tennessee and Arizona.
It said the temperatures could get "dangerously hot."Video: Drought hits cattle ranchers' wallets (on this page)
Weather.com published a map of the U.S. showing areas at risk of severe thunderstorms Saturday, with an area from Iowa to the Mid-Atlantic at risk.
"A powerful derecho developed Friday over Indiana and marched eastward across Ohio, the Virginias and into Maryland and Delaware, causing widespread wind damage with gusts over 90 miles per hour in some cases," it added.
"Another round of widespread damaging winds may materialize in these hard-hit areas," it warned. "A second area of severe weather is possible for the central and northern Plains, but those storms may be more isolated in nature."Slideshow: Summertime living (on this page)
West Virginia Gov. Earl Ray Tomblin declared a state of emergency after more than 500,000 customers in 27 counties were left without electricity because of the storm.
More than 20 elderly residents at an apartment home in Indianapolis were displaced when the facility lost power due to a downed tree.
In Ohio, the storms damaged property and toppled three tractor trailers on Interstate 75 near Findlay.
"I'm in Columbus, Ohio," Brittney Mettke posted on msnbc.com's Facebook page. "I haven't had power since 6 pm yesterday and it's about 100 degrees now. It's pretty tough with a toddler. More storms tonight, and temps in the high nineties all week. They are saying about to week to restore power."
"We were hit with 80+mph winds and rain last evening," added Suzanne Peterson Helt, who said she was from Ohio, too. "They say there weren't any toronados near us but when the rain travels horizontally down the street and the roof is missing from the local Subway ... I have to wonder. So many fallen trees at power poles that the State Route I live on was closed for hours while they cleaned up the mess."
The Associated Press and Reuters contributed to this report.