(CNN) -- First came the heat, then killer storms, followed by more summer sizzle.
Millions of people across nine states were reeling without power Saturday to deal with thermostat-popping temperatures after fierce thunderstorms pounded parts of the Midwest and Mid-Atlantic.
At least 10 people were killed and three states -- Virginia, West Virginia and Ohio -- declared states of emergency.
After the rains passed, governors cautioned people to stay cool as temperatures climbed again and to stay out of the way of emergency crews working to clear debris and restore power.
"Last night's thunderstorms caused the broadest non-hurricane related power outage in Virginia history," said Virginia Gov. Bob McDonnell.
"This is not a one-day situation; it is a multi-day challenge," he said.
Joseph Rigby, president of the electric company Pepco, said it could be a week before power is fully back up in some areas of Washington.
"Given the damage, you can understand this is going to take some time," he said. "The wild card is the weather."
The storms raced east Friday from Indiana through Ohio and into West Virginia and the nation's capital. There was a chance that more storms could move through the region Saturday night.
More than 3.7 million homes were without power Saturday, including nearly 1 million in Virginia alone and 26,000 as far west as the southern Chicago-Joliet area of Illinois.
Saturday morning, the storms' fury -- winds gusted as high as 80 miles per hour -- was visible with downed trees and debris littering roads.
Catherine Estelle Ford of Scottsville, Virginia, died when a tree fell on her after she stepped out of her car and tried to make a phone call, according to the Albemarle County Police Department.
Five others died in Virginia due to downed trees, according to the governor.
A 71-year-old woman in Montgomery County, Maryland, was also crushed by a tree that crashed onto her home, according to Lucille Baur, a public information officer for the county police.
In Washington, a man and his wife were electrocuted when they stepped outside to check on downed trees and power lines, said police spokesman Araz Alali. The man was killed and his wife was is in critical condition with burns. She is expected to survive, Alali said.
In New Jersey, two boys, ages 7 and 2, were killed in Parvin State Park in Salem County when a pine tree snapped and fell on their campsite, the New Jersey Department of Environmental Protection said in a news release.
The boys were cousins from Millville and when the storm began, their families had huddled together in a single tent that ended up being crushed by the tree.
The power outages and debris littering roads led to traffic disruptions and other headaches in several areas.
Amtrak service between Washington and Philadelphia was expected to be restored by some time Saturday after the storm downed trees and wires across tracks.
In the nation's capital, 19 intersections were without traffic lights at one point Saturday.
The storm's fury was felt online, as well, when digital clouds were knocked out by real ones. Power outages knocked out some Amazon Cloud services in Virginia, taking down sites that rely on them, including Netflix, Pinterest and Instagram.
"Pinterest.com is not currently available due to server outages related to storms on the east coast," the company said. "Unfortunately, several high-traffic sites are being affected. Thank you for your patience -- we'll have you pinning again as soon as possible!"
And they did. The site was up and running Saturday.
But in many ways, the aftermath of the storm was compounded by a forecast of another sweltering summer day.
One in three Americans were baking Saturday in an area of nearly 600,000 square miles experiencing unusually warm weather.
In storm-affected areas, many people had no electricity to run fans, air-conditioning and refrigerators.
Even in places where power was not disrupted, people with no air-conditioning were advised to spend the day in a library, a cooling center or some other such place to avoid heat exhaustion.
This was after temperatures tipped the 100-degree mark in several cities Friday, including St. Louis, Richmond, Nashville, Washington and Atlanta, the National Weather Service said.
In many places, however, it felt much hotter than the official thermostat reading.
"If you don't have a good pair of boots, it'll burn clear through to your feet," said roofer Zach Bruner in Evansville, Indiana, where he said the 103-degree temperatures were spiking to 130 on the job site.
The bad news? Relief is nowhere in sight as the extreme heat is expected to continue through the weekend and beyond.
Atlanta opened five cooling centers in anticipation of another day of triple-digit heat.