The eastern U.S. on Monday was hammered by the fourth consecutive day of stifling heat after a weekend of violent storms that killed 15 people and knocked out power to millions.
Nearly 2 million people were still without power Monday, with the biggest concentration of outages in the Washington, D.C. area. Power companies warned it could take several days to restore electricity completely in some areas.
The U.S. government told federal workers in the Washington area they could take unscheduled leave or work from home on Monday and Tuesday.
Two of the largest property insurers, USAA and Nationwide, said they had received more than 12,000 claims in total from the weekend storms. Most were for house damage.
The storms capped a costly June for insurers, which were already facing losses of at least $1 billion from a hailstorm that ripped through Dallas.
"Hot and hotter will continue to be the story from the plains to the Atlantic Coast for the next few days," the National Weather Service said.
From June 25 to July 1, nearly 2,200 record temperatures were either broken or matched, the National Oceanic and Atmospheric Administration (NOAA) said Monday. For the 30 days of June, the number rose to more than 3,200.
Accuweather meteorologist Alex Sosnowski said the number of records broken was very unusual. He said that while some aspects of the heat wave are unknown, much of it is because of a lack of snow cover during the late winter on America's plains.
Instead of the sun's heat melting snow, it instead heated the ground, which in turn warmed the air. The increase in temperature even made crops grow ahead of schedule until now; Sosnowski said the lack of rainfall has stunted crops' growth.
Sosnowski added that while some areas are not unusually warm, namely New England and the Northwest, the center of the country will experience high temperatures for the next several weeks, possibly into August.
Five states had more than 100 record temperatures broken in June. Texas had 237 records broken, followed by Colorado (226), Kansas (164), Missouri (126), and Arkansas (115).
Monday brought another grim challenge when many embarked on a difficult commute over roads with darkened stoplights.Slideshow: Storms, extreme heat hit millions (on this page)
To alleviate congestion around Baltimore and Washington, federal and state officials gave many workers the option of staying home Monday. Federal agencies will be open in Washington, but non-emergency employees have the option of taking leave or working from home. Maryland's governor also gave state workers wide leeway for staying out of the office.
There were more than 500 signal outages in Maryland on Sunday afternoon, including more than 400 in hard-hit Montgomery County outside the nation's capital, according to the State Highway Administration. There were 100 signal outages in northern Virginia late Sunday afternoon, and 65 roads were closed, although most were secondary roads.
"If you have to drive or need to drive, leave yourself a lot of extra time," Maryland State Highway Administration spokesman Charlie Gischlar said. "There's going to be delays."
And it was set to remain unseasonably sweltering, with heat warnings continuing into Monday after hundreds of daily high temperature records were broken throughout the region over the weekend, the National Weather Service said.
Temperatures approached or exceeded 100 degrees in many storm-stricken areas, and utility officials said the power will likely be out for several more days.
The heat was set to linger over the East Coast for a few days at least, Weather.com reported.
"High pressure in the upper atmosphere will shift back to the central states, focusing the core of above-average temperatures over that region of the country eastward into mid-Atlantic,” it reported. "Minneapolis, Minn., St. Louis, Mo., Chicago, Ill. and Washington, D.C. are just a few of the cities that should prepare for a sultry week ahead."
Emergencies were declared in Maryland, Ohio, Virginia, West Virginia and Washington, D.C., on Saturday because of damage from storms that unleashed hurricane-force winds across and a 500-mile stretch of the mid-Atlantic region.
The storms' rampage came as sweltering temperatures topped 100 Fahrenheit in several southern cities, including Atlanta, where the mercury hit an all-time record of 106 degrees on Saturday and reached 105 on Sunday.
Over two dozen cities across 10 states set or tied all-time record high temperatures on Friday and Saturday, including Columbia, South Carolina; Knoxville, Tennessee; and Raleigh, North Carolina.
The Associated Press, Reuters, NBC News, msnbc.com staff and weather.com contributed to this story.