Relentless heat was forecast for much of the eastern United States for a fourth straight day on Monday, with about 2.2 million customers without power after violent storms and soaring temperatures killed at least 15 people.
Power companies warned it could take several days to restore electricity completely in some areas as much of the United States sweltered in a record-breaking heat wave.
"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.
Emergencies were declared in Maryland, Ohio, Virginia, West Virginia and Washington because of damage from a rare "super derecho" storm packing hurricane-force winds across a 700-mile (1,100 kilometre) stretch from the Midwest to the Atlantic Ocean.
About 2.2 million homes and businesses from Illinois to New Jersey were still without power Monday morning, with the biggest concentration of outages in the Washington area.
With power lines down across the region, the U.S. government told federal workers in the Washington area they could take unscheduled leave or work from home.
Many schools and local governments also canceled programs or were closed because of outages.
The storms came amid a record-setting heat wave that has seen temperatures top 100 Fahrenheit (38 Celsius) in several southern cities. In Atlanta, the mercury hit an all-time record of 106 degrees (41 C) on Saturday and reached 105 on Sunday.
From St. Louis to Washington, temperatures were forecast to hit more records on Monday.
Excessive heat warnings and advisories continued on Monday over much of the mid-Mississippi Valley and southern states.
Strong to severe thunderstorms were possible across the southern mid-Atlantic region and north-central United States, the National Weather Service said.
DAMAGE TO POWER GRIDS
Thunderstorms and high winds battered eastern North Carolina on Sunday afternoon, causing three more deaths on top of at least 12 caused by the deadly storms and heat in several states on Saturday.
Powerful storms that brought wind gusts of up to 90 mph on Sunday knocked out power to more than 200,000 Commonwealth Edison customers in northeastern Illinois. About 100,000 remained without power on Monday, the utility said.
Utilities in Ohio, Virginia and Maryland described damage to their power grids as catastrophic.
FirstEnergy utilities in states from Ohio to New Jersey had about 314,000 customers without power. The company said it expected to restore electricity to its Maryland, Ohio and Pennsylvania customers by Tuesday and Wednesday, but it could be late in the week before power was restored to all its customers in West Virginia.