A massive heat wave that has enveloped the US Midwest pushed into the Northeast on Friday, ushering in temperatures that could top 100 degrees Fahrenheit (38 degrees C) in Washington and prompting utilities to take steps to prevent power outages.

The huge blob of warm air is likely to blanket the region, home to a third of the US population, through Sunday with little overnight relief, said meteorologist David Roth of the National Weather Service's Weather Prediction Center.

"There are 124 million people under a heat advisory or excessive heat warning - that's a third of the population," Roth said.

As of Friday, the heat wave sprawled from Kansas to the Atlantic Coast, and from South Carolina north to Maine. It is expected to intensify today and tomorrow.

Utilities in the eastern half of the United States expect to have enough resources to meet power demand on Friday but asked consumers to turn down air conditioners to avoid putting stress on the system, which could cause outages.

"I'm very confident," Consolidated Edison Inc President Tim Cawley said when asked at a news conference if the utility, which serves New York City, could quickly respond to any outages in the country's most populous city. He said 4,000 employees were poised to work 12-hour shifts over the weekend.

On Saturday, parts of Manhattan lost power for hours, darkening Broadway theaters, halting subways and closing restaurants and shops in a partial blackout blamed on a faulty piece of equipment.

Early on Friday, as the heat intensified in downtown Madison, Wisconsin, 11,600 homes and businesses lost power after fires erupted at two substations near the state capitol. With temperatures forecast to hit 93F (34C) later in the day, Governor Tony Evers asked nonessential state workers to stay home. By midafternoon, power had been restored to all but about 3,500 customers, according to Madison Gas and Electric .

On the East Coast, temperatures on Friday were forecast to reach near 100F (38C) in Washington, 97F (36C) in Philadelphia and 91F (33C) in New York, where it would feel more like 110F (43C) with high humidity, Roth said.

On Saturday, the forecast called for 99F (37C) in New York and Philadelphia and 100F (38C) in Washington. Much the same was in the forecast for Sunday.

"It's crazy," said Fonik Bitaly, 28, who was entering his sixth hour of work dressed as a costumed character, Batman, in New York City's Times Square. "It's really, really hard to be outside right now. You need to drink lots of water or you'll be in serious trouble."

The dangers posed by extreme heat and humidity prompted officials to scrap outdoor competitions, including Saturday's horse races at Saratoga Race Course in upstate New York and Sunday's New York City Triathlon.

"As soon as you get outside, it's like 'Boom!' said Loig Loury, 32, who moved to New York from Paris last year. "The heat attacks you."

To keep cool during past heat waves, suburban children typically ran under lawn sprinklers and city kids frolicked in the spray of fire hydrants, but the New York City Fire Department warned that special spray caps that firehouses hand out should be used to avoid creating a hazard.

"If you open a fire hydrant without these caps, you endanger your neighbors because the water pressure drops and our firefighters are not able to fight fires," FDNY Commissioner Daniel Nigro wrote on social media.