MADISON, Wis. — Tens of millions of people across the Midwest and East endured dangerously hot temperatures again on Sunday as a rare June heat wave that gripped much of the U.S. was expected to last into the coming week.

Most of the northeastern quadrant of the country from Minnesota to Maine was under some type of heat advisory. So were parts of Arkansas, Tennessee, Louisiana and Mississippi, the National Weather Service said.

The weather service office in Wakefield, Virginia, warned of sweltering conditions through Wednesday.

“Please plan ahead to take frequent breaks if you must be outside, stay hydrated and provide plenty of water and shade for any outdoor animals,” the office said on X.

Sunday marked the second consecutive day of extreme heat across the Midwest and East Coast. Heat indices on Saturday hit 103 degrees Fahrenheit in Chicago and 101 in Madison, Wisconsin, turning that city’s annual naked bike ride into a sticky and sweaty affair.

Meteorologists say a phenomenon known as a heat dome, a large area of high pressure in the upper atmosphere that traps heat and humidity, is responsible for the extreme temperatures.

‘I just want to sit in my air conditioning’

Lynn Watkins, 53, is the director of Sacred Hearts Day Care in Sun Prairie, a Madison suburb. She said that she tried to sit outside Saturday to grill but it was so hot she had to go inside. She plans to cancel all outdoor activities at the day care today with highs around 93 forecast.

“I can’t stand being outside when it’s like this,” she said. “I just want to sit in my air conditioning.”

The temperature reached 80 in the Chicago area by 7:30 a.m. Sunday. Forecasts called for heat indices of between 100 and 105.

The heat index in Pittsburgh was expected to top 105. The temperature in Columbus, Ohio, was 77 at 8:30 a.m. Highs there were expected to reach 97 with a heat index around 104.

Forecasts called for a heat index of 100 in Philadelphia on Sunday, with a 108 heat index today.

Philadelphia declares a heat emergency: The city’s public health department declared a heat emergency Sunday lasting until Wednesday evening. Officials directed residents to air-conditioned libraries, community centers and other locations, and set up a “heat line” staffed by medical professionals to discuss conditions and illnesses made worse by the heat. At Lincoln Financial Field, officials said each fan attending Sunday’s FIFA World Cup match would be allowed to bring in one 20-ounce plastic bottle of water.

With temperatures in the mid-80s, Maryland’s Rehoboth Beach was crowded Sunday.

“It’s only going to get worse,” said beachgoer Vak Kobiashvili. “People are trying to get out to the beach before it’s too hot to really even manage to be outside.”

Kobiashvili said even his dog didn’t want to be outside.

“East Coast weather, at least from my perspective, is just very sweaty in the summer,” he said. “It’s that walking through a swamp kind of feeling.”

Forecasters warned the heat index in Cromwell, Connecticut, would reach 105 on Sunday, which would make life brutal for golfers Tommy Fleetwood and Keegan Bradley as they compete during the final round of the Travelers Championship. The index on the TPC River Highlands course stood at 98 degrees at 3:18 p.m. Fans sought shelter under trees and on air-conditioned benches. Many lined up for water at a hydration station near the ninth green.

Karin Skalina of New York had been in the sun-soaked bleachers by the eighth green and eventually sought relief on a ventilated cooling bench. “Didn’t work,” she said.

“(We’re) trying to follow the shade,” Skalina said.

Courtney Kamansky of Newington, Connecticut, had been to the tournament before and came prepared with extra water bottles. Asked if she was able to find shade, she pointed to her umbrella and said, “I bring it with me.”