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

Weather service offices throughout the region warned of sweltering and sometimes life-threatening 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 service office in Wakefield, Virginia, said on X.

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.

Thunderstorms slam New York State

Twin 6-year-old girls were among three people killed when thunderstorms brought trees down onto homes in central New York before dawn Sunday, according to the Oneida County Sheriff’s Office. Several inches of rain fell over just a few hours, inundating the small town of Kirkland.

A neighbor, Jared Bowman, said he ran next door to help the twins' mother escape through a window after a giant maple crashed through the roof around 4 a.m.

“She was yelling, ‘Get my kids out!’” Bowman told the Post-Standard.

A 50-year-old woman died when a tree hit a house nearby, sheriff's officials said. The streets were littered with electrical lines and thousands in the region were without power.

Gov. Kathy Hochul declared a state of emergency in 32 counties due to the strong storms and forecast extreme heat.

‘I just want to sit in my air conditioning’

Sunday marked the second straight day of extreme heat across the Midwest and East Coast. Heat indexes on Sunday hit 103 F (39.4 C) in Chicago and 101 F (38.3 C) on Saturday in Madison, Wisconsin, turning that city’s annual naked bike ride into a sticky and sweaty affair.

Lynn Watkins, 53, director of Sacred Hearts Day Care in Sun Prairie, a Madison suburb, said that she tried to sit outside to grill but it was so hot she had to go inside. She plans to cancel all outdoor activities at the day care on Monday with highs around 93 F (33.8 C) forecast.

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

The heat index in Pittsburgh reached 105 F, and hovered around 104 F (40 C) in Columbus, Ohio.

Forecasts in Philadelphia called for a heat index of 108 F (42.2 C) on Monday.

Philadelphia declares a heat emergency

The city’s public health department declared a heat emergency through 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.

With temperatures in the mid-80s, Delaware'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 F on Sunday, making for a steamy day during the final round of the PGA Travelers Championship. 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, 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.”

Heat to persist into the coming week with highest temperatures shifting eastward

Cincinnati Reds shortstop Elly De La Cruz got sick Saturday while playing in extreme heat against the Cardinals in St. Louis. Seattle Mariners reliever Trent Thornton, facing the Cubs in Chicago, also fell ill.

The heat is expected to persist this week with the highest temperatures shifting eastward. New York City is expected to see highs around 95 F (35 C) on Monday and Tuesday. Boston is on track for highs approaching 100 F (37.7 C) on Tuesday, and temperatures in Washington, D.C., were expected to hit 100 F on Tuesday and Wednesday.

Mark Gehring, a National Weather Service meteorologist in Sullivan, Wisconsin, said this level of heat is not uncommon during the summer months in the U.S., although it usually takes hold in mid-July or early August. The most unusual facet of this heat wave is the sheer amount of territory sweltering under it, he said.

“It's basically everywhere east of the Rockies,” he said, referring to the Rocky Mountains. “That is unusual, to have this massive area of high dewpoints and heat.”

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This story has been updated to correct that Rehoboth Beach is in Delaware, not Maryland.

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Associated Press journalists Christopher Weber in Los Angeles; Holly Ramer in Concord, New Hampshire; Mingson Lau in Rehoboth Beach, Delaware; and Jimmy Golen in Cromwell, Connecticut, contributed to this report.