MADISON, Wis. >> Tens of millions of people across the Midwest and East endured dangerously hot temperatures again 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 NY

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 Sunday hit 103 F in Chicago and 101 F 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 Monday with highs around 93 F 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 in Columbus, Ohio.

Forecasts in Philadelphia called for a heat index of 108 F on Monday.

Heat emergency

Philadelphia’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.”