Tens of millions of people across the US Midwest and East have endured dangerously hot temperatures again, with a sprawling June heatwave gripping much of the US 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.

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.

Flooding in Canastota, New York, from a severe summer storm. – AP

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 neighbour, 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 4am.

“She was yelling, ‘Get my kids out!’” Bowman said.

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.

Governor 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 103F (39.4C) in Chicago and 101F (38.3C) on Saturday in Madison, Wisconsin.

Children run through water sprayed by a fire truck to cool off in St Louis. – AP

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 93F (33.8C) 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 105F, and hovered around 104F (40C) in Columbus, Ohio.

Forecasts in Philadelphia called for a heat index of 108F (42.2C) 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 centres 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 105F 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.

Seattle Mariner Trent Thornton is helped off the field after suffering from heat exhaustion in Chicago. – AP

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 95F (35C) on Monday and Tuesday. Boston is on track for highs approaching 100F (37.7C) on Tuesday, and temperatures in Washington DC, were expected to hit 100F 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 US, 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.”