CONCORD, N.H. (AP) — A heat wave extending from the Midwest to New England moved closer to a breaking point Thursday, with millions of people sweating it out for another day.

The National Weather Service said the heat wave was expected to peak in the eastern Great Lakes and New England on Thursday, and in the Ohio Valley and Mid-Atlantic on Friday or the weekend.

Heat index readings, which combine temperature and humidity, were expected to surpass 100 degrees (37.7 C) in many locations, possibly setting some all-time records, the weather service said. Officials warned that record overnight temperatures would prevent natural cooling and allow the heat danger to build up indoors.

In a study published Thursday, a group of international scientists said human-caused climate change has dialed up the heat and drastically increased the odds of experiencing the killer heat that’s been baking the Southwestern United States, Mexico and Central America this month. Last year, the U.S. saw the greatest number of heat waves — abnormally hot weather lasting more than two days — since 1936.

It’s made for an unusually early start to extreme summer heat in northern New England this year. The city of Caribou, Maine, just 10 miles from the Canadian border, saw a record 103 degrees (39.4 C) on the heat index Wednesday, warmer than Miami.

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“I’ve seen this maybe one time before where it’s been this hot in June,” said Hannah Embelton, 22, a server at an ice cream store in Caribou, adding that customers were staying away from the soft serve options because they melt too quickly.

“We usually never get the brunt of all this heat and humidity because we are so north. Just how hot it is, that is all everyone is talking about,” she added.

It was already close to 90 degrees in Concord, New Hampshire, Thursday morning as John Dupont opened his kettle corn stand at the city’s 50th annual Market Days Festival. He and his daughter set up two fans and were prepared to drape icy towels around their necks as the temperature climbed.

“This year is a little challenging because of all the heat. Our kettle gets up to 150,000 BTUs,” he said.

Some options for relief were limited by Juneteenth activities: Cities that opened cooling centers advised that some public libraries, senior centers and pools where residents could beat the heat were closed for Wednesday’s holiday.

In Indianapolis, a daytime homeless service center picked up slack from closed cooling centers and food pantries. Horizon House served about 200 people a day this week, but the number jumped to 300 for lunch during its Juneteenth festival. It also gave out roughly 450 bottles of water.

“Today was downright swampy, ”said Courtney Kay Meyers, director of development and communications. “We’re really struggling to keep up with water and demand for water.”

Officials have urged people to limit outdoor activities when possible and to check in with family members and neighbors who may be vulnerable to the heat.

In New York, state parks have free admission Thursday, and select state-run pools and beaches opened early for swimming, Gov. Kathy Hochul said. New York City beaches were open but public swimming pools there don’t open until next week. The city has a list of hundreds of air conditioned sites that are free and open to the public.

“The humidity is pretty insane,” said Anne-Laure Bonhomme, a health coach who was sightseeing in New York with her family.

Chicago broke a 1957 temperature record Monday with a high of 97 degrees (36.1 C). A cold front was expected to bring relief to areas near Lake Michigan on Thursday and Friday, the weather service said.

Ocean waters are warmer as well, particularly in the Gulf of Mexico, where the season’s first named storm, Alberto, was dumping heavy rain and flash flooding along a stretch of the coast from Mexico to Louisiana. Hurricane season this year is forecast to be among the most active in recent memory.

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