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. In central New Hampshire, Angela Wilcox took her two children and two nephews boating Thursday in search of the coldest swimming spot on Squam Lake, where they’ve stayed at the rustic Rockywold Deephaven Camps for 16 years.

“This is the hottest it’s ever been, especially in June,” Wilcox said. “We’re kind of shocked.”

Camp workers were delivering large blocks of ice to the cabins for refrigeration. A tradition at the camp for more than a century, the ice is harvested from the lake each winter and then packed with sawdust in an insulated storage hut. The method has kept fresh ice available throughout the summer and into the fall.

Further south in Concord, it was already close to 90 degrees 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.

In Burlington, Vermont, Jack Hurlbut said he’s never been so hot in his life.

“It sucks,” the 28-year-old said. “I live in Vermont for a reason, you know what I mean?”

Hurlbut, who is experiencing homelessness, was hanging out with some others on a church lawn outside of the city’s public library, which was serving as a cooling center. He’d slept nearby the night before and said he was trying to catch the shade without getting kicked out.

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’s beaches were available but its public swimming pools 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.

Many school districts in New Jersey switched to early dismissals as the school year winds down, and at least two rescheduled their high school graduation ceremonies due to concerns about excessive heat and humidity.

Some relief is on the way: A cold front is moving into areas near Lake Michigan on Thursday and Friday, the weather service said. Chicago broke a 1957 temperature record Monday with a high of 97 degrees (36.1 C).

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.

(AP)