Jerome Quirion of Sherbrooke, Quebec, Canada, wades back to shore after venturing into the chilly Atlantic Ocean with his 18-month-old daughter, Amelie, while vacationing, in Old Orchard Beach on Tuesday. Robert F. Bukaty/Associated Press

TOLEDO, Ohio — A blistering heat wave Wednesday extended from the Midwest to New England, leaving millions of people sweltering through the Juneteenth holiday, including in places like northern Maine where they rarely experience such conditions this early in the year.

The city of Caribou, Maine, just 10 miles from the Canadian border, saw a record 103 degrees on the heat index, which combines heat and humidity. The region was under a heat advisory until Wednesday evening. Several residents said they were used to temperatures in the 70s and 80s in June and rarely this humid.

“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.

Even before noon on Wednesday, the heat index in Maine soared to 100 degrees in Portland and Waterville with 103 in Sanford and Fryeburg, according to News Center Maine meteorologist Jason Nappi. Thursday is forecast to be just as hot.

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.

Over at Moose River Campground, about 13 miles from the border, owner Lisa Hall was fixing a cabin faucet amid the sizzling temperatures and said such conditions are more common in mid July or early August.

“I am sweating like crazy and it’s way too hot,” she said.

The dangerous temperatures were expected to peak in the eastern Great Lakes and New England on Wednesday and Thursday, and in the Ohio Valley and Mid-Atlantic on Friday and Saturday, the National Weather Service said. Heat index readings were expected to reach 100 to 105 degrees in many locations.

“We are seeing a ridge of upper level high pressure, which is bringing all this heat from the southern United States,” said Kyle Pederson, meteorologist with the National Weather Service in Boston.

“That is just causing lots of hot temperature in the mid- to upper 90s and the heat index to reach over 100. It’s just going to make it feel warmer than it is outside,” he said. “You will really feel that humidity and feel the heat quicker.”

The conditions were expected to scale back some Juneteenth activities and limit options for relief. Cities that opened cooling centers this week advised that Wednesday’s Juneteenth holiday meant some public libraries, senior centers and pools where residents could beat the heat would be closed.

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.

People walk through the wading pool at Deering Oaks Park in Portland on Wednesday. Mainers looked for ways to deal with a heat wave on Wednesday which brought temperatures in the high 80s with high humidity. Gregory Rec/Staff Photographer

In New York, state parks have free admission Wednesday and 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 were closed until next week. The city has a list of hundreds of air conditioned sites that are free and open to the public. Public libraries, which have been used as cooling centers during other heat waves, were closed Wednesday because of the Juneteenth federal holiday.

Anne-Laure Bonhomme, a 43-year-old health coach, was visiting sites in New York with her family. “The humidity is pretty insane,” she said.

People and even zoo animals were forced to find ways to thwart the muggy weather.

An organization that provides produce to areas with limited access to fresh food in Columbus, Ohio, prepared frozen towels and packed cold water for their workers.

“Hydration is the key,” said Monique McCoy, market manager for the Local Matters Veggie Van.

High humidity lingered over Indianapolis Wednesday making the deceptively cloudy day warm and unenjoyable. Parks and walking trails were sparsely populated despite the holiday.

The Bennett family considered taking their 7-year-old and 4-month old to the pool or a splash pad on their day off of work, but decided it would still be too warm for the kids. Instead, Kayla and Sarah drove from Muncie to the Indiana State Museum in downtown Indianapolis to visit the indoor exhibits in the air conditioning.

A recent study found that climate change is making heat waves move more slowly and affect more people for a longer time. Last year, the U.S. saw the greatest number of heat waves – abnormally hot weather lasting more than two days – since 1936.

Chicago broke a 1957 temperature record Monday with a high of 97 degrees. A cold front was expected to bring relief to areas near Lake Michigan on Thursday and Friday, the National Weather Service in Chicago said.

In California, wildfires erupted east of San Francisco in the state’s historic Gold Country region and in the mountains of northern Los Angeles County after what had been a quiet start to fire season. Wildfires in southern New Mexico damaged 500 buildings Tuesday in a mountain village of 7,000 people that had been evacuated with little time to spare.

Meanwhile, a fresh batch of tropical moisture was bringing an increasing threat of heavy rain and flash flooding to the central Gulf Coast. Hurricane season this year is forecast to be among the most active in recent memory.

Anthony Izaguirre in Albany, New York, and Isabella Volmert in Indianapolis contributed to this story.