Six-year-old James Charles Liptak and his dog Rumble keeps cool from the oppressive heat with a walk in the water steps by PNC Park on the North Shore area of Pittsburgh on Monday. Pittsburgh Post-Gazette via AP

TOLEDO, Ohio — Stifling heat blanketed tens of millions across United States on Tuesday, forcing people and even zoo animals to find ways to cool down as summer arrives in what promises to be a sweltering week.

Extreme heat alerts stretched from Iowa to Ohio and even into the upper reaches of Michigan on Tuesday, canceling youth sports camps, nature walks and festivals across the region. The National Weather Service said the dangerous heat wave was expected to make its way across the country and into Maine until at least Friday.

An organization that provides produce to areas with limited access to fresh foods in Columbus, Ohio, prepared frozen towels for their workers in case of overheating and packed cold water to stay hydrated.

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

In Toledo, Ohio, the city canceled a weekly fitness event and a neighboring suburb called off a street fair as temperatures reached the mid-90s. A food bank in upstate New York canceled deliveries for Wednesday out of concern for its staff and volunteers.

Schools in New York canceled field trips Tuesday to the Rosamond Gifford Zoo in Syracuse, where workers turned on water misters for visitors and the animals. Elephants and other animals were getting chunks of ice in their pools, said Ted Fox, the zoo's executive director.

“Most of the species love them,” Fox said. “Even the tigers love to lick the ice and put their heads on them when it's this warm.”

Cities that opened cooling centers this week advised that some public libraries, senior centers and pools where residents could beat the heat will be closed Wednesday because of the Juneteenth holiday.

The blast of extreme temperatures before the official start of summer came a little too early for many.

“This is hot for just moving in to summer, so I'm hoping that we're going to see the downward trend in the temperature here soon because this is a warm one,” said Krista Voltolini, who was selling produce at a farmer's market in Columbus.

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 most 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 Fahrenheit. Hot and muggy conditions will continue this week with peak heat indexes near 100 F at times, the National Weather Service in Chicago said in a post on the social platform X.

Much of the Midwest and Northeast were under heat warnings or watches, with officials urging 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, Gov. Kathy Hochul activated the National Guard to assist in any heat emergencies that develop over the next several days.

“This is a time of significant risk, and we're doing our best to make sure that all lives are protected,” Hochul said Tuesday.

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.

While much of the U.S. swelters, late-season snow was forecast for the northern Rockies, with parts of Montana and north-central Idaho under a winter storm warning into Tuesday. As much as 20 inches was predicted for higher elevations around Glacier National Park.

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.

John Cornack a volunteer with Frick Park Clay Courts Tennis Club (FPCCTC), revitalizes the red clay tennis courts in Frick Park in the Pittsburgh neighborhood of Regent Square, on Monday. Cornack is a retired brain injury specialist from Michigan who takes care of the community tennis courts through the overwhelming heat by taking precautions with proper head gear. Pittsburgh Post-Gazette via AP

Ethan Hickman uses a towel to wipe off sweat as he takes a break from unloading a stiflingly hot trailer of fireworks outside Powder Monkey Fireworks ahead of the opening of the stand Monday, June 17, 2024, in Weldon Spring, Mo. Associated Press

Workmen with the Architect of the Capitol office, perform maintenance on the irrigation system in a park near the Senate, at the Capitol in Washington, Tuesday, June 18, 2024. Extreme heat is expected to break records for tens of millions of people in the United States this week. Associated Press