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Much of Iowa and big chunks of the Midwest were under an through at least Tuesday. Temperatures were forecast to reach the 90s, with heat index values, or “feels-like� temperatures, expected to top 100 degrees (37.8 degrees Celsius) in the region, Ansorge said.

Visiting Des Moines with family, Rachel Washburn searched for things to do with kids during a heat wave. They landed at a water sprayground before lunch, where her children played tag in the cool water to escape a heat index forecast to reach 109 degrees Fahrenheit (43 degrees Celsius).

“My kids were quite shocked at the heat and humidity,� said Washburn of her seven children ages 18 months to 17 who are used to more temperate weather in Bemidji, Minnesota. “We were hoping for some good weather, but we'll make do.�

Some of the worst conditions are expected by Thursday and Friday as the heat moves through the Ohio Valley, the Mid-Atlantic and into the Northeast, with the potential for some record-high temperatures, said Scott Kleebauer, a meteorologist with the Weather Prediction Center in College Park, Maryland.

On Monday morning, Tom and Cindy Youngblood walked around an outdoor sculpture park in Des Moines, enjoying brief moments of shade and a helpful breeze. The couple, both 67 and from Rogers, Arkansas, returned from a camping trip in Wisconsin and chose a hotel over their camper van Sunday.

“We did not want to camp last night because we knew it would be too hot,� Cindy Youngblood said.

Kleebauer said one of the center’s biggest recommendations is to stay hydrated and have access shady areas and air conditioning.

“It just so happens to be coinciding with a time frame where a lot of people are away and a lot of people are going away for vacation� during the Fourth of July holiday week, he said.

Extreme heat has also , where temperature records were set and many heat-related deaths were reported in France.

People can be caught off guard by the first heat wave of the year, said Dr. Roy Elrod, chief of staff at DMC Detroit Receiving Hospital.

“You’re happy winter’s gone, you’re ready to enjoy the summer, you’ve just been aching for it,� Elrod said. “And so, I think we slip into kind of a position where we think it’s got to be OK.�

But heat-related injuries can happen in a matter of minutes, especially to those who don’t prepare for the weather by hydrating, wearing light clothing, avoiding the hottest times of the day and minimizing exposure to the sun, he said.

“We’re just not always prepared for it and it just takes an incident that rattles you and shakes you up that you understand that it can get serious very quick,� he said.

In the Midwest, some camp programs on Monday rearranged their schedules. Other outdoor activities, like a farmers market in Michigan and a drive-in theater in Minnesota, were canceled on Monday because of the heat.

In Flint, Michigan, the city activated four cooling centers through Wednesday, with the potential to extend operations if the heat persists.

The University of Wisconsin-Madison said it was closing 23 buildings to the public starting Tuesday, allowing only limited access to 11 others. It was relocating some summer classes after a broken water line at its cooling plant earlier this month severely reduced the ability to provide air conditioning across campus.

Temperatures approaching 90 degrees and high humidity didn’t stop Toni Kreutzer, 28, from taking a walk along the shores of Lake Mendota in Madison, Wisconsin, with her 13-year-old dog Chester.

“I like it hot,� Kreutzer said. "I just don’t like the humidity."

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McCormack reported from Concord, New Hampshire. Associated Press reporters Scott Bauer in Madison, Wisconsin, and Haya Panjwani in Washington, D.C., contributed.