CHICAGO -- An oppressive and potentially deadly summertime mix of sizzling temperatures and high humidity baked a large swath of the United States Sunday, pushing heat indexes in dozens of cities to dangerous levels.
Forecasters warned the heat wave will persist through much of the week and cautioned residents in more than three dozen states to take extra precautions.
Heat advisories and warnings were in place in 17 states from Texas to Michigan as temperatures and humidity combined to make it uncomfortable to be outside.
The National Weather Service posted excessive-heat warnings for much of the nation's midsection, including Illinois, Wisconsin, Minnesota, Iowa, Missouri, Kansas, Nebraska, Oklahoma, and North Dakota, where forecasters predicted heat indexes could hit 115 degrees, as well as for the South.
"This will likely be the most significant heat wave the region has experienced in at least the last five years," the weather service said.
Kristina Pydynowski, a senior meteorologist at AccuWeather, said the heat wave is likely to affect more than 40 states.
All of the states will experience temperatures of 90 degrees or higher, Ms. Pydynowski said, and "a large number of them will bake above 100 degrees for days on end."
One National Weather Service forecaster called the heat wave unrelenting and said relief should not be expected soon: A so-called "heat dome" over the region isn't moving much.
In Oklahoma City, forecasters expected Sunday to be the 27th day this year with the temperature at 100 or above.
The city is on pace to break its record for such days -- 50 set in 1980 -- with triple-digit heat possible through September.
It's even worse in western Oklahoma, where temperatures at 110 or above have been common in recent weeks.
In Enid, asphalt at a major intersection along U.S. Highway 412 buckled Saturday night from the heat.
In Chicago, city officials said a half-dozen cooling centers would remain open this week, as temperatures as high as 105 were forecast in Illinois.
Cooling centers were open in Detroit to help residents who don't have air conditioning at home.
Others were heading toward water for relief, including 65-year-old welder Marcellus Washington, who wore a floppy cloth fishing hat and sunglasses as he walked through Chene Park on the Detroit River that marks the border with Canada.
"A day like this, you can't beat it," Mr. Washington said. "It's a heavenly day. It's God's weather."
Others who had to be outside took precautions.
Capt. Dan Murphy of the North Dakota National Guard said several hundred soldiers deployed for flood-fighting efforts in the Dakotas were required to take mandatory rest breaks in the shade.
"It's hot in those vests and uniforms," Captain Murphy said. "These are soldiers. They can't just strip down to T-shirts and shorts."
Officials at the Cornhusker State Games, an amateur sports festival in Nebraska, had crews bring extra water and ice for participants.
"It takes a physical toll on anyone out there," said Dave Minarik, the event's executive director.
Police said heat may have played a role in the death of a 55-year-old man at a camp for the homeless in Springfield, Mo., on Saturday.
Police found him in a small tent after others at the camp raised alarm.
An autopsy is scheduled for Monday.
While the heat wave is currently focused on the High Plains and Mississippi Valley, it is expected to press east by the middle of the week, the weather service said.
By midweek many locations on the East Coast will have heat index values approaching or exceeding 100 degrees, including Washington, the weather service said.