CHICAGO — An oppressive and potentially deadly summertime mix of sizzling temperatures and high humidity baked a large swath of the country again on Sunday, pushing afternoon heat indexes in dozens of cities to dangerous levels.
Forecasters warned that the heat wave would persist through much of the coming week and cautioned residents in more than three dozen states to take extra precautions.
The National Weather Service posted excessive heat warnings for much of the country's midsection, including Illinois, Wisconsin, Minnesota, Iowa, Missouri, Kansas, Nebraska, and Oklahoma, as well as South and North Dakota, where forecasters predicted heat indexes could hit 115 degrees.
"This will likely be the most significant heat wave the region has experienced in at least the last five years," the weather service said.
Cities especially hard hit by the heat included Rapid City, S.D.; Springfield, Ill.; and Minneapolis, where AccuWeather.com meteorologists were predicting that long-standing high-temperature records would fall this week.
Kristina Pydynowski, a senior meteorologist at AccuWeather, predicted that the heat wave will affect more than 40 states.
All the states will see temperatures of 90 degrees Fahrenheit or higher, she said, and "a large number of them will bake above 100 degrees for days on end."
The scorching weather is the latest in a series of meteorological problems to best the Midwest in recent months.
The list includes the devastating tornado that ripped through Joplin, Mo., in late May, killing nearly 160 people and destroying more than 8,000 homes and other structures, as well as the ongoing flooding along the Missouri River, which has triggered weeks of evacuations and other emergency measures from Montana through Missouri.
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.
In Chicago, where high heat and humidity warnings were twinned with an alert for poor air quality, temperatures were expected to hit 95 degrees in the afternoon, creating heat indexes as high as 105.
In Minnesota, the heat wave was expected to continue through Wednesday with possible thunderstorms in some parts. Highs in the Twin Cities area could reach 94 degrees on Sunday, and 97 degrees from Monday through Wednesday.
The weather service is projecting possibly six consecutive days of temperatures at 90 degrees or higher in the Twin Cities, the longest stretch to far this year, but short of records, meteorologist Jim Richardson said.
"Basically, today through Wednesday looks to be the warmest regime and humidity levels up there as well," Richardson said.
By midweek many locations on the East Coast will have heat index values approaching or exceeding 100 degrees, including Washington D.C., the weather service said.
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