CHICAGO An oppressive and potentially deadly summertime mix of sizzling temperatures and high humidity baked a large swath of the United States on Sunday, pushing afternoon heat indexes in dozens of cities to dangerous levels.
Forecasters warned the heatwave 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 nation's midsection, including Illinois, Wisconsin, Minnesota, Iowa, Missouri, Kansas, Nebraska, Oklahoma, as well as South and North Dakota, where forecasters predicted heat indexes could hit 115 degrees Fahrenheit.
"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, South Dakota; Springfield, Illinois; and Minneapolis, Minnesota, where AccuWeather.com meteorologists were predicting long-standing high-temperature records would fall this week.
Kristina Pydynowski, a senior meteorologist at AccuWeather, said the heatwave would likely 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, Missouri, 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 by 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 Minneapolis-St. Paul area could reach 94 degrees F 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, (Sunday) through Wednesday looks to be the warmest regime and humidity levels up there as well," Richardson said.
By mid week many locations on the East Coast will have heat index values approaching or exceeding 100 degrees, including Washington, D.C., the weather service said.
In St. Louis, temperatures are expected to reach 96 degrees on Sunday, 98 on Monday and touch the 100-degree mark on Tuesday. The heat index values are expected to range from 105 to 115 degrees.
In the Kansas City area, heat index values are expected to range from 105 to 110 degrees on Sunday and then average about 110 degrees through Friday with oppressive heat and humidity. (Reporting by James B. Kelleher and David Bailey; Editing by Maureen Bavdek)