CHICAGO (Reuters) - High temperatures and humidity were expected to bake a large swath of the country once again on Sunday and to send afternoon heat indexes in dozens of cities to dangerous levels, where forecasters predicted they would remain through much of the coming week. The National Weather Service posted excessive heat warnings for much of the country'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. "This will likely be the most significant heat wave the region has experienced in at least the last five years," the weather service said. The heat wave is expected to press east by the middle of the week, the weather service said. In Chicago, where the high heat and humidity warnings were twinned with a poor air quality alert, forecasters expected temperatures to hit 95 degrees Fahrenheit 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 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.