It's been a steamy summer in the Midwest, where heat indexes are once again rising to dangerous levels and are forecasted to last through the week. The National Weather Service has posted extreme-heat warnings from Illinois to Oklahoma, where indexes could climb as high as 115 degrees. The Twin Cities could possibly face six consecutive days of temperatures greater than 90 degrees. Forecasters have said that humidity levels will be at their worst in the Midwest today through Wednesday. By midweek, the wave is expected to push east, stifling Washington, D.C., with heat indexes of 100 degrees or higher.