Extreme heat is building across the Central U.S., and it doesn't look like it will let up for the region until Thursday or Friday. Excessive heat warnings, heat advisories, and excessive heat watches blanket the central U.S. from Texas north to Minnesota.
Temperatures will top 100°F in some locations, and heat indices could surpass 115°F. Expect that overnight lows might not get below 80°F—values that are 15 to 20 degrees above normal for much of the Upper Midwest. The heat is expected to remain in place for most of the week, but will begin to shift eastward by Thursday or Friday.
n the Upper Midwest, enough moisture will be present to make the heat unbearable. The "heat index" is often referred to as the "feels-like" temperature—how hot it feels to your skin. When temperatures soar, humidity can exacerbate the heat. The Midwest has had just enough rain in the past couple of weeks to make the air more humid than it is down south, which is why the heat index will be so high in this region. The air moisture is also caused by agriculture. We often see high humidity where lots of corn is grown, especially Iowa.
from a weather blog:
http://www.wunderground.com/blog/angelafritz/comment.html?entrynum=3