From the National Weather Service, Lincoln office

HOT and HUMID conditions will prevail for much of this week. Tuesday looks to be the hottest day, as highs climb around 100 in most of the area. Highs near 100 will be possible again in the south and southeast on Wednesday. Heat index values will climb into the 100 to 105 degree range Monday through Wednesday. Thunderstorm chances return on Wednesday as a cold front drops south into central Illinois. The front will stall out across central and southeast Illinois...so storm chances will continue through Friday.

[Copied from National Weather Service, Lincoln office]