Heavy rainfall events can make heat waves much, much worse. Lots of moisture & high dew points may cut your actual air temperature, but it feels much hotter & more uncomfortable with the amount of humidity.
The heavy rainfall that we have exprienced recently all over the region will work with an expanding upper ridge (dome of heat with origins in the Southwest U.S. & Chihuahuan Desert of Mexico) next week to cause these wet soils to evaporate water................rapidly. Corn & other vegetation will also be actively growing over a massive area & growing at a very fast clip given heat & the soil moisture. Corn especially transpires lots of water into the air when actively growing in moist to wet soils (a.k.a. corn sweat). This will all only enhance the moisture entering the air. It is like a crock pot of steaming food with the lid on.
That said, although temperatures may "only" run 89-94, given it looks like a scenario of 77-81 dew points pooling over the area with a persistent south to southwest wind. That wind will also be blowing over extremely wet, corn-lush central Illinois, only increasing the dew points here. Such dew points will cause heat indices to max out as high as 110 in the area & overnight lows to remain at 75-80, unless cooling t'storms drop that number to 71-73 (there will be some storms around on the edge & in the dome of heat that fire).
Although current data does not suggest 95-100 actual air temperature late next week, there have been instances where very intense droughts to our southwest have caused extreme heat (107-115) to build in the Plains. This heat then has expanded northeastward over soils here that are saturated by heavy rainfall with actively-growing corn & vegetation.
This heat then became loaded of moisture & its intensity sent temperatures to 96-102 with dew points at 77-82. Record high heat indices of 120 have occurred here in this way. Examples are July 1995, July 1999, even August 2005.............there was even such an event in 1905 with numerous deaths from "heat prostration" in Lafayette & Kokomo.
2010 saw a similar situation where we were just hammered with wave after wave of severe storms (including one derecho) with flooding rainfall early June to late July on the edge of a historic drought & heat wave to our south & southwest. At times, the heat dome would overspread part of our area, send us to 92-95, but dew points reached 81, resulting in extreme heat indices of +110. Here at WLFI, we peaked at 113 that summer for a heat index with one evening seeing the temperature at 88 with a dew point of 82 after a brief t'storm. That made it feel like 108 with steam/fog written in my weather notes for the observation here at the station.