It will be downright uncomfortable until temperatures warm Monday afternoon, ameliorating some of the humidity with rising daytime temperatures, although heat index readings could still approach 100 - 105 degrees Monday afternoon in areas that see more sunshine than clouds. Pittsfield's observation station recorded a surface dewpoint temperature of 81 degrees late Sunday evening. This was the highest reading to be found in the entire state of Illinois at that particular time on Sunday and also the highest dew point temperature found in the upper Midwest during that hour (not a record, just a situational observation). These humidity levels will remain in place at least until a cold front brings some relief by Tuesday.
Given the moist air mass and a proximal warm front, chances for showers and thunderstorms will remain non-zero across the entire Tri-states through late Monday morning. Chances should increase again late Monday in advance of the cold front. There is a low chance for thunderstorms to produce strong winds and small hail across the far southwestern Tri-States region on Monday, as NOAA's Storm Prediction Center has highlighted areas roughly along and west of a line from Paris, MO to Novelty, MO in a slight risk for severe storms.