We've got a pattern change underway which will make this the warmest feeling week of the year.
It begins with the humidity. Dew points - the measure of the moisture in the atmosphere - are on the rise Sunday. Up 5° to 8° from Saturday. They will continue to climb the next couple of days, eventually reaching the lower and perhaps middle 70°s later this week.
Anytime the dew point gets above 60° it begins to feel uncomfortable. Once it climbs in to the upper 60°s to lower 70°s, it feels very tropical, or as I like to call it "muggly".
Combine the higher humidity with higher temperatures - high temperatures will be in the upper 80°s to near 90° - and peak heat indices will reach the upper 90°s. The highest heat index should come Tuesday and Wednesday topping out around 97°/98°.
PATTERN CHANGE - The "Ring of Fire"
While winds at the surface turn out of the southwest, winds aloft will turn out of the northwest as air flows around a developing "hot dome" in the upper levels of the atmosphere in the central/western United States. This pattern, called the "Ring of Fire", typically allows thunderstorm complexes to develop in the evening in the northern United States, then slide southeast during the overnight.
The first complex may develop tonight. High-resolution computer models suggest a complex will fire in Minnesota, then dive southeast to northern Illinois/northwestern Indiana by early Monday morning.
The complex should be in the process of weakening as it reaches central Indiana. The best chance for rain/storms from the complex will be between 7am and 11am Monday.
The atmosphere will begin to "recover" during the early afternoon and a few scattered thunderstorms will be possible late afternoon through the evening.
While I am not expecting widespread severe weather, a few thunderstorms may be able to produce strong wind gusts both during the morning and afternoon.