It felt just like July yesterday with highs topping 90° in a few spots. It hit 91° in Romeoville and 92° in Rockford. A cold front coming through today will usher in much cooler Canadian air. Highs this weekend will be nearly 10° below average. Lows by Sunday morning may dip into the upper 30s well west and northwest of the city. The timing of this autumnal air mass coincides well with the official start of fall on Sunday at 3:44 PM.
It doesn't look like this cool down will last long. We should be near 80° by Wednesday and be into the 80s by the end of next week. The longer range (6 to 10 day and 8 to 14 day) forecasts from the Climate Prediction Center have us outlooked for above average temperatures overall through the end of the month and into early October (see above image).