Illinois weather in September
One place stands in for Illinois here — these are Chicago's numbers, not a statewide average.
Based on 1 weather station, in Chicago. One station — Chicago, where Lake Michigan runs the thermostat; downstate summers come hotter, and the far south of the state sees gentler winters.
Chicago & the Lakefront
Highs near 76°, nights around 57°, and 3.2 in of rain in a typical September.
| Place | Highs / lows | Wet days | The short version |
|---|---|---|---|
| Chicago | 76° / 57° | 6 | Summer hangs on and the beaches quietly empty out. |
Each place above has its own page for the full story — what the month feels like, what to pack, and whether the timing is right.