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The temperatures in Illinois in September are comfortable with low of 16°C and and high up to 24°C.
You can expect about 3 to 8 days of rain in Illinois during the month of September. It’s a good idea to bring along your umbrella so that you don’t get caught in poor weather.
Our weather forecast can give you a great sense of what weather to expect in Illinois in September 2025.
If you’re planning to visit Illinois in the near future, we highly recommend that you review the 14 day weather forecast for Illinois before you arrive.
| Month | Temperatures | Rainy Days | Dry Days | Snowy Days | Rainfall | Weather | More details |
|---|---|---|---|---|---|---|---|
| January | -2° / -7° | 5 | 11 | 15 | 39 mm | Awful | Illinois in January |
| February | 0° / -6° | 4 | 11 | 14 | 41 mm | Awful | Illinois in February |
| March | 7° / 1° | 6 | 19 | 6 | 55 mm | Bad | Illinois in March |
| April | 12° / 5° | 7 | 21 | 3 | 79 mm | Awful | Illinois in April |
| May | 19° / 12° | 7 | 24 | 0 | 98 mm | Ok | Illinois in May |
| June | 24° / 17° | 7 | 23 | 0 | 128 mm | Good | Illinois in June |
| July | 27° / 20° | 7 | 24 | 0 | 114 mm | Good | Illinois in July |
| August | 27° / 19° | 5 | 26 | 0 | 100 mm | Perfect | Illinois in August |
| September | 24° / 16° | 4 | 26 | 0 | 64 mm | Perfect | Illinois in September |
| October | 16° / 10° | 5 | 25 | 0 | 65 mm | Good | Illinois in October |
| November | 9° / 3° | 4 | 22 | 4 | 40 mm | Bad | Illinois in November |
| December | 3° / -2° | 5 | 16 | 10 | 38 mm | Awful | Illinois in December |