Illinois weather in September
September is one of the best months to visit Illinois: highs of 22°C (71°F) and about 86 mm (3.4 in) of rain on average. Nights average 16°C (62°F) and cloud cover 46% (NASA POWER, 2001–2025 averages). It is the 6th driest month of the year and the 3rd warmest. Compared with August, it is cooler and drier; October brings highs of 15°C (59°F) and 99 mm (3.9 in) of rain.
Figures are long-term averages for Chicago, the largest city in Illinois. See below for other places.
Is September a good time to visit Illinois?
best month Weather score 76/100.
Yes. September is one of the best months for weather in Illinois.
Does it rain a lot in Illinois in September?
Some rain is normal: about 86 mm (3.4 in) falls in an average September. It is the 6th driest month of the year.
How warm is Illinois in September?
Days average 22°C (71°F) and nights 16°C (62°F).
Illinois weather by month
| Month | High | Low | Rain | Cloud | Weather |
|---|---|---|---|---|---|
| January | 0°C (32°F) | -5°C (23°F) | 48 mm | 71% | least pleasant |
| February | 0°C (33°F) | -5°C (23°F) | 54 mm | 70% | least pleasant |
| March | 5°C (41°F) | -1°C (31°F) | 62 mm | 66% | least pleasant |
| April | 10°C (50°F) | 4°C (39°F) | 95 mm | 63% | least pleasant |
| May | 15°C (59°F) | 9°C (47°F) | 117 mm | 64% | fair |
| June | 21°C (70°F) | 15°C (59°F) | 102 mm | 56% | best |
| July | 25°C (77°F) | 20°C (68°F) | 95 mm | 48% | best |
| August | 25°C (77°F) | 20°C (68°F) | 105 mm | 48% | best |
| September | 22°C (71°F) | 16°C (62°F) | 86 mm | 46% | best |
| October | 15°C (59°F) | 10°C (49°F) | 99 mm | 58% | fair |
| November | 9°C (47°F) | 4°C (38°F) | 61 mm | 63% | least pleasant |
| December | 3°C (37°F) | -2°C (29°F) | 58 mm | 73% | least pleasant |
Sources: 2001–2025 monthly averages from NASA POWER (MERRA-2 reanalysis). Page updated . How we rate months
Weather across Illinois in September
Weather can differ a lot within Illinois: coasts, hills and mountains each have their own climate.
| Place | High | Low | Rain | Weather |
|---|---|---|---|---|
| Chicago | 22°C (71°F) | 16°C (62°F) | 86 mm | best |
| Aurora | 24°C (76°F) | 13°C (55°F) | 87 mm | best |
| Rockford | 23°C (74°F) | 12°C (53°F) | 93 mm | best |
| Joliet | 25°C (76°F) | 13°C (55°F) | 85 mm | best |
| Naperville | 24°C (76°F) | 13°C (55°F) | 87 mm | best |
| Peoria | 26°C (78°F) | 13°C (56°F) | 86 mm | best |
| North Peoria | 26°C (78°F) | 13°C (56°F) | 86 mm | best |
| Elgin | 24°C (76°F) | 13°C (55°F) | 87 mm | best |
| Waukegan | 23°C (74°F) | 13°C (55°F) | 94 mm | best |
| West Town | 22°C (71°F) | 16°C (62°F) | 86 mm | best |
| Champaign | 26°C (79°F) | 14°C (56°F) | 78 mm | best |