September in O'Fallon averages a daily high of 26.8°C with nights around 13.8°C. Expect roughly 7.6 wet days totalling 79.5mm. Comfortable conditions for being outdoors all day.
Across the year, the best time to visit O'Fallon is September–October.
Based on ST PETERS 2 SE, MO US (10.7km), 1995–2024. 11 years of September data.