January in O'Fallon averages a daily high of 4.2°C with nights around -6°C. Expect roughly 8 wet days totalling 63.4mm. A hard month to visit for comfort alone.
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 January data.