March in O'Fallon averages a daily high of 13.5°C with nights around 0.5°C. Expect roughly 10.9 wet days totalling 92.2mm. Generally workable, with some weather to plan around.
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. 12 years of March data.