In September, O'Fallon typically sees daytime highs of 27.3°C and overnight lows around 13.8°C. Compared to the annual average high of 18.9°C, September runs +8.5°C.
September in O'Fallon averages a daytime high of 27.3°C and an overnight low of 13.8°C.
September runs +8.5°C compared to O'Fallon's annual average high of 18.9°C, ranking #4 of 12 months by warmth.
Plan for daytime highs near 27.3°C and cooler evenings around 13.8°C.