O'Fallon Weather Forecast

    • 73
    • 94

    Slight Chance Thunderstorms

    • 72
    • 94

    Mostly Cloudy

    • 66
    • 90

    Rain Showers Likely

    • 59
    • 78

    Rain Showers Likely

    • 54
    • 71

    Mostly Cloudy

    • 55
    • 74

    Partly Sunny

Today 18 Sep St. Charles

Sunny. A slight chance of Showers and Thunderstorms this afternoon. Hot with Highs in the mid 90s. Northeast Winds around 5 Mph. Chance of rain 20 percent. Heat index Values up to 108.

Tonight 18 Sep St. Charles

Partly cloudy. A slight chance of Showers and Thunderstorms in the evening. Humid with Lows in the lower 70s. Southeast Winds around 5 Mph. Gusts up to 20 Mph in the evening. Chance of rain 20 percent. Heat index Values up to 103 early in the evening.

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O'Fallon Temperature Statistics

September Temperature

St. Louis, Spirit Of St. Louis Airport (10.6 miles)

  • Lowest 8 September, 2026

    59.6 °F

  • Average September

    81.2 °F

  • Highest 5 September, 2026

    102.7 °F

Climate (2021–2026)

St. Louis, Spirit Of St. Louis Airport (10.6 miles)

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  • Average Low 2021–2026

    46.1 °F

  • Average 2021–2026

    57.6 °F

  • Average High 2021–2026

    69 °F
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