O'Fallon Weather Forecast

    • 75
    • 98

    Hot

    • 73
    • 99

    Hot

    • 69
    • 93

    Mostly Sunny

    • 69
    • 91

    Mostly Sunny

    • 77
    • 97

    Hot

    • 76
    • 98

    Slight Chance Thunderstorms

Today 4 Sep St. Charles

Sunny, hot with Highs in the upper 90s. Southwest Winds around 5 Mph. Heat index Values up to 108.

Tonight 4 Sep St. Charles

Mostly clear in the Evening, then becoming partly cloudy. Lows in the mid 70s. South Winds around 5 Mph in the Evening, becoming light and variable. Heat index Values up to 105 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 1 September, 2026

    71.7 °F

  • Average September

    86.3 °F

  • Highest 1 September, 2026

    99.6 °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

    57.5 °F

  • Average High 2021–2026

    68.9 °F
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