O'Fallon weather by month
What a typical year looks like in O'Fallon, Missouri: average highs and lows, rain and snow, and the warmest and coldest days on record. Normals are the 1991–2020 averages at ST LOUIS SPIRIT OF ST LOUIS AP, 15 km from O'Fallon; records cover 1987–2026.
The year at a glance
- Precipitation a year
- 39.8 in1010 mm
- Snow a year
- Rare
- Last spring freeze
- Apr 11
- First fall freeze
- Oct 21
- Heating degree days
- 4,650
- Cooling degree days
- 1,446
Freeze dates are medians (half of years earlier, half later). Degree days use a 65°F base; HVAC design temperatures here are 4°−16° (heating, 99%) and 98°37° (cooling, 1%).
Month by month
O'Fallon in January
Highs average 41°5° and lows 22°−6°, with 2.4 in60 mm of precipitation in a typical January.
- Record high
- 76°24° (2002)
- Record low
- −16°−27° (1999)
- Heating degree days
- 1038
- Cooling degree days
- 0
O'Fallon in February
Highs average 47°8° and lows 26°−3°, with 2 in50 mm of precipitation in a typical February.
- Record high
- 84°29° (2024)
- Record low
- −12°−24° (2014)
- Heating degree days
- 808
- Cooling degree days
- 0
O'Fallon in March
Highs average 57°14° and lows 34°1°, with 3.5 in88 mm of precipitation in a typical March.
- Record high
- 94°34° (2026)
- Record low
- 1°−17° (2002)
- Heating degree days
- 598
- Cooling degree days
- 6
O'Fallon in April
Highs average 68°20° and lows 44°7°, with 4.6 in116 mm of precipitation in a typical April.
- Record high
- 94°34° (2002)
- Record low
- 21°−6° (1990)
- Heating degree days
- 288
- Cooling degree days
- 28
O'Fallon in May
Highs average 77°25° and lows 55°13°, with 4.6 in116 mm of precipitation in a typical May.
- Record high
- 95°35° (2012)
- Record low
- 31°−1° (1992)
- Heating degree days
- 88
- Cooling degree days
- 120
O'Fallon in June
Highs average 86°30° and lows 64°18°, with 4.3 in109 mm of precipitation in a typical June.
- Record high
- 109°43° (2012)
- Record low
- 45°7° (1988)
- Heating degree days
- 6
- Cooling degree days
- 300
O'Fallon in July
Highs average 90°32° and lows 68°20°, with 3.3 in84 mm of precipitation in a typical July.
- Record high
- 108°42° (2012)
- Record low
- 50°10° (2013)
- Heating degree days
- 0
- Cooling degree days
- 426
O'Fallon in August
Highs average 88°31° and lows 66°19°, with 3.4 in87 mm of precipitation in a typical August.
- Record high
- 106°41° (2007)
- Record low
- 46°8° (2004)
- Heating degree days
- 2
- Cooling degree days
- 370
O'Fallon in September
Highs average 81°27° and lows 57°14°, with 3 in77 mm of precipitation in a typical September.
- Record high
- 102°39° (2026)
- Record low
- 32°0° (1989)
- Heating degree days
- 51
- Cooling degree days
- 162
O'Fallon in OctoberThis month
Highs average 70°21° and lows 45°7°, with 3.1 in79 mm of precipitation in a typical October.
- Record high
- 94°34° (2006)
- Record low
- 23°−5° (2013)
- Heating degree days
- 271
- Cooling degree days
- 32
O'Fallon in November
Highs average 56°13° and lows 34°1°, with 3.3 in83 mm of precipitation in a typical November.
- Record high
- 85°29° (1989)
- Record low
- 5°−15° (1991)
- Heating degree days
- 593
- Cooling degree days
- 2
O'Fallon in December
Highs average 45°7° and lows 26°−3°, with 2.3 in59 mm of precipitation in a typical December.
- Record high
- 77°25° (2025)
- Record low
- −22°−30° (1989)
- Heating degree days
- 907
- Cooling degree days
- 0