O'Fallon, IL Weather & Climate Normals
What a normal year in O'Fallon actually looks like
On the 30-year record NOAA keeps for O'Fallon, IL, the comfort score comes out to 43/100, strongly seasonal: Comfortable weather is limited to a shorter window; summers, winters, or precipitation swing wide. Only 4 months a year land in the 55–75°F range this site treats as mild.
How much O'Fallon's weather actually swings
Average temperatures in O'Fallon run from 34°F in Jan up to 79°F in Jul, a 45.4°F seasonal swing, one of the widest gaps in this dataset.
O'Fallon, month by month
| Month | Avg high | Avg low | Precip | Snow |
|---|---|---|---|---|
| Jan | 43°F | 24°F | 2.56in | 3.4in |
| Feb | 49°F | 28°F | 2.16in | 3.3in |
| Mar | 59°F | 36°F | 3.44in | 1in |
| Apr | 71°F | 46°F | 4.9in | 0in |
| May | 79°F | 56°F | 5.05in | 0in |
| Jun | 87°F | 65°F | 4.62in | 0in |
| Jul | 90°F | 68°F | 4.03in | 0in |
| Aug | 89°F | 65°F | 3.92in | 0in |
| Sep | 83°F | 57°F | 3.35in | 0in |
| Oct | 73°F | 47°F | 3.12in | 0in |
| Nov | 58°F | 37°F | 3.6in | 0.6in |
| Dec | 46°F | 28°F | 2.61in | 2.9in |
If you're timing a visit, Apr, May, Sep, and Oct are O'Fallon's mildest stretch; if you're moving there, you should plan around Jul's heat and Jan's cold instead.
How wet O'Fallon actually gets
Over a typical year, O'Fallon sees about 43.36in of precipitation a year across roughly 100 days with measurable rain or snow, with 11.2in of snow a year.
O'Fallon against the rest of the country
Out of 4876 cities scored here, O'Fallon comes in at 3984; within IL alone it ranks 235 of 330. Population is about 32,169.
Checking the actual forecast
These figures come from Belleville Siu Rsch, IL US, the NOAA station 6.7 miles from O'Fallon, averaged over 30 years, not tomorrow's forecast.
More on how this is worked out
Why some cities show no snow or rain figure · What a 30-year climate normal actually is