O'Fallon weather by month
Average highs run from 40°F in January to 90°F in July, and January nights average 24°F. It gets 42 inches of rain and melted snow a year and 17 inches of snow. Expect about 47 days a year at 90°F or hotter. Nights drop below freezing about 85 times a year. The most comfortable months are June and September to October.
Best time to visit O'Fallon
For comfortable weather, go in June and September to October. In those months the average high stays between 62°F and 86°F, nights average 42°F or warmer, and rain stays under 4.5 inches for the month.
Month by month
| Month | High | Low | Precip. | Snow | Wet days | Days 90°F+ | Nights below 32°F | Feels |
|---|---|---|---|---|---|---|---|---|
| January | 40°F | 24°F | 2.6 in | 5.7 in | 5 | 0 | 24 | cold |
| February | 46°F | 28°F | 2.2 in | 4.3 in | 5 | 0 | 19 | cold |
| March | 57°F | 37°F | 3.5 in | 2.3 in | 7 | 0 | 10 | cool |
| April | 68°F | 47°F | 4.7 in | 0.2 in | 8 | 0 | 1 | mild |
| May | 77°F | 58°F | 4.8 in | 0.0 in | 8 | 2 | 0 | warm |
| June | 86°F | 67°F | 4.5 in | 0.0 in | 7 | 11 | 0 | warm |
| July | 90°F | 71°F | 3.9 in | 0.0 in | 6 | 16 | 0 | hot |
| August | 88°F | 69°F | 3.4 in | 0.0 in | 5 | 13 | 0 | hot |
| September | 81°F | 61°F | 3.0 in | 0.0 in | 5 | 5 | 0 | warm |
| October | 69°F | 49°F | 3.2 in | 0.0 in | 6 | 0 | 1 | mild |
| November | 56°F | 37°F | 3.4 in | 0.9 in | 5 | 0 | 9 | cool |
| December | 45°F | 29°F | 2.5 in | 3.2 in | 5 | 0 | 20 | cold |
| Year | 67°F avg | 48°F avg | 41.7 in | 16.6 in | 71 | 47 | 85 |
"Wet days" are days with at least 0.1 inch of precipitation. "Feels" is a plain-language label for the average high. Normals for 1991-2020 from NOAA NCEI, station St Louis Lambert Intl. Airport, MO (USW00013994), 19.8 miles from the city center. Averages describe typical conditions, not a forecast.
Sunrise, sunset and daylight in O'Fallon
The shortest day is December 21, with 9h 26m of daylight; the longest is June 21, with 14h 53m. The earliest sunset comes around December 2 (4:40 pm) and the latest around June 20 (8:31 pm). The sun sets after 8 pm on 99 days a year.
| On the 15th of | Sunrise | Sunset | Daylight |
|---|---|---|---|
| January | 7:19 am | 5:05 pm | 9h 45m |
| February | 6:54 am | 5:40 pm | 10h 46m |
| March | 7:14 am | 7:09 pm | 11h 56m |
| April | 6:26 am | 7:39 pm | 13h 13m |
| May | 5:50 am | 8:08 pm | 14h 18m |
| June | 5:37 am | 8:29 pm | 14h 52m |
| July | 5:50 am | 8:27 pm | 14h 37m |
| August | 6:16 am | 7:57 pm | 13h 40m |
| September | 6:44 am | 7:10 pm | 12h 26m |
| October | 7:12 am | 6:24 pm | 11h 11m |
| November | 6:45 am | 4:48 pm | 10h 03m |
| December | 7:14 am | 4:41 pm | 9h 27m |
Calculated for 2026 at the center of O'Fallon with NOAA's solar equations, in local clock time (Central Time, including daylight saving time where it applies). Times shift by a minute or two from year to year.
Is O'Fallon getting warmer?
The last 10 years averaged 2.2°F warmer than the 20th century. See the climate stripes for O'Fallon.Frequently asked questions
What is the warmest month in O'Fallon?
July is the warmest month, with an average high of 90°F and an average low of 71°F (NOAA 1991-2020 normals).
What is the coldest month in O'Fallon?
January is the coldest, with average lows of 24°F and highs of 40°F.
What is the rainiest month in O'Fallon?
May gets the most precipitation, 4.8 inches on average. February is the driest at 2.2 inches.
Does it snow in O'Fallon?
Yes. O'Fallon averages 17 inches of snow a year.
When is the best time to visit O'Fallon?
For comfortable weather, June and September to October. Those months average highs between 62 and 86°F with lows of 42°F or more and modest rain.
Thinking of moving to O'Fallon?
See what homes, rent and everyday prices cost, who lives there and how it compares with nearby cities on the full O'Fallon profile.
Compare the weather: O'Fallon vs St. Louis, O'Fallon vs Columbia, O'Fallon vs Springfield, O'Fallon vs Peoria.