Historical weather data, all-time temperature records, and monthly climate averages for O'Fallon, IL. Explore typical weather conditions by week for trip planning, and see how today compares to the historical record.
April 4th in O'Fallon runs mild on average, with highs near 63° and lows around 41°. But the date has seen real swings — a balmy 84° back in 2011 and a brutal 23° low in 1987. There's about a 1-in-3 chance of rain today, though snow is essentially off the table.
O'Fallon's weather history is not for the faint of heart — the thermometer has touched 110° in July 1954 and plunged to -27° in January 1977, a swing of 137 degrees. A single June day in 1957 dumped 8.15 inches of rain, and a January 1982 snowstorm buried the area under 14.8 inches.
Summer is the main event here, with highs averaging 89-90° in mid-July and a stretch of 80s that runs from May through September. If you're cold-averse, avoid mid-January when average highs barely crack 39°. Rain peaks in late June, averaging over an inch in a single week.