December weather forecast in O'Fallon

In December, the weather in O'Fallon is typically cold. Average daily high temperature in December is about 5 degrees above zero. On average, O'Fallon has 13 partly cloudy, 8 sunny, 5 rainy, and 5 cloudy days in December. The rains are usually heavy. December as well as January, February, and March is considered the most uncomfortable time to visit O'Fallon.