December in San Bernardino is the rainiest and the coldest month of the year. In December, the weather in San Bernardino is typically warm. Average daily high temperature in December is about 11 degrees above zero. On average, San Bernardino has 17 sunny, 10 partly cloudy, 3 rainy, and 1 cloudy day in December. The rains are usually light. December as well as January and February is considered the most uncomfortable time to visit San Bernardino.