December in Santa Rosa is the cloudiest and the coldest month of the year. In December, the weather in Santa Rosa is typically warm. Average daily high temperature in December is about 14 degrees above zero. On average, Santa Rosa has 11 rainy, 9 sunny, 6 partly cloudy, and 5 cloudy days in December. The rains are usually heavy. December as well as January and February is considered the most uncomfortable time to visit Santa Rosa.