January in Santa Rosa is the rainiest month of the year. In January, the weather in Santa Rosa is typically warm. Average daily high temperature in January is about 14 degrees above zero. On average, Santa Rosa has 11 rainy, 10 sunny, 6 partly cloudy, and 4 cloudy days in January. The rains are usually heavy. January as well as February and December is considered the most uncomfortable time to visit Santa Rosa.