January in Santa Rosa is the rainiest and the warmest month of the year. In January, the weather in Santa Rosa is typically warm. Average daily high temperature in January is about 23 degrees above zero. On average, Santa Rosa has 22 rainy, 5 sunny, and 4 partly cloudy days in January. The rains are usually heavy. January as well as February, March, April, and November is considered the most uncomfortable time to visit Santa Rosa.