December in San Mateo is the coldest month of the year. In December, the weather in San Mateo is typically warm. Average daily high temperature in December is about 13 degrees above zero. On average, San Mateo has 13 sunny, 13 partly cloudy, and 5 rainy days in December. The rains are usually light. December as well as January, February, and March is considered the most uncomfortable time to visit San Mateo.