Weather will likely play an important role when you plan your trip to San Mateo, and keep in mind that cheaper options are usually available when the weather isn't as good. The hottest months are usually February and January with an average temp of 69°F, while the coldest months are November and December with an average of 67°F. The rainiest months in San Mateo are May, October, August, and April, with each month seeing an average of 22 inches of rainfall.