Weather is probably a major factor when you plan your trip to Santa Rosa, and keep in mind that cheaper options are usually available when the weather isn't as good. The hottest months are usually January and December with an average temp of 76°F, while the coldest months are July and June with an average of 60°F. The rainiest months in Santa Rosa are October, May, November, and September, with each month seeing an average of 8 inches of rainfall.