Weather will likely play an important role when you plan your trip to San Manuel, and keep in mind that cheaper options are usually available when the weather isn't as good. The hottest months are usually May and June with an average temp of 81°F, while the coldest months are January and February with an average of 73°F. The rainiest months in San Manuel are October, July, September, and August, with each month seeing an average of 11 inches of rainfall.