Weather will likely play an important role when you plan your trip to San Juan Cosala, and keep in mind that cheaper options are usually available when the weather isn't as good. The hottest months are usually May and April with an average temp of 74°F, while the coldest months are January and December with an average of 65°F. The rainiest months in San Juan Cosala are July, August, September, and June, with each month seeing an average of 7 inches of rainfall.