Weather will likely play an important role when you plan your trip to Yamakawa Onsen, and keep in mind that cheaper options are usually available when the weather isn't as good. The hottest months are usually August and July with an average temp of 69°F, while the coldest months are January and February with an average of 37°F. The rainiest months in Yamakawa Onsen are June, July, August, and September, with each month seeing an average of 14 inches of rainfall.