Planning for the weather is a good way to make your trip to Yamakawa Onsen relaxing and enjoyable, especially when you're splurging. 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.