If you're hoping to get lost in the rain on your romantic getaway, check out these details about year-round temperatures in Kumamoto: The hottest months are usually August and July with an average temp of 77°F, while the coldest months are January and February with an average of 46°F. The rainiest months in Kumamoto are June, July, August, and May, with each month seeing an average of 13 inches of rainfall.