Planning for the weather is a good way to make your trip to Karuizawa relaxing and enjoyable, especially when you’re splurging. The hottest months are usually August and July with an average temp of 67°F, while the coldest months are January and February with an average of 31°F. The rainiest months in Karuizawa are July, August, June, and September, with each month seeing an average of 10 inches of rainfall.