Planning for the weather is a good way to make your trip to Shiojiri 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 30°F. The rainiest months in Shiojiri are July, October, September, and June, with each month seeing an average of 7 inches of rainfall.