Weather will likely play an important role when you plan your trip to Yonaizawa, but bear 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 temperature of 20°C, while the coldest months are January and February, with an average of -1°C. The rainiest months in Yonaizawa are July, August, September and October, with each month seeing an average of 237 mm of rainfall.