Planning for the weather is an important factor for making your trip to Karatsu a success, especially when you're splurging. The hottest months are usually August and July, with an average temperature of 25°C, while the coldest months are January and February, with an average of 9°C. The rainiest months in Karatsu are July, June, August and September, with each month seeing an average of 241 mm of rainfall.