In April, the weather in Karuizawa is typically warm. Average daily high temperature in April is about 13 degrees above zero. On average, Karuizawa has 13 rainy, 7 sunny, 7 partly cloudy, and 3 cloudy days in April. The rains are usually heavy. April as well as January, February, March, and June is considered the most uncomfortable time to visit Karuizawa.