September in Koslan is the rainiest month of the year. In September, the weather in Koslan is typically cold. Average daily high temperature in September is about 9 degrees above zero. On average, Koslan has 21 rainy, 8 cloudy, and 1 sunny day in September. The rains are usually moderate. September as well as January, February, April, and May is considered the most uncomfortable time to visit Koslan.