September in Koloa is the warmest month of the year. In September, the weather in Koloa is typically warm. Average daily high temperature in September is about 26 degrees above zero. On average, Koloa has 23 sunny, 4 partly cloudy, and 3 rainy days in September. The rains are usually light. September as well as January, June, and October is considered the most comfortable time to visit Koloa.