September in Wailuku is the warmest month of the year. In September, the weather in Wailuku is typically warm. Average daily high temperature in September is about 26 degrees above zero. On average, Wailuku has 23 sunny, 6 partly cloudy, and 1 rainy day in September. The rains are usually light. September as well as April, May, June, and July is considered the most comfortable time to visit Wailuku.