September in Hilo is the warmest month of the year. In September, the weather in Hilo is typically warm. Average daily high temperature in September is about 23 degrees above zero. On average, Hilo has 16 partly cloudy, 7 sunny, 4 rainy, and 3 cloudy days in September. The rains are usually light. September as well as January, May, June, and August is considered the most comfortable time to visit Hilo.