In September, the weather in Koh Lanta Yai is typically warm. Average daily high temperature in September is about 27 degrees above zero. On average, Koh Lanta Yai has 25 rainy, 2 cloudy, 2 partly cloudy, and 1 sunny day in September. The rains are usually heavy. September as well as April, May, June, and July is considered the most uncomfortable time to visit Koh Lanta Yai.