September in New Amsterdam is the sunniest and the hottest month of the year. In September, the weather in New Amsterdam is typically hot. Average daily high temperature in September is about 31 degrees above zero. On average, New Amsterdam has 19 rainy, 6 sunny, and 5 partly cloudy days in September. The rains are usually heavy. September as well as January, February, March, and April is considered the most uncomfortable time to visit New Amsterdam.