September in Blenheim is the rainiest month of the year. In September, the weather in Blenheim is typically warm. Average daily high temperature in September is about 12 degrees above zero. On average, Blenheim has 11 sunny, 11 partly cloudy, 7 rainy, and 1 cloudy day in September. The rains are usually light. September as well as June, July, and August is considered the most uncomfortable time to visit Blenheim.