September in Palo Alto is the sunniest and the warmest month of the year. In September, the weather in Palo Alto is typically warm. Average daily high temperature in September is about 23 degrees above zero. On average, Palo Alto has 19 sunny, 10 partly cloudy, and 1 rainy day in September. The rains are usually light. September as well as June, July, August, and October is considered the most comfortable time to visit Palo Alto.