September in Boulder averages a daily high of 26.4°C with nights around 9.4°C. Expect roughly 7.8 wet days totalling 53.6mm. Comfortable conditions for being outdoors all day.
Across the year, the best time to visit Boulder is September–October.
Based on BOULDER, CO US (2.5km), 1995–2024. 30 years of September data.