In September, Boulder typically sees daytime highs of 20.9°C and overnight lows around 4.7°C. Compared to the annual average high of 13.6°C, September runs +7.3°C.
September in Boulder averages a daytime high of 20.9°C and an overnight low of 4.7°C.
September runs +7.3°C compared to Boulder's annual average high of 13.6°C, ranking #4 of 12 months by warmth.
Plan for daytime highs near 20.9°C and cooler evenings around 4.7°C.