In September, Diamond Bar typically sees daytime highs of 30.5°C and overnight lows around 15.2°C. Compared to the annual average high of 24.1°C, September runs +6.4°C.
September in Diamond Bar averages a daytime high of 30.5°C and an overnight low of 15.2°C.
September runs +6.4°C compared to Diamond Bar's annual average high of 24.1°C, ranking #3 of 12 months by warmth.
Plan for daytime highs near 30.5°C and cooler evenings around 15.2°C.