In September, Seattle typically sees daytime highs of 20.7°C and overnight lows around 10.9°C. Compared to the annual average high of 15.2°C, September runs +5.6°C.
September in Seattle averages a daytime high of 20.7°C and an overnight low of 10.9°C.
September runs +5.6°C compared to Seattle's annual average high of 15.2°C, ranking #3 of 12 months by warmth.
Plan for daytime highs near 20.7°C and cooler evenings around 10.9°C.