December in Seattle averages a daily high of 8.5°C with nights around 2.6°C. Expect roughly 18 wet days totalling 129.8mm. A hard month to visit for comfort alone.
Across the year, the best time to visit Seattle is July–August.
Based on SEATTLE BOEING FIELD, WA US (6.9km), 1995–2024. 26 years of December data.