May in Seattle averages a daily high of 18.8°C with nights around 9.6°C. Expect roughly 11 wet days totalling 44.8mm. Generally workable, with some weather to plan around.
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 May data.