December in Napa averages a daily high of 14.8°C with nights around 4.6°C. Expect roughly 12 wet days totalling 145.3mm. Generally workable, with some weather to plan around.
Across the year, the best time to visit Napa is April–October.
Based on NAPA STATE HOSPITAL, CA US (2.8km), 1995–2024. 30 years of December data.