December in Brentwood averages a daily high of 13.6°C with nights around 4.7°C. Expect roughly 8.7 wet days totalling 74mm. Generally workable, with some weather to plan around.
Across the year, the best time to visit Brentwood is April–May.
Based on ANTIOCH PUMPING PLANT NUMBER 3, CA US (7.6km), 1995–2024. 24 years of December data.