December in Costa Mesa averages a daily high of 17.9°C with nights around 10.4°C. Expect roughly 5.7 wet days totalling 39.8mm. Comfortable conditions for being outdoors all day.
Across the year, the best time to visit Costa Mesa is May–November.
Based on NEWPORT BEACH HARBOR, CA US (5.3km), 1995–2024. 28 years of December data.