December in South San Francisco averages a daily high of 14.3°C with nights around 7.6°C. Expect roughly 11.4 wet days totalling 117.3mm. Generally workable, with some weather to plan around.
Across the year, the best time to visit South San Francisco is May–October.
Based on SAN FRANCISCO INTERNATIONAL AIRPORT, CA US (5.4km), 1995–2024. 30 years of December data.