The warmest month in the city, September is definitely a good time to visit San Francisco. Just bring your layers. What is the weather like in San Francisco in September? Expect temperatures around high 60s (19-22'C) by day to mid-50s (11-14'C) by night. Pleasant weather, coupled with diminishing crowds (tourists love summer), it's a good time to visit the city. Mind the famous San Francisco fog in early mornings and…