December is not a favourable month for a trip to San Francisco unless you happen to take delight in pouring rains. To make it worse, December is also known to be very cold and dark. However the temperatures never drop to freezing and it never snows, so San Francisco winter is quite mild.
The peak of the rainfalls occurs in January. This is also the coldest month of the year. Seasonal benefits include the abundance of Point Reyes Whales in surrounding waters and blooming plum trees.
February is still cold and wet, in fact, one of the most humid months a year, hence not very amiable for tourists. The only seasonal pleasure is perhaps mustard blooms found all across the vineyards.