One of the big challenges with global warming is dealing with the large variances in annual data, which have always been an issue with weather forecasting. But along with global warming, most U.S. cities are also getting wetter. And in spite of all the drought news of recent years, San Francisco is no exception.
Since 1945, average rainfall in San Francisco has increased 0.4 inches per decade according to NOAA. But 2013 was exceptionally dry, with just 3.3 inches, and 2015's only seen 3 inches so far (though Dec 2014 had over 11 inches).
Much of the drought is also due to the low snowpack in the Sierras. But here again, the data shows no long term trend towards less snow in that mountain range, just high seasonal variations. So there's very little evidence the current drought has anything to do with global warming.