Main findings:

California’s driest 12-month period on record occurred during 2013/14, and although global warming has very likely increased the probability of certain large-scale atmospheric conditions, implications for extremely low precipitation in California remain uncertain.

Specifically, extreme geopotential height values in this region, which are a defining metric of this type of atmospheric configuration, occur much more frequently in the present climate than in the absence of human emissions.

Full report:

EEE 2013 = Explaining Extreme Events of 2013 from a Climate Perspective