Droughts along the western coast of the United States are 'predominantly (84%) driven by internal atmospheric variability' and cooling sea surface temperatures
California is no stranger to drought; it is a recurring feature of the state's climate. They recently experienced a five year event from 2012-2016; other notable historical droughts included 2007-09, 1987-92, 1976-77, and off-and-on dry conditions more than a decade in the 1920s and 1930s. Presently, they are in year three of a serious drought. 1
Looking back in time it's evident that things could be much worse. Beginning about 1,100 years ago, what is now California baked in two droughts, the first lasting 220 years and the second 140 years. Each was much more intense than the mere six year dry spells that afflict modern California from time to time. 2