In 2013, a high-pressure zone formed over the Pacific Ocean, diverting precipitation towards Alaska. While these zones are common, they normally change position quickly. The recurring high-pressure “ridge” is unprecedented in modern weather records in that it remained in place for many months at a time, held by a large, static bend in the jet stream.
RRR trends and projections
An April 2016 study investigating changes during California's October to May "rainy season” identifies a significant increase in the occurrence of atmospheric patterns associated with certain precipitation and temperature extremes over the 67-year period. The study identifies in particular that both thermal expansion and sea level pressure trends contribute to a notable increase in anomalous northeastern Pacific ridging patterns similar to that observed during the 2012–2015 California drought.