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California's driest 12-month period on record occurred during 2013114, 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.

The event: 2013114 drought in California. Nearly the entire state of California experienced extremely dry conditions during 2013 (Fig. 2.1a). Statewide, 12-month accumulated precipitation was less than 34% of average (Fig. 2.1b), leading to a wide range of impacts. In early 2014, state and federal water agencies announced that agricultural water users in the Cen- tral Valley would receive no irrigation water during 2014 (DWR 2014; USER 2014), and that a number of smaller communities throughout California could run out of water entirely within a 90-day window (USDA 2014a). Low rainfall, unusually warm temperatures, and stable atmospheric conditions affected the health of fisheries and other ecosystems (CDFW 2014), cre- ated highly unusual mid-winter wildfire risk (CAL FIRE 2014), and caused exceptionally poor air quality (BAAQMD 2014). Such impacts ultimately resulted in the declaration of a state-level "drought emergency" and the federal designation of all 58 California coun- ties as "natural disaster areas" (USDA 2014b).

The California drought occurred in tandem with a highly persistent region of positive geopotential height (GPH) anomalies over the northeastern Pacific Ocean (Fig. 2.1e,h), nicknamed the "Ridiculously Resilient Ridge" in the public discourse. Anomalous geostrophic flow induced by these highly unusual GPH gradients was characterized by weakened westerly zonal winds over the Pacific, strengthened zonal flow over Alaska (Fig. 2.Id), and a couplet of poleward-equatorward meridional wind anomalies centered in the northeastern Pacific around 135°W (Fig. 2.1g). This amplified atmospheric configuration displaced the jet stream well to the north, leading to greatly reduced storm activity and record-low precipitation in California (Fig. 2.1a,b).

California typically experiences strong seasonality of precipitation, with the vast...