In western states, precipitation is gauged not on the calendar year, but on a water year that begins on October 1st (when rivers and reservoirs are naturally at their lowest) and ends on September 30th. At this time last year, it appeared like California was destined for a sixth year of drought, but forecasts quickly took a different turn. Throughout the winter of 2016–2017, a parade of more than 30 atmospheric rivers rolled through California, including three that were classified as “extreme.” According to a recent study, such atmospheric rivers will take a more prominent role in California’s already highly variable climate in the future due to climate change (Gershunov et al. 2017).

California was blanketed by ample snow this year, like the 47.6 feet measured at Donner Pass. January was particularly ferocious, with back-to-back storms hitting the Tahoe region and piling up over 23 feet of snow at Squaw Valley ski resort. Storms raised Lake Tahoe by 6.5 feet, the most change experienced in one season! The statewide snow water equivalent was 163% of average on April 1, 2017, the benchmark date used to measure peak snowpack. While the snowpack was impressive, it was the contribution of rain in the Northern Sierra that really set the records. The Northern Sierra Precipitation 8-station Index measured 94.7 inches of precipitation (189% of average), making this the wettest year on record, and beating the 1982–1983 winter by six inches! The Central and Southern Sierra Precipitation Indices didn’t set any records but came close, finishing at 179% and 160% of average, respectively. 

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