2017 California floods

Significant flooding affected parts of California in the first half of 2017. Northern California saw its wettest winter in almost a century, breaking the previous record set in the winter of 1982–83. Flooding related to the same storm systems also impacted parts of western Nevada and southern Oregon. Damage to California roads and highways alone was estimated at over $1 billion. The flooding occurred at the end of one of California's worst droughts on record, and many parts of the state were unprepared to handle the huge volume of rain and snow. However, the heavy precipitation also helped to refill drought-impacted surface water supplies, including many major lakes and reservoirs. The impact on groundwater reserves was limited. Governor Jerry Brown declared the drought officially over on April 4, 2017.

The high-amplitude ridge off the West Coast that characterized the 2011–17 California drought, dubbed the "Ridiculously Resilient Ridge", was replaced by a persistent presence of anomalous troughs impacting California. Another feature in the 2013–2015 winters was the extreme temperature contrast between a warm western U.S. and a cold eastern continent. These anomalous temperature and circulation patterns were referred to as the North American winter “dipole”.Figure (a) shows the climatological geopotential height (Z) overlaid with its eddy component, in which the dipole centers are located (indicated by X and +). The dipole basically describes the wintertime stationary waves over North America, which contribute to the mean temperature difference between the climatologically warmer western U.S. and colder eastern half. Therefore, an amplification of the stationary wave would enhance such a temperature difference, like in 2013-2015 winters, while a weakening of the stationary wave would reverse the situation, like in 2016–2017 winter. Indeed, in winter 2016–2017 this dipole was apparently reversed.

In January the Russian River in Sonoma and Mendocino Counties rose 3 feet (0.91 m) above flood stage, inundating about 500 houses. Over 570,000 customers of Pacific Gas and Electric lost power in Northern and Central California during the event. Over 3,000 people in the Guerneville area were evacuated. The community of Forestville, and the nearby Laguna de Santa Rosa flooded, blocking roads and agricultural lands. Flooding in the Russian River valley inundated vineyards, causing millions of dollars in crop damage.

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