Periods of heavy rainfall caused by multiple atmospheric rivers in California between December 31, 2022 and March 25, 2023 resulted in floods that affected parts of Southern California, the California Central Coast, Northern California and Nevada. The flooding resulted in property damage and at least 22 fatalities. At least 200,000 homes and business lost power during the December-January storms and 6,000 individuals were ordered to evacuate.
The floods were widely reported by media as an example of how climate change is increasing extreme changes in weather, especially cycles of precipitation and drought. Scientists interviewed by Los Angeles Times said that further study is needed to determine the connection and California has recorded similar events almost every decade since records started in the 19th century. Other scientists have emphasized that floods were caused by ocean warming, directly related to climate change. Scientist Kevin Trenberth declared that "the interaction between the warming ocean and the overlying atmosphere (...) is producing these prodigious rainfalls that have occurred in so many places around the world recently". Climate change is intensifying the water cycle. This brings more intense rainfall and associated flooding, as well as more intense drought in many regions. It has been both predicted by scientists and observed in the last years and documented by the IPCC (International Panel for Climate Change 6th assessment report). Before the rains started, California had been in an extreme drought.
Due to the storms, Governor Gavin Newsom declared a state of emergency on January 4, 2023. President Joe Biden then declared a state of emergency in 17 California counties on January 9, 2023. That same day, two lawmakers sent a letter urging President Biden to declare a state of emergency for San Luis Obispo County and Santa Barbara County. Biden approved a major disaster declaration for Santa Cruz, Sacramento and Merced counties on January 14. Monterey, San Luis Obispo and Santa Barbara counties were added a few days later to the declaration. Later, Ventura County was approved disaster relief. Biden surveyed the damage with Newsom on January 19.
Impacts
The storms causing the 2022–2023 California floods reached Nevada, Arizona, and Utah.
California
Over 40 state parks in California were completely closed in January, and one national park was also closed, Redwood National Park. Dozens of Amtrak trains were delayed, ran on modified schedules, or cancelled entirely due to the floods as well.
San Francisco Bay Area
Oakland set a record for 24 hour rainfall at of rain on December 31, while San Francisco recorded its second wettest day with of rain. The Santa Cruz Branch Rail Line sustained major damage. US 101 was flooded in South San Francisco, California, while SR 84 was closed due to landslides and flooding in Fremont. This forced the Oakland Zoo to close until at least January 17. Flooding and road washouts were widely reported. Flooding was exacerbated by the series of storms as they exceeded the soil’s capacity to soak up water.
A 2-year-old boy in Occidental died from his injuries in January after a tree fell on his family house. A weather station in Nicasio recorded a wind gust of during what forecasts described a "bomb cyclone" on January 4.
On March 11, 2023, SR 84 between Portola Road and Skyline Boulevard was closed indefinitely due to damage from a landslide, resulting in the road buckling and leaving behind large cracks.
Trocadero was severely damaged as a result of the storm. A shelter-in-place order was issued for the area around 555 California Street after glass panels started being blown off the side of the building.
Flooding shut down US 101 in Gilroy, as well as I-580 in Oakland. It was reported that trees and power lines had been downed by the storm with an Amtrak commuter train hit by a tree and derailed near Porta Costa, though the train remained upright and no one was injured according to officials. On March 21, a ground stop was imposed at San Francisco International Airport.
Sacramento Valley
A levee along the Cosumnes River broke in January, resulting in the SR 99 being flooded. Evacuations were ordered in Wilton. Several people were trapped in their cars and had to be rescued. Three people were killed from flooding on the roads. The levee failures were traced to a private property.