From late Dec. 2022 into Jan. 2023, a series of nine “atmospheric rivers” dumped a record amount of rain and mountain snow across the western U.S. and Canada, hitting California particularly hard. More than 32 trillion gallons of water rained down across the state alone, and the moisture also pushed into much of the Intermountain West. 

The San Francisco Bay area experienced its wettest three-week period in 161 years—the last time rainfall totals there were greater, Abraham Lincoln was president. Additionally, according to California’s Department of Water Resources, snowpack throughout the Sierra Nevada mountains increased to between 186 to 269 percent of normal, measured on Jan. 23, 2023.  

The parade of storms caused significant flooding in areas of the Central Valley, Salinas Valley, and Santa Cruz Mountains, along with power outages and mudslides. The perpetual deluge resulted in at least 21 deaths and prompted more than 1,400 rescues throughout the state. California’s Geological Survey mapped more than 700 reported landslides due to rainfall. 

Atmospheric rivers are relatively long, narrow bands of highly concentrated water vapor flowing high above us in the atmosphere, and are the largest transport mechanisms of freshwater on Earth. Atmospheric rivers move with the weather, usually occurring under particular combinations of wind, temperature, and pressure conditions. When atmospheric rivers make landfall, they often release significant amounts of precipitation in the form of rain or snow over relatively short periods of time. 

Based on satellite observations, an atmospheric river is typically greater than 1,245 miles (~2,000 km) long, less than 620 miles (1,000 km)—typically 250 to 375 miles (~400 to 600 km)—wide, and averages 1.8 miles (3 km) in depth. A study by Ralph et al. (2013) found that typical atmospheric river conditions last around 20 hours over an area on the coastline.