This article was published on: 02/16/23 10:54 AM
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.
Key Points
–Starting on December 26, 2022, a series of 9 atmospheric rivers (ARs) brought significant amounts of rain, snow, and wind to California and other parts of the western United States over a 3-week period.
–80% of a full seasonal snowpack was deposited in California during these storms. Statewide, precipitation over these 3 weeks was 11.2 inches, which is 46% of a full water year.
–The AR events have been a big boost to mountain snowpack across the West, where snow water equivalent (SWE) totals are well above normal for this time of year except for many parts of the Cascades and the Northern Rockies, where accumulation has slowed since the beginning of January.
–Given that it is still early in the snow accumulation season, water year totals could either be moderate if the rest of the winter and spring are dry, or relatively high if precipitation continues.
–According to Natural Resources Conservation Service SNOTEL, as of end of day January 22, 2023, the snow water equivalent for the California Region is 215%, the Great Basin is 206%, and the Upper and Lower Colorado River Basins are 146% and 218%, respectively.
–These recent storms improved drought conditions by increasing soil moisture throughout much of the West, especially in California. The amount of water stored in many reservoirs increased, but some are still well below historical averages for this time of year.
–Despite the drought improvements in many areas, long-term drought persists in parts of the West. Reservoir storage deficits, such as those within the Colorado River system, and groundwater and soil moisture deficits (especially in the Northwest) that have built up over many months to years will require additional precipitation to overcome.
–Pockets of extreme (D3) and exceptional (D4) drought continue to persist in Utah, Nevada, and central and eastern Oregon.
–The long sequence of ARs that made landfall in California and the amounts of precipitation that fell over this relatively short period of time caused flooding, dangerous travel conditions, and debris flows.
–The NOAA Climate Prediction Center’s Seasonal Outlook shows chances of drought removal or improvement for central and northern California, Oregon, Idaho, and the northern Rockies, with drought remaining in southern California, Nevada, and Utah. The current forecasts indicate AR activity could pick up again in early February, but the storm tracks are still uncertain.