An atmospheric river is a long, narrow corridor of concentrated moisture in the atmosphere that can transport vast amounts of water vapor from the tropics to other regions – on average, an atmospheric river can transport as much water as 20 Mississippi Rivers.
Atmospheric rivers are a key feature in the California water cycle. Half of California’s water supply comes from atmospheric rivers, often in powerful downpours that can hit in rapid succession. On the flip side, fewer atmospheric rivers generally leads to California progressing into drought. California often doesn’t get out of a drought until an atmospheric river arrives; seven of the last 12 droughts ended with atmospheric rivers.
On average, California usually sees about five or six atmospheric rivers, but some years it can be as few as one or two. Researchers are continuing to study why some years have so many, and others, so few.
Similar to hurricanes or tornadoes, a rating scale for atmospheric rivers has been developed that ranks atmospheric rivers by its physical characteristics, such as water vapor or wind speed, and the level of potential destruction it might cause. However, unlike other rating systems, the atmospheric river system incorporates the idea that atmospheric rivers can be beneficial, hazardous, or both. On the low end of the scale, AR Cat 1 events rated as primarily beneficial and at the high end, AR Cat 5 events primarily hazardous.
Climate change is amplifying the severity of many extreme weather events, including atmospheric rivers, particularly in California. Research by USGS scientists and their collaborators has revealed that, over the past 70 years, there has been a trend of increasing water vapor transport to the West Coast linked to warming ocean surfaces. While atmospheric rivers are not expected to become more frequent, California’s precipitation patterns are projected to become more erratic, with a greater amount of water concentrated in fewer, more intense atmospheric river events.
WEB RESOURCES
- Center for Western Weather & Water Extremes: CW3E provides weather predictions, climate projections, atmospheric river forecasts, interactive maps, research and publications, and workshops and collaborations.
- NOAA Atmospheric River Portal: Access information, images, analyses, diagnostics of current conditions, forecasts, and projects related to atmospheric rivers (ARs).
Story map from the Center for Western Weather & Water Extremes
Coverage of atmospheric rivers on Maven’s Notebook
THE CONVERSATION: How heat on the Tibetan Plateau fuels powerful storms in California, half a world away
By Yongkang Xue, University of California, Los Angeles The Tibetan Plateau may be on the other side of the world from California, but what happens on this high-altitude landscape in Asia can set off a chain of events that fuels powerful atmospheric rivers with flooding rainfall and heavy snow hitting the U.S. West Coast. My team of atmospheric scientists discovered this connection, and we explain how it works in a new study in the journal Science Advances. California’s unexpected rainfall […]
SCRIPPS: How does El Niño impact the weather?
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NASA data feeds river forecasts as snow drought effects linger
By Emily DeMarco, NASA As the effects of the 2026 snow drought in the western United States carry into summer, NASA Earth data is feeding machine-learning forecasts that inform decisions about water, power, and public safety in Washington state. Tacoma Power, a Washington public utility, is using a U.S. technology company’s river-flow forecasts during a year of water extremes on the Cowlitz River. The utility’s largest hydroelectric project uses water stored behind Mayfield and Mossyrock dams to generate enough electricity […]
UC RIVERSIDE: Slowing Atlantic current fueling stronger California storms
Study shows how ocean circulation will reshape global rainfall By Jules Bernstein, UC Riverside A slowing Atlantic Ocean current is projected to intensify powerful storms in California while reducing snowfall over Greenland, according to a new University of California, Riverside study. The Atlantic Meridional Overturning Circulation, or AMOC, works like a giant conveyor belt in the ocean, moving warm water from the tropics northward to heat places like Europe, then cycling the cooled, denser water back south along the ocean […]
Click here for all coverage of atmospheric rivers on Maven’s Notebook.