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
SFEWS: State of Bay-Delta Science 2025, plus endangered smelt indices and freshwater inflows
From the San Francisco Estuary & Watershed Science Journal The latest issue of San Francisco Estuary & Watershed Science (Vol. 23, Issue 2) highlights strategies for advancing science-based decision-making amidst climate change and extreme weather, explores the impacts of catastrophic wildfires on water quality, and examines the role of atmospheric rivers in California’s evolving hydroclimate and flood risks. Additionally, the issue features research on protecting endangered smelt species through hydrodynamic and ecological modeling and reconstructs historical freshwater inflows to the […]
EOS: Tree rings record history of jet stream-related climate extremes
Persistent spatial patterns of summer weather extremes in the northern hemisphere recorded in tree ring growth records provide a thousand-year history of jet stream ‘wave5’ dynamics. by Susan Trumbore The degree to which global warming will affect atmospheric dynamics and, therefore, extreme weather is still uncertain. Broadman et al. [2025] find a clever way to reconstruct the history of one dynamical pattern that occurs when the jet stream forms five peaks and troughs around the Northern Hemisphere (referred to as […]
INSIDE CLIMATE NEWS: Atmospheric rivers may be diminishing on the West Coast and surging in the East, study finds
Over a 20-year period, atmospheric rivers could double the amount of rain falling on part of the Southeast, the researchers found. By Chad Small, Inside Climate News This article originally appeared on Inside Climate News, a nonprofit, non-partisan news organization that covers climate, energy and the environment. Sign up for their newsletter here. It makes sense that atmospheric rivers would flood West Coast headlines as well as its coastlines. Eighty percent of all West Coast flood damage is attributable to […]
USGS: Causes and consequences of flooding in California’s Central Valley
From USGS Ecosystems Land Change Science Program: USGS researchers are leveraging advances in satellite-based monitoring to better characterize and predict flood inundation, which is essential for effective emergency response and water management planning. Atmospheric rivers (ARs) are long corridors of water vapor transport that extend from the tropics to mid-latitudes. These meteorological phenomena can bring large amounts of rain or snow to the western United States each year. In locations like California, AR storms are responsible for the largest magnitude […]
DESERT RESEARCH INSTITUTE: Wet soils increase flooding during atmospheric river storms
A new study examined decades of atmospheric river storms across the West Coast to pinpoint the conditions that lead to catastrophic flooding. By the Desert Research Institute Atmospheric rivers are responsible for most flooding on the West Coast of the U.S., but also bring much needed moisture to the region. The size of these storms doesn’t always translate to flood risk, however, as other factors on the ground play important roles. Now, a new study helps untangle the other drivers […]
Click here for all coverage of atmospheric rivers on Maven’s Notebook.