Atmospheric Rivers: Background and Forecasting

Atmospheric Rivers: Background and Forecasting

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INSIGHTi

Atmospheric Rivers: Background and
Forecasting
Updated December 4, 2023
An atmospheric river (AR) consists of a long band of water vapor moving through the atmosphere,
typically resulting in heavy precipitation over land (Figure 1). Improved AR observations and
understanding (especially for large ARs) may facilitate flood preparedness and response and water supply
management. This is especially true in some snowpack-dominated watersheds. ARs significantly
influence U.S. West Coast water conditions, producing on average 30%-50% of the region’s annual
precipitation, and sometimes more. For example, from late December 2022 through mid-January 2023,
ARs delivered about half of California’s average annual precipitation, and more ARs followed in the
spring.
ARs may have implications for other U.S. regions, as well. For instance, ARs from the Gulf of Mexico
contributed to central U.S. flooding in 1983 and 2008 and southern U.S. flooding in 2016. Congress may
direct AR research and the use of AR information through authorizations and appropriations for agencies,
such as the National Oceanic and Atmospheric Administration (NOAA) and U.S. Geological Survey
(USGS), which conduct and support AR science, and the U.S. Army Corps of Engineers (USACE), which
manages water resources and flood risks.
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Figure 1. Atmospheric River Extending From Hawaii to the U.S. West Coast, 2014

Source: NOAA, “Atmospheric Rivers.”
Notes: The light blue area denotes a plume of water vapor. An AR originating in the tropics near Hawaii that brings water
vapor to the U.S. West Coast is sometimes called a Pineapple Express.
ARs typically form in tropical regions when winds over the ocean draw water vapor into narrow bands.
AR interactions with land features, such as mountain ranges (Figure 2), or certain atmospheric conditions
cause the water vapor to move upward in the atmosphere and then fall as heavy rain or snowfall. When
ARs slow down over a particular area or occur in rapid succession, the resultant precipitation can lead to
flooding, mudslides, landslides, and debris flows, especially in areas that have experienced wildfires in
recent years. In some cases, ARs can help improve or “bust” drought conditions.

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Figure 2. An Example of Atmospheric River Formation

Source: NOAA, “What Is an Atmospheric River?”
According to some estimates, multiple ARs are in motion around the Earth at any given time, with 90% of
the planet’s atmospheric water vapor concentrated in four to five ARs at a time. Scientists have begun
categorizing ARs based on their maximum water vapor transported over a certain space and time (Figure
3; e.g., the U.S. West Coast December 2022-January 2023 ARs ranged from Cat 1 to Cat 4).

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Figure 3. Atmospheric River Strength Categories

Source: USGS, “Rivers in the Sky: 6 Facts You Should Know About Atmospheric Rivers.”
Notes: This rating scale, adapted from a 2019 article, uses 250 kg m-1 s-1 (i.e., 250 kilograms of water vapor per meter
per second) intervals to categorize atmospheric rivers by their transport of water vapor, as well as duration as measured
in hours.
Detection and Forecasting
Federal and nonfederal agencies and institutions employ a range of methods to observe and forecast ARs
and their short- and longer-term impacts, including on snowpack, rivers and streams, and subsequent
vegetation growth. Observations (e.g., wind, temperature, water vapor content) come from satellites,
radar, and aircraft- or ocean-based missions (e.g., Figure 4). For example, scientists working on
improving prediction of land-falling ARs in the western United States have established an Atmospheric
River Reconnaissance (AR Recon) partnership. AR Recon activities include coordinating and sharing AR
observations on the U.S. West Coast, and more recently in the Gulf of Mexico and southeastern United
States. A limited number of land-based AR monitoring stations also have been deployed to collect data
not well captured by other observing systems.

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Figure 4. Tracks of Atmospheric River Reconnaissance Flights
Deploying Dropsondes on January 27, 2018

Source: Courtesy photo provided by U.S. Air Force, “Hurricane Hunters Complete Atmospheric River Reconnaissance.”
Notes: Dropsondes are weather reconnaissance devices that measure conditions as the devices fall from an aircraft at
altitude over water. For a summary of atmospheric river reconnaissance flights and dropsondes, see Center for Western
Weather and Water Extremes, “Atmospheric River Reconnaissance.”
NOAA and others use these observations in forecasting models. NOAA’s National Weather Service uses
the model outputs to issue outlooks and warnings for AR-related weather events, such as rain, snow, wind,
high surf, flooding, thunderstorms, and tornadoes (e.g., Figure 5).

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Figure 5. Flood Watch, Warning, and Advisory Summary for California
Due to Precipitation from December 26, 2022, to January 16, 2023, Atmospheric Rivers

Source: NOAA, National Weather Service, San Francisco Bay Area, CA, Weather Forecast Office, “A Parade of Storms
Impact Bay Area.”
Emergency managers and infrastructure operators also may use AR information. For example, managers
may use AR forecasts, along with other information, to inform when to release water from reservoirs—
known as Forecast Informed Reservoir Operations (FIRO)—to reduce flood risk and enhance water
supplies.
As understanding of ARs improves, scientists are exploring how ARs may change with a warming
climate. Some research (as noted by the USGS and NOAA) suggests a warmer climate may alter U.S.
West Coast ARs’ frequency, intensity, and location. While globally ARs in a warming climate may be
associated with more precipitation, the effects on precipitation may not be uniform along the U.S. West
Coast. Potential changes to AR-related rain and snowfall may further encourage improved understanding
and forecasting of ARs for watersheds that receive significant AR precipitation.
Additional Considerations
In recent years, Congress has supported AR-related activities through appropriations for NOAA to
observe and predict ARs (explanatory statement accompanying P.L. 117-328) and authorization for
USACE to expand its FIRO efforts to other river basins (P.L. 116-260, §157, Division AA; P.L. 117-263,
Division H, Title LXXXI, §8303), among other ways. The 118th Congress may consider assessing federal
efforts to advance the understanding and forecasting of ARs and their impacts, including on flood risk and
water supplies.

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Author Information

Eva Lipiec
Nicole T. Carter
Analyst in Natural Resources Policy
Specialist in Natural Resources Policy

Disclaimer
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IN12094 · VERSION 3 · UPDATED