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1 [1] (December 10, 2025)

handle is hein.crs/govesyf0001 and id is 1 raw text is: 'Congressional Research Service 0 Updated December 10, 2025 Atmospheric Rivers: Background and Forecasting 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 of 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). According to scientists, 67 ARs made landfall over the U.S. West Coast in the 2024 water year (October 1, 2023, through September 30, 2024). 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. Federal support for AR research and the use of AR forecasting may be an issue in congressional authorizations and appropriations across various agencies. These include the National Oceanic and Atmospheric Administration (NOAA) and the U.S. Geological Survey (USGS), which conduct and support AR science and forecasting. It also includes those federal agencies operating water resource projects for flood risk reduction and/or water supply purposes, such as the U.S. Army Corps of Engineers (USACE). Figure I. Schematic of an 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 impacted by wildfires in recent years. In some cases, ARs also can help improve or bust drought conditions. Figure 2. Examnle of Atmosnheric River Formation Source: CRS, adapted from NOAA, What Are Atmospheric Rivers? 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. Scientists have begun categorizing ARs based on their maximum water vapor transported over a certain space and time (Figure 3; e.g., the 2024 water year U.S. West Coast ARs ranged from weak to extreme). Figure 3. Atmospheric River Strength Categories ou Category Exud tionnow b Primarily hazardous 70 r1 (i. 2 hEir Mae ranas masued o hazardous, tto-las beneficial 7% 250 Baanof hazardous Drtn at (orcst Nr tr an d vapor arn ensmeasrnei ou Deth ion b v and forecast Ri n NoAAce USS, antd fothe paG Rtners mplo y: ange o metodstionosev and Forecastis nd hi hotn