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1 [1] (January 13, 2025)

handle is hein.crs/govesbd0001 and id is 1 raw text is: Congressional Research Service Informing the legislative debate since 1914 0 January 13, 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 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). 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. The level of federal support for AR research and the use of AR forecasting may be an issue in congressional authorizations and appropriations for 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, and the U.S. Army Corps of Engineers (USACE), which manages flood risks associated with various federal water resource projects. Figure I. 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 can help improve or bust drought conditions. Figure 2. Example of Atmospheric River Formation Source: CRS adapted from 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. 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 2024 water year ARs ranged from weak to extreme). Figure 3. Atmospheric River Strength Categories Category Exce tional Primarily hazardous o 1250 Extreme4 Mostly hazardous, 1000 also beneficial Q 750 Balance of hazardous aT E t and beneficial >.g500 E 'W Wea k 2 Primarily beneficial, 3- 20 - ----also hazardous E Notanatmosphricivew, 24 48 72 Primarily beneficial Duration (hours) Source: CRS adapted from 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- Is-I (i.e., 250 kilograms of water vapor per meter per second) intervals to categorize ARs by their transport of water vapor and duration as measured in hours. Detection and Forecasting NOAA, USGS, and other partners employ a range of methods to observe and forecast ARs and their short- and

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