Eva Lipiec and Nicole T. Carter updated (4 December 2023) this CRS Insight report: 'Atmospheric Rivers: Background and Forecasting'.
Download CRS_Insight_Atmos_Rivers_Background_Forecastig_4Dec2023
Lots of graphics; click on them to enlarge them. In the online versions there are some links, They are not active in the copy I have pasted below. ARs are now hitting close to home here in the Pacific Northwest and will likely be here for a few more days. At the bottom I have some personal remembrances from my days livein in the Sierra Nevada in Truckee, CA, just off I-80 below Donner Summit.
Introduction
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.
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.
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).
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.
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)
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.
Personal Reflections on ARs
I joke that if I had used the term 'AR' in one of my hydrology classes 30-40 years ago I probably would have had my hydrologic credentials stripped! 'Pineapple Express' was perfectly all right!
I lived in Truckee, CA (Sierra Nevada) in the mid-late 1980s. I recall that we got whacked with ARs for over a week in February. I could not get to my job in Reno because I-80 was treacherous driving and I could not even get to I-80 from my house! My wife and I spent a lot of time shoveling snow off the roof. It became apparent to me why mountain homes were not flat-roofed but had sloping metal roofs. My wife and I would watch the evening weather report on the Sacramento TV stations showing the storms backed up to Hawaii. We did not have the images you see above.
The folks who lived 'down below' (i.e., Sacramento and SFO) could not easily get to their vacation homes. That summer there were a lot of 'squashed' homes. Cars, too.
In between the storms we got a lit of skiing in (downhill and Nordic) and even some snow camping.
Flash forward to December 2023. Instead of living at 6000+ feet amsl in Truckee I'm at about 250 feet amsl in Corvallis, OR. Still getting hammered by the Pineapple Express, but it's falling as rain; the white stuff is several thousand feet above me in the Cascade Range. Too bad my skiing days are behind me!
Enjoy!
"A man's character isn't his fate; a man's fate is the joke his life plays on his character." - @Philip_Roth quoted in @Harpers via @TheWeek
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