Atmospheric Rivers (ARs) are relatively narrow regions in the atmosphere and are responsible for most of the horizontal transport of water vapor outside of the tropics. While ARs come in many shapes and sizes, those that contain the largest amounts of water vapor, the strongest winds, and stall over watersheds vulnerable to flooding, can create extreme rainfall and floods. These events can disrupt travel, induce mud slides, and cause catastrophic damage to life and property. However, not all ARs cause damage – most are weak, and simply provide beneficial rain or snow that is crucial to water supply.
Improved monitoring, observation-based process understanding and prediction of ARs provide the critical knowledge needed by flood control managers, water supply authorities, and reservoir operators to mitigate the risks of major flood events while being able to take advantage of these heavy rainfall events as drought busters.
NOAA and its partners conduct targeted field campaigns using satellite measurements, offshore aircraft reconnaissance, and land-based AR observatories to guide model forecast system development, leading to improvements in the prediction of AR intensity and duration to support water resource management decisions.
On average, about 30-50% of annual precipitation in the west coast states occurs in just a few AR events, thus contributing to water supply.
In the strongest cases ARs can create major flooding when they make land-fall and stall over an area.
A strong AR transports an amount of water vapor roughly equivalent to 7.5–15 times the average flow of liquid water at the mouth of the Mississippi River.
ARs are a default feature in the entire global water cycle, and are tied closely to both water supply and flood risks, particularly in the Western U.S.
A well-known example of a type of strong AR that can hit the U.S. West Coast is the "Pineapple Express," due to their apparent ability to bring moisture from the tropics near Hawaii to the U.S. west coast.
Improved understanding of ARs and their importance has come from more than a decade of scientific studies using new satellite, radar, aircraft & other observations & major weather model improvements.