Atmospheric River Information Page

mountain rainstorm photo
Photo by Robert Leroux
Atmospheric Rivers (AR) are relatively narrow regions in the atmosphere that 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.  Learn more...

Quick Facts

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.
ARs are a primary 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.
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.
On average ARs are 400-600 km wide.
ARs move with the weather and are present somewhere on the earth at any given time.
Improved understanding of ARs and their importance has come from roughly a decade of scientific studies using new satellite, radar, aircraft and other observations and major numerical weather model improvements.