Atmospheric Rivers: Overview and Situational Awareness Communication Using the AR Scale

Atmospheric rivers (AR) are essentially rivers in the sky – rivers of water vapor pushed along by the wind. An average AR is roughly 500 miles wide and 1000-2000 miles long. ARs have been studied scientifically since the 1990s, were defined in the Glossary of Meteorology in 2018, and have been the subject of hundreds of scientific papers. They have inspired new programs like Atmospheric River Reconnaissance, which is now called for in federal legislation, and creation of the Center for Western Weather and Water Extremes, founded in 2013.

They have become part of the public lexicon, especially in the Western US where they are well known as the storm type responsible for both major flooding, and for vital water supply. This is not surprising because an average AR transports as much water (as vapor) as 25 times what the Mississippi river discharges into the Gulf of Mexico and are thus often associated with heavy precipitation. Whether a water year in the west includes major flooding or instead turns to drought hinges on just a few AR storms each winter. Strong ARs striking vulnerable regions cause 84% of all FEMA flood damages in the west, while weaker ones contribute substantially to water supply. To aid situational awareness, the AR Scale was created in 2019 and is being used increasingly to communicate with the public and decision makers. This presentation will briefly summarize ARs and describe how the AR Scale has been used in communicating weather and water risks.