Atmospheric Rivers Explained: How 'Rivers in the Sky' Bring Floods — and Water — This Winter

Winter brings atmospheric rivers — long, narrow corridors of moisture that behave like rivers in the sky. These features can deliver enormous benefits by refilling reservoirs and snowpacks, but they can also cause devastating floods, landslides and debris flows when they make landfall.

What is an atmospheric river (AR)?

An atmospheric river (AR) is a concentrated band of water vapor transported by winds from tropical or subtropical regions into higher latitudes. Often thousands of miles long but only a few hundred miles wide, ARs are visible in satellite imagery and model forecasts and are sometimes described simply as a "river in the sky."

How common are they — and where do they occur?

There are typically four to five active ARs on Earth at any given time. They are most frequently tracked across the North Pacific and make a critical contribution to the U.S. West Coast climate; in a typical winter, roughly 24 ARs make landfall along the West Coast. ARs also form over the Atlantic and South Pacific and affect the Caribbean, eastern U.S., southern South America, western Europe, southeast Asia and New Zealand.

How much water do ARs carry?

The water vapor transport is staggering: NOAA's Earth System Research Laboratory notes a strong AR can carry up to 15 times the average liquid flow at the mouth of the Mississippi River. A classic estimate from research finds an AR can move moisture on the order of the Amazon River — about 176,000 tons per second. Globally, ARs account for roughly 90% of the moisture moved from the tropics into higher latitudes.

Rating atmospheric rivers

In 2019 scientists introduced an AR intensity scale similar in concept to the hurricane scale. It ranges from AR1 (beneficial and weak) to AR5 (exceptional and hazardous) and is based on the amount of moisture transported and how long the AR persists over an area. Slow-moving, very wet ARs score highest and typically pose the greatest flood risk.

Hazards: when ARs become dangerous

When strong ARs hit land they can produce prolonged heavy rain that causes flash flooding, landslides, levee breaches and catastrophic erosion. Orographic lift — when moist air is forced upward by hills or mountains — amplifies rainfall on the U.S. West Coast. Studies have linked ARs to many major flood events: about 80% of California's major floods are tied to ARs even though ARs account for only 17% of West Coast storms.

Warm ARs are also 2.5 times more likely to create "rain-over-snow" events in the Sierra Nevada, which can melt snowpack prematurely and reduce summer water storage while increasing near-term flood risk. If heavy rain falls on recently burned slopes, fast-moving debris flows of mud and rock can endanger communities below.

Benefits: ARs as essential water sources

At the same time, ARs are vital for replenishing water supplies. Near parts of the U.S. West Coast, up to half of average annual precipitation can come from just a handful of AR events. Their return has ended many droughts — one study found ARs helped end up to 74% of Pacific Northwest droughts and 40% of California droughts between 1950 and 2010. For example, 31 ARs between fall 2022 and spring 2023 contributed to a significant recovery of California reservoirs.

When temperatures are low enough for precipitation to fall as snow, ARs can produce some of North America's heaviest multi-day snow totals — sometimes exceeding 100 inches in a week in the Sierra or Cascades — which greatly benefits spring and summer water supply.

Economic impacts and notable events

ARs are a major cause of costly flood damage in the western U.S.; one analysis attributed an average of about $1.1 billion per year (over a 40-year period) to AR-driven flooding. A late-2023 AR that tapped Caribbean moisture helped fuel a powerful East Coast storm that caused roughly $1.3 billion in damage from Florida to Maine. Worldwide, ARs have fueled damaging floods and mudslides in Chile (2023), accompanied Storm Desmond across the U.K., Ireland and Norway (2015), and contributed to notable U.S. events such as the Nashville floods and the 2010 East Coast "Snowmageddon."

Forecasting and research

Forecasting of ARs has improved thanks to better numerical models, satellites and targeted field campaigns. Programs like the Atmospheric River Reconnaissance deploy aircraft, buoys and dropsonde-equipped missions (e.g., WC-130J flights) to collect moisture and wind data and improve forecasts that can give communities days of advance notice.

Bottom line

Atmospheric rivers are powerful, often double-edged components of the global water cycle: they are essential for delivering much of the precipitation that sustains regions, yet they can produce extreme flooding and other hazards when conditions align. Continued research and improved forecasting are critical to maximizing their benefits while minimizing their risks.

Author note: This article synthesizes findings from meteorological research and operational forecasts to explain why atmospheric rivers matter for water resources and natural hazards this winter.