What Is an Atmospheric River? Rivers in the Sky Explained

By
Harris
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18

Key Points

  • NOAA describes atmospheric rivers as long, narrow “rivers in the sky” that carry most of the water vapor moving outside the tropics.
  • An average one carries about as much water vapor as the Mississippi River’s flow at its mouth; exceptionally strong ones carry up to 15 times more.
  • The “Pineapple Express” is a well-known strong atmospheric river that can carry tropical moisture from near Hawaii to the U.S. West Coast.
  • Scripps’ CW3E scale ranks them from AR 1 (weak, primarily beneficial) to AR 5 (exceptional, primarily hazardous) by vapor transport and duration.
  • Most bring helpful rain and snow, but the strongest can trigger floods and mudslides, which the National Weather Service flags with Flood Watches and Warnings.

An atmospheric river is a long, narrow band of concentrated water vapor that flows through the lower atmosphere, a bit like a river in the sky. When one runs into land, especially mountains, that vapor can fall as heavy rain or snow. They supply much of the West’s water and cause many of its worst floods.

This guide explains how atmospheric rivers form, what the “Pineapple Express” is, how scientists rate them from 1 to 5, where they hit and how to prepare, based on NOAA, National Weather Service (NWS), Scripps Institution of Oceanography and U.S. Geological Survey (USGS) information.

What is an atmospheric river?

In NOAA’s explainer What are atmospheric rivers?, the agency describes them as “relatively long, narrow regions in the atmosphere” that transport most of the water vapor outside the tropics. An average atmospheric river carries about as much water vapor as the average flow of water at the mouth of the Mississippi River, and exceptionally strong ones can move up to 15 times that amount.

The American Meteorological Society (AMS), which formally defined the term in 2017, calls it “a long, narrow, and transient corridor of strong horizontal water vapor transport.” Its glossary adds that atmospheric rivers are the largest “rivers” of fresh water on Earth, carrying on average more than double the flow of the Amazon.

How does an atmospheric river form and move water?

Per the AMS glossary, an atmospheric river is typically tied to a low-level jet stream ahead of the cold front of an extratropical cyclone, the kind of mid-latitude low that drives many winter storms. The moisture comes from tropical and/or mid-latitude sources, and most of the transport happens in the lower atmosphere.

The vapor falls out when something lifts the air. The glossary notes that atmospheric rivers frequently produce heavy precipitation where they are forced upward, for example by mountains. That is why coastal ranges and the Sierra Nevada wring so much water out of these storms: moist Pacific air rises up the slopes, cools and condenses into rain, or snow at higher elevations.

Scientists measure strength with integrated water vapor transport (IVT), the amount of water vapor moving horizontally through a column of air, in kilograms per meter per second. Minimal atmospheric river conditions begin at an IVT of 250. A bomb cyclone is also a midlatitude low; our explainer on how a bomb cyclone rapidly intensifies covers that storm type.

What is the Pineapple Express?

The Pineapple Express is the best-known nickname for a strong atmospheric river. NOAA describes it as one capable of bringing moisture from the tropics near Hawaii to the U.S. West Coast. It is a nickname, not a separate storm type, and not every atmospheric river reaching California, Oregon or Washington is a Pineapple Express.

How are atmospheric rivers rated? The AR 1 to AR 5 scale

In February 2019, a team led by F. Martin Ralph, director of the Center for Western Weather and Water Extremes (CW3E) at Scripps Institution of Oceanography, published a scale in the Bulletin of the American Meteorological Society to rank atmospheric rivers. Co-authors included NWS forecasters. Unlike hurricane or tornado scales, it accounts for storms that are helpful as well as hazardous.

The scale uses two ingredients at a given location: the maximum IVT during the event (as a 3-hour average) and how long atmospheric river conditions last. The intensity thresholds are:

  • Weak: 250 to 500 kg per meter per second
  • Moderate: 500 to 750
  • Strong: 750 to 1,000
  • Extreme: 1,000 to 1,250
  • Exceptional: 1,250 or more

A storm that keeps IVT above 250 continuously for 24 to 48 hours gets the category that matches its peak intensity. If it lasts less than 24 hours, it is downgraded one category; if it lasts longer than 48 hours, it is upgraded one category. Duration matters because strong vapor transport plus a long stay over one place causes the greatest impacts. Scripps illustrated each level with past storms:

Category Typical balance Historical example cited by Scripps
AR 1 (Weak) Primarily beneficial Feb. 2, 2017, California: lasted 24 hours at the coast, modest rainfall
AR 2 (Moderate) Mostly beneficial, somewhat hazardous Nov. 19 to 20, 2016, Northern California: 42 hours, several inches of rain that helped refill low reservoirs after drought
AR 3 (Strong) Balance of beneficial and hazardous Oct. 14 to 15, 2016: 36 hours, 5 to 10 inches of rain, some rivers rose to just below flood stage
AR 4 (Extreme) Mostly hazardous, also beneficial Jan. 8 to 9, 2017: 36 hours, up to 14 inches of rain in the Sierra Nevada, at least a dozen rivers reached flood stage
AR 5 (Exceptional) Primarily hazardous Dec. 29, 1996 to Jan. 2, 1997, Central California: over 100 hours, more than $1 billion in damages

The scale describes an area roughly 50 kilometers wide, not every hillside. CW3E posts experimental forecasts on its Atmospheric River Scale page. The AR scale is a heads-up tool, not an official NWS alert like those explained in our hurricane watch vs. warning guide.

Benefits vs. hazards: why atmospheric rivers matter

NOAA stresses that not all atmospheric rivers cause damage. Most are weak systems that bring beneficial rain or snow that is crucial to the water supply. Scripps notes they often make the difference between flood and drought years in California.

The benefits

Snowpack is a big part of the story. NOAA points to a series of atmospheric rivers from Dec. 10 to 22, 2010, that brought 11 to 25 inches of rain to parts of the West Coast, from western Washington to Southern California. Those same storms helped the Sierra Nevada collect 75 percent of its annual snow by Dec. 22, the first full day of winter.

The hazards

The strongest atmospheric rivers can produce extreme rainfall and floods, often by stalling over flood-prone watersheds. NOAA says they can disrupt travel, trigger mudslides and cause catastrophic damage to life and property. Scripps reports that roughly 80 percent of levee breaches in California’s Central Valley are associated with landfalling atmospheric rivers.

Local conditions can make a storm worse. The scale’s authors wrote that stronger atmospheric rivers can become mostly hazardous if they strike an area that is already vulnerable, such as wildfire burn scars or ground that is already wet.

The USGS ARkStorm scenario, patterned after California’s 1861 to 1862 storms, imagines a series of atmospheric rivers hitting the West Coast over several weeks. The USGS estimated that such a storm could flood thousands of square miles, cause thousands of landslides, and cost on the order of $725 billion. The geologic record shows six California megastorms more severe than 1861 to 1862 in the last 1,800 years.

Where do atmospheric rivers hit in the United States?

The West Coast is the main target. Scripps says atmospheric rivers bring most of its heaviest rains and floods. Research cited by Scripps found that the Oregon coast receives the most AR 4 (extreme) storms, about one per year on average. Washington sees an extreme atmospheric river about every two years, the San Francisco Bay Area about every three years, and Los Angeles about every 10 years. The strongest storm to hit the Southern California coast in a typical year usually falls in the AR 2 to AR 3 range.

They are not only a western problem. NOAA’s Geophysical Fluid Dynamics Laboratory says they occur frequently over the eastern U.S. and contribute substantially to heavy precipitation, and a 2024 study it highlighted found a significant rise in winter atmospheric river frequency there over four decades. NOAA Hydrometeorology Testbed researchers matched atmospheric rivers to about 41 percent of heavy precipitation days in the Southeast, with the link showing up mainly outside summer. Seasonal patterns matter too, as our guide to how El Niño and La Niña shape U.S. winters covers that wider picture.

How NOAA and the National Weather Service forecast and warn

NOAA studies atmospheric rivers with satellites, radar, research aircraft and computer models. It says this research helps NWS forecasters issue warnings for potential heavy rain and flooding in areas prone to atmospheric river impacts as many as five to seven days in advance.

The NWS has no special “atmospheric river warning.” It uses its standard flood alerts:

  • Flood Watch (be prepared): conditions are favorable for flooding. It does not mean flooding will occur, but it is possible.
  • Flood Warning (take action): flooding is imminent or occurring.
  • Flash Flood Warning (take action): a flash flood is imminent or occurring. If you are in a flood-prone area, move immediately to high ground.

How to prepare for an atmospheric river

Ready.gov calls floods the most common U.S. disaster and recommends these steps:

  • Know your risk. Check FEMA’s Flood Map Service Center, knowing flooding does not follow map lines.
  • Get alerts. Sign up for your community’s warning system, and use the Emergency Alert System and NOAA Weather Radio.
  • Check insurance early. Homeowner’s policies do not cover flooding, and a National Flood Insurance Program policy typically takes up to 30 days to take effect.
  • Prepare the house. Clear drains and gutters, move valuables higher and gather supplies for several days.
  • Never drive into water. Ready.gov warns that six inches of moving water can knock you down and one foot can sweep a vehicle away. Turn Around, Don’t Drown.

If you live below a burn scar or on a steep slope, follow local officials closely during heavy rain.

The bottom line

An atmospheric river is a narrow stream of water vapor that drops rain or snow when it reaches land. Most are weak and refill reservoirs and snowpack, while the strongest and slowest can cause major floods and mudslides. The CW3E scale shows which storms are likely helpful or dangerous, and NWS Flood Watches and Warnings tell you when to act. For current alerts and storm coverage, follow the LiveNewsWorld U.S. Weather Live center.

Sources