Atmospheric River: The Term Everyone Tosses Around In the Winter

Winter is here, and with it comes the term that everyone throws around: atmospheric river.

Want to sound smart in front of your friends? We are here to help.

Let's start with the basics.

The definition: An atmospheric river is a long, narrow plume of moisture transported by winds from the tropics. Think of it like a river in the sky.

Going forward, we will abbreviate it AR.

They often stretch from the tropics or the subtropics into higher latitudes, often thousands of miles long. These thin ribbons of humid air can be identified and tracked in satellite imagery and computer model forecasts.

These features can be beneficial, crucial, but also damaging, especially for those near the West Coast of the United States.

The term was first used in a 1994 study and is now widely used by meteorologists.

They're common: On average, there are four to five active ARs on Earth at one time.

They are most often tracked over the North Pacific Ocean. In a typical winter, an average of 24 ARs make landfall along the U.S. West Coast, according to three separate studies. The "Pineapple Express" is one well-known type of AR. That's a moisture band lined up from near Hawaii to the West Coast of North America ahead of a strong Pacific cold front at least once or twice each fall, winter or spring.

But ARs also happen over the Atlantic and South Pacific Oceans, often affecting parts of the Caribbean, eastern U.S., southern South America, western Europe, southeast Asia and New Zealand.

The water they transport is mind-boggling: According to NOAA's Earth System Research Laboratory (ESRL), a strong AR can transport as much water vapor as up to 15 times the average flow of liquid water at the mouth of the Mississippi River. A 1998 MIT study estimated the moisture flux of an AR is roughly the same as that of the Amazon River, about 176,000 tons per second.

Taken together, ARs are responsible for 90% of the moisture tapped from the tropics into higher latitudes globally.

They can be rated like hurricanes: In 2019, scientists at the Center for Western Weather and Water Extremes and the National Weather Service unveiled a rating scale for atmospheric rivers analogous to the Saffir-Simpson hurricane wind scale. The scale is based on two criteria: how much moisture the AR transports and how long it could last over a given area. It ranges from AR1 (weak and beneficial) to AR5 (exceptional and hazardous).

The slowest-moving, wettest ARs are rated highest and tend to be the most hazardous when that copious moisture is wrung out over land. Fast-moving, less moist ARs are rated lowest and tend to be more beneficial than dangerous.

When hazardous ARs make landfall: When stronger atmospheric rivers reach land, heavy rainfall triggers serious impacts such as flash flooding, landslides, even levee breaks. That's especially the case when this moisture is lifted by hills and mountains, such as along the U.S. West Coast.

A 2006 study found that atmospheric rivers were responsible for all seven floods of northern California's Russian River in that seven-year period of study.

About 80% of California's major flood events can be traced to ARs, even though ARs accompany only 17% of West Coast storms, according to NASA.

If this rain falls on areas recently burned by wildfires, flows of mud and rocks known as debris flows can damage homes and threaten lives when they move quickly downslope.

But isn't all this moisture wrung out over California's Sierra a good thing, given the state's dependence on melting snowpack for water supply?

Only if it falls as snow.

A 2016 study by NASA, NOAA, UCLA, and Scripps Institution of Oceanography found that atmospheric river events are 2.5 times more likely to produce "rain over snow" events, or rain falling on existing Sierra snowpack.

They're also essential and can be beneficial: Parts of the world depend on ARs to maintain their water supply.

For instance, up to half of the average annual precipitation near the West Coast of the U.S. comes from just a handful of ARs. A lack of ARs can tip an area dependent on them into drought.

The return of ARs can reverse that. Up to 74 percent of Pacific Northwest droughts and up to 40 percent of California droughts from 1950 to 2010 were ended by an AR, according to a 2013 study.

Despite some damaging events, 31 ARs from fall 2022 through spring 2023 lead to a miracle replenishment of reservoirs in California after years of drought.

If an AR event isn't too warm to produce rain on top of existing snowpack, it can be beneficial.

Atmospheric rivers can dump some of the heaviest multi-day snowfalls you'll find anywhere in North America over the Sierra and Cascades, sometimes amounting to over 100 inches of snow in a week or less.

This can turn worry into relief among local water authorities when a so-called "miracle March" AR event boosts Sierra snowpack after a disappointing winter.

Atmospheric rivers are, in fact, vital to the Earth's water cycle. A 2017 NASA study estimated ARs contribute 22 percent to the total flow of water across the planet's land surfaces.

They can be costly disasters: Atmospheric rivers are the main perpetrators of flood damage in the western U.S., a 2019 study concluded. Examining data from the National Flood Insurance Program, the study also found ARs caused an annual average of $1.1 billion in flood damage in the West in the 40-year period ending in 2017.

A recent AR from the Caribbean Sea fueled a powerful East Coast storm in the week before Christmas 2023 that inflicted $1.3 billion damage from Florida to Maine.

Other examples of atmospheric rivers around the world:

- ARs in 2023 brought flooding and mudslides to drought-stricken Chile, which had been suffering from a multi-year drought.

- One impressive AR over 5,000 miles long accompanied Storm Desmond in December 2015, triggering massive flooding in parts of the U.K., Ireland, even Norway.

- The Nashville May 2010 flood was found to have had a contribution from an atmospheric river which began in the eastern tropical Pacific basin.

- An AR even chipped its moisture into the infamous "Snowmageddon" mid-Atlantic snowstorm from Feb. 5-6, 2010.

- The East Asian procession of heavy rain events from late spring into summer - known in China as the Mei-Yu season - often gets a boost from atmospheric rivers.

They're predictable: Thanks to advances in scientific research, numerical modeling and satellite technology, meteorologists can detect and forecast atmospheric river events sometimes a week or more in advance.

Given their impact, ARs remain an active area of research.

The Atmospheric River Reconnaissance program fielded by the Scripps Institution of Oceanography utilizes hurricane hunter aircraft and data from buoys to improve forecasts.

Dropping sensors into atmospheric rivers over the Pacific Ocean, the WC-130J aircraft missions collect data on moisture and winds, with the goal of improving forecasts, which are typically based heavily on satellite data only.

Jonathan Erdman is a senior meteorologist at weather.com and has been covering national and international weather since 1996. His lifelong love of meteorology began with a close encounter with a tornado as a child in Wisconsin. He studied physics at the University of Wisconsin-Madison, then completed his Master's degree working with dual-polarization radar and lightning data at Colorado State University. Extreme and bizarre weather are his favorite topics. Reach out to him on X (formerly Twitter), Threads, Facebook and Bluesky.