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December 14, 2021

Atmospheric rivers have been in the news a lot over the past couple of months, from a late October atmospheric river that brought record-breaking rainfall across Northern California to a mid-November storm that led to catastrophic flooding in Washington. A new atmospheric river storm is hitting the Western U.S. now and more are likely on their way. But what exactly is an atmospheric river?

Atmospheric rivers aren’t a new phenomenon on the West Coast, but this type of storm has drawn greater attention in recent years as scientists have learned more about how they work.

Here are 6 things to know about atmospheric rivers as the West’s wet season continues:

1. Atmospheric rivers transport water vapor from the tropics towards the poles.

Streams of water, with water vapor indicated by color, move over the globe, with atmospheric river plumes streaming away from the equator
Animation showing AR plumes over the Pacific during January 2012.

The formation of an atmospheric river starts near the equator. The sun heats the earth most directly at the equator, and these warm temperatures cause water to evaporate and rise into the atmosphere.

Some of that water vapor is pulled away from the equator by atmospheric circulation, forming a narrow band that transports the water vapor to other regions like a conveyer belt. Atmospheric rivers flow in the lowest part of the atmosphere, only about half a mile to a mile above the ground. When they reach the coasts and flow inland over mountains, the atmospheric river is pushed upwards, causing much of that water vapor to condense and fall to the ground as rain or snow, creating an atmospheric river-driven storm.

2. Atmospheric rivers are the largest “rivers” of fresh water on Earth.

While atmospheric rivers are pretty different from rivers of liquid water down on the ground, they transport enough water to deserve their moniker as rivers. Studies of atmospheric rivers over the Pacific have found that they transport water vapor at a rate equal to 7–15 times the average daily discharge of the Mississippi River. They can be hundreds to thousands of miles long, and though they are narrow in the context of weather systems, "narrow” can mean up to 300 miles across! 

Atmospheric rivers are always flowing somewhere on Earth, even though they don’t consistently stay in one place like rivers on the ground. At any given time, 90% of the water vapor moving toward the poles is concentrated in about 4-5 atmospheric rivers across the globe. Together, these narrow bands of flowing water vapor cover less than 10% of the circumference of the planet. 

Atmospheric river storms can affect people around the country and the world. Scientists estimate that atmospheric rivers provide over half of the mean annual runoff on the east and west coasts of North America, France, northern Spain and Portugal, the United Kingdom, southeastern South America, southern Chile, Southeast Asia, and New Zealand.

World map showing visualization of atmospheric rivers all over the world in February 2017
Atmospheric rivers occur all over the world in this global view from February, 2017.


3. There’s a rating system for atmospheric rivers like there is for hurricanes.

Like the scales for hurricanes and other hazards, the rating scale for atmospheric rivers is based both its physical characteristics (wind speed for hurricanes, quantity of water vapor for atmospheric rivers) and on the level of destruction it causes.  

While other rating systems are focused solely on the hazards of the event, the atmospheric river system incorporates the idea that these events can be beneficial, hazardous, or both. On the low end of the scale, AR Cat 1 events rated as primarily beneficial and at the high end, AR Cat 5 events primarily hazardous.

CHart showing atmospheric river rating scale from 1-5, with higher numbers primarily hazardous and lower ones primarily beneficial. Higher levels of water vapor transport and longer durations result in higher numbers.
A scale that categorizes atmospheric river events based on the maximum instantaneous integrated water vapor transport (IVT) associated with a period of atmospheric river conditions (i.e., IVT ≥ 250 kg m–1 s–1) and the duration of those conditions at a point. 

Atmospheric river storms can be beneficial in places like drought-stricken California—up to 50% of California’s annual precipitation can come from atmospheric rivers, and atmospheric rivers can bring enough water to end a drought. USGS research has found that 33%–74% of droughts on the West Coast between 1950 and 2010 were broken by the arrival atmospheric river storms (the October atmospheric river eased but did not end California’s current drought, however). On the other hand, high-intensity atmospheric rivers can be as destructive as hurricanes and lead to widespread flooding, landslides, and debris flows.  

The atmospheric rivers that hit Northern California on October 24 and the Northwest on November 15 have both been rated 5, AR Cat 5 (Exceptional): Primarily hazardous.


4. Though an atmospheric river can help extinguish fall fires, they can increase the hazard of past and future wildfires.

When a severe wildfire burns on a hillside, little vegetation remains, and the slope is vulnerable to flash floods and debris flows. Fires can also make the top layer of soil non-absorbent for a short time after the fire, so that water runs right down.

As a result, the rain brought by atmospheric rivers, in combination with more localized weather patterns, can lead to especially hazardous conditions near burn scars.

Illustration showing how a forested watershed ability to absorb and filter precipitation changes after a fire
Before a fire, forests act like a sponge and a water filter, meaning that rainwater can recharge drinking water supplies and only needs minimal treatment before use. After a fire, forests respond to rainfall as if the ground is covered in a layer of plastic wrap. Water cannot penetrate into the soil and huge amounts of surface runoff from rainstorms carry ash, sediment and other pollutants downstream into streams and reservoirs.

USGS scientists regularly conduct post-fire debris-flow hazard assessments for select fires in the Western U.S. not long after the fire burns. The hazard maps produced during these assessments help officials identify potentially dangerous conditions so they can take action to protect lives and property before and during extreme weather events. For example, USGS hazard maps of the 2020 Bond Fire informed response during subsequent atmospheric river storms in early 2021. 

Atmospheric rivers can influence the impacts of future fires, too. In 2017, USGS scientists studying this topic found that atmospheric rivers could actually increase the area burned by fires in the year following an event, especially in the most arid parts of the interior Southwest. Though wet winters can lead to higher soil moisture in the short term and increase snowpack in the mountains, wet winters also mean a lot of vegetation growth at lower elevations. Much of that growth is invasive grasses that dry out quickly come summertime and become highly flammable fuels for fast-moving wildfires.

Mud and water and a big boulder rush down a hillside
Post-wildfire flooding and debris flow in a small canyon above the Las Lomas debris basin in Duarte, the winter after the the June 2016 Fish Fire in Los Angeles County, California. 

5. An atmospheric river mega-storm could be California's other “Big One.”

Visualization of the ARkStorm Scenario.

If you live on the West coast, you’ve likely heard about “the big one” or even “the really big one,” phrases that refer to potential major earthquake events along the faults of California and the Pacific Northwest. But there’s another “big one” you may not have heard of: according to USGS natural hazards scientists, an atmospheric river-driven mega-storm that could cause catastrophic damage is plausible, if not inevitable, for California. Such a storm could cause extensive flooding across the state, raising environmental health concerns, causing thousands of landslides, disrupting critical infrastructure for days or weeks and causing 350 billion dollars in damages and 290 billion dollars in business interruption losses.

USGS scientists have developed ARkStorm, a hypothetical, scientifically realistic future winter storm scenario, to figure out all the details of what such an event would look like. ARkStorm (for Atmospheric River 1,000) was designed to be similar in intensity to the California winter storms of 1861 and 1862, the largest and longest California storms in the historic record and the cause of the Great Flood of 1862. This type of storm would produce precipitation at levels only experienced on average once every 500 to 1,000 years.

sepia-colored poen and ink drawing of people in boats floating down a city street surrounded by buildings.
Artist's drawing of flooded streets in Sacramento, California (view up K Street from the levee) during the flood of 1862.

6. Atmospheric rivers are expected to increase in intensity in California due to climate change.

Human-caused climate change is increasing the intensity of many extreme weather events, and atmospheric rivers are no exception, at least in California. Research by USGS scientists and partners has found that over the past 70 years, there is a pattern of increasing water vapor transport onto the West Coast associated with ocean surface warming. Atmospheric rivers aren’t predicted to become more frequent, but California’s precipitation will become more volatile, with more water concentrated into a smaller number of higher-intensity atmospheric river events. 

High-intensity atmospheric river storms can cause a lot of damage, and there are likely to be more such storms in our future. But with the help of USGS science, we have the information and tools to prepare for even a “big one.” Unlike earthquakes or fires, scientists can predict the timing and strength of atmospheric rivers several days in advance, allowing people to stock up on emergency food and water, make preparations for shelter, and avoid high-risk areas.  

Over the long term, the studies like the ARkStorm Scenario can help raise awareness of a future big storm and inform major logistical planning and infrastructure development, helping people prepare for major atmospheric river storms and limit their destruction. 

View from the sky looks down on a roadway that runs along a coastal cliff, part of the road has washed away.
An atmospheric river hit the central California coast and stalled there between January 26 and 28, 2021 — with catastrophic consequences. Rainwater washed dead trees, ash, mud, and rock downslope from the nearby watershed, scorched by the Dolan Fire in Los Padres National Forest in the fall of 2020. Drain pipes that run below Highway 1 were rapidly clogged with the debris and were eventually overwhelmed. The roadway was no match for the overflowing culverts, resulting in a massive collapse of the rocky cliff.

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