What is an atmospheric river?

An atmospheric river is poised to open the floodgates, with torrential rains and gusty winds expected to thrash the West Coast starting Wednesday. California is expected to bear the brunt of the storms as the sudden onslaught of moisture could trigger mudslides and deadly flooding.

Parts of coastal Oregon and California are under high-wind warnings, while some of the same areas, including northern and central California, are under flood watches. The region is expecting a downpour of anywhere from a few inches of rain to between 6 and 8 inches. And wind speeds are expected to blow between 20 and 40 mph with gusts peaking at 60 mph.

This is just the first in back-to-back atmospheric rivers expected to impact the region.


An atmospheric river is a plume of moisture, or water vapor that moves from the tropics and is dumped over a region as heavy rain or snow. Atmospheric rivers are narrow bands that tend to occur on the eastern side of strong mid-latitude storms.

The average atmospheric river carries a huge amount of water vapor: It can be nearly equivalent to the average flow of water at the mouth of the Mississippi River, according to the National Oceanic and Atmospheric Administration. Larger ones can carry 15 times the water volume of the Mississippi River.


These rivers in the sky can happen anywhere on the globe, including the U.S. East Coast, but they are the main source of heavy rainfall on the U.S. West Coast. The west coasts of continents tend to have clear, uninterrupted streams of moisture.

The moist air from the tropics hits various mountain ranges on the west coast and cools as it rises. The cool air forms clouds and then creates intense precipitation on the mountains’ eastern side, said Ryan Torn, the chair of the atmospheric and environmental sciences department at the University at Albany. And the more moisture in the air, the more precipitation.

“It basically like there’s a sponge and you’re starting to just squeeze the water out of it,” Torn said. The most damaging atmospheric river, the firehose-like floods, are the ones that move down the mountain and stall in one place, he said.


An average of seven to 10 strong atmospheric rivers make landfall in California each year, according to Matthew Igel, an adjunct professor in the University of California at Davis’s department of land, air and water resources.

El Niño is a naturally occurring climate pattern characterized by abnormally warm ocean waters in the tropical Pacific that drives heavy rains and storms.

A beastly subtropical jet stream, supercharged by the El Niño and unusually warm waters, is pushing the atmospheric river to California’s coast, Torn said. Many atmospheric river plumes occur over the ocean. The already heavy showers could get an extra boost of water power from El Niño.

El Niño years tend to bring a higher level of moisture and make atmospheric rivers along the West Coast more frequent.

“If you take the same storm and you put it in an El Niño year, it’s going to give you more water because the atmosphere is holding more water during that time,” Torn said.

However, Igel is unsure whether El Niño is the sole driver behind more precipitation and higher storm intensity.

“Weather is just chaotic,” he said. “And how much is being determined by El Niño and how much is being determined by just random chance? It is difficult to say.”


Many experts believe there are still questions about how climate change could impact storm patterns in the Pacific Ocean and their frequency.

But, some speculate there are three main ways that climate change could affect atmospheric rivers in the future:

-More rain: Experts say a warmer atmosphere in the future will hold more moisture, which could lead to more precipitation and an intensification of storms.

-More frequent and more intense: Atmospheric rivers already account for nearly 90 percent of California’s flood damage. Research suggests that as temperatures warm, atmospheric rivers could occur in closer succession and drop more rainfall in a season.

-Less snow: In the same way atmospheric rivers are known for bringing heavy rain to coastal and central areas, it’s crucial for bringing snow to the northern Sierra Nevada. But as temperatures warm, some research suggests that instead of snow, the rivers could bring rain to those regions too.


Studies have found that atmospheric rivers generally last 20 hours over an area along the coastline.

Torn said a single atmospheric river can move around in the ocean for up to five days. And that an atmospheric river can cause a single location along the coast to be inundated with heavy rainfall from 12 to 72 hours.