What is an atmospheric river?

(KDVR/NEXSTAR) — Have you ever heard of a weather pattern called the “atmospheric river”? The powerful and prolonged condition is gearing up to move over the western region of the U.S. through Jan. 4.

An atmospheric river is basically a conveyor belt of moisture from the Pacific Ocean — and while they’re not on land like typical rivers, they do contain enough water to be classified as rivers, U.S. Geological Survey explains. 

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Technically, an atmospheric river, or AR, is a channel of water vapor that gets picked up near Hawaii, then transported by atmospheric wind directly into the West. 

You might’ve heard the phrase, “Pineapple Express,” which is another term for an AR, according to the National Oceanic and Atmospheric Administration.

Atmospheric rivers are channels of water being transported via atmospheric wind. (AP Images)

Once they meet land, AR moisture is released, then lifted higher into the atmosphere where it develops into heavy rain/snow. This is particularly evident in mountainous areas of the west. While certain landscapes can benefit from AR moisture — parts of the Sierra Nevada mountain range receive 30-50% of their annual precipitation from ARs — too much of it can cause problems.

Earlier this week, an atmospheric river sparked a cascade of issues in areas of California and Oregon.

The National Weather Service’s forecaster/meteorologist William Churchill told The New York Times that while California can usually benefit from the extra precipitation, “too much all at once” creates risk in areas damaged by wildfires. Here, scorched debris creates the possibility of mudslides, according to Churchill.

USGS elaborates on this point, explaining that fire damage dries out soil, making it less absorbent. When heavy rains begin, water slides right off — creating the potential for excess runoff and flash flooding. Additionally, this can present hazards for future fires, USGS scientists say, since excess moisture can cause dry, weedy vegetation to grow where it’s not supposed to — giving wildfires even more dry foliage to burn through.

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While the term “atmospheric river” might be new to many, the phenomenon’s effect on the weather is pretty constant: There’s always one happening somewhere across the globe, USGS explains.

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