What is an atmospheric river?

By Alex Sosnowski, AccuWeather senior meteorologist

The term "atmospheric river" is a flashy name for a plume of tropical moisture in the atmosphere that can result in heavy rainfall or snowfall in a narrow swath.

Use of the term is most common along the West Coast of the United States, but the setup can occur anywhere that persistent winds can transport moisture from the tropics to a mid-latitude location.

Pedestrians make their way across a street in the pouring rain in downtown Los Angeles on Wednesday, March 21, 2018. The storm was fed by a long plume of subtropical moisture called an atmospheric river. (AP Photo/Richard Vogel)

Impacts from an atmospheric river can range from beneficial rainfall in drought-stricken areas to significant travel disruptions, flooding, mudslides and excessive snowfall.

Atmospheric rivers essentially act like rivers in the sky and can carry the same amount of water as the near the mouth of the Mississippi River, according to the National Weather Service.

However, not all atmospheric rivers are created equal.

At times, this high-speed conveyor belt of water may act like a giant fire hose. A foot of rain and disastrous flooding may occur a narrow swath, when the system stalls.

When the atmospheric river gradually shifts its position or is not so intense, light to moderate rainfall may be sprayed over a broad area.

Rounds of strong winds may also accompany an atmospheric river.

A more specific version of an atmospheric river is a setup sometimes referred to as a "pineapple express."

In this case, a plume of tropical moisture extends from near Hawaii to California or anywhere along the west coast of North America, according to AccuWeather Senior Meteorologist Ken Clark.

"The term pineapple express was used before the term atmospheric river became popular," Clark said.

An extreme pineapple express setup may bring drought-busting rain, tremendous flooding and mudslides to lower elevations of California, including desert areas. Yards of snow may pile up in the Sierra Nevada, and travel over the passes may grind to a halt.

Atmospheric rivers can also wreak havoc along the U.S. East Coast. For example, disastrous flooding was sparked in South Carolina, while Hurricane Joaquin churned off the coast in October 2015.

An atmospheric river flooded Charleston, South Carolina, on Oct. 3, 2015. News of record-setting rainfall was only tempered by the forecast that the direct effects of powerful Hurricane Joaquin would remain offshore of the United States. (AP Photo / Chuck Burton)

Even though the core of Joaquin remained well offshore, upper-level winds transported a steady plume of moisture, like a river in the sky, from the tropical system to a non-tropical system over the interior Southeastern states.

South Carolina experienced record flooding as more than 20 inches of rain fell on some communities spanning Oct. 1-5.

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