'Pineapple Express' Is Expected to Hit Calif. Again, But What Is It? - NBC Bay Area

'Pineapple Express' Is Expected to Hit Calif. Again, But What Is It?

"Think of it as a river in the sky. It can be as large as 200 miles"

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    National Weather Service

    Another "Pineapple Express" is set to hit the Bay Area this week, and no, it's not the 2008 film starring Seth Rogan and James Franco or the marijuana strain.

    The Pineapple Express is a type of Atmospheric Rivers (ARs) which are condensed water vapor in the atmosphere that move with the weather, according to the National Weather Service.

    "Think of it as a river in the sky," NBC Los Angeles forecaster Belen De Leon said. "It can be as large as 200 miles."

    The Pineapple Express is aptly named for the moisture it brings from the tropics near Hawaii to the U.S. West Coast.

    But are Atmospheric Rivers good or bad? The answer is that it depends. 

    ARs can be beneficial to California's water supply, but it can also lead to devastating floods, according to the NWS. In early 2017, extreme precipitation from several ARs contributed to the flood risk crisis at Oroville Dam, the NWS said.

    The rainfalls from ARs are responsible for 30-50 percent of all the precipitation that occurs in California, Oregon and Washington, NWS said.

    The NWS has issued a flood warning in the North Bay area and Santa Cruz Mountains from late tonight through Saturday morning in anticipation for the incoming Pineapple Express.

    Idil Bereket of the San Francisco Public Utilities Commission described San Francisco as "the bottom of the bowl" and when it rains heavily, the area can experience flooding.

    The potential for flooding will be a concern in the Central Valley and in parts of the Sierra Nevada, where storm runoff could combine with melting snow at lower elevations. Snowfall will be limited to high elevations because of the storm's warmth.

    The Pineapple Express was also one of the reasons for last winter's historic levels of precipitation in California. 

    The last AR to hit the coast was in Southern California in March

    The Associated Press contributed to this report.

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