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The Pineapple Express is an airborne river. Another one is drenching California.

Heavy rain, snow, floods, and potential debris flows are hitting central and northern California.

Tropical moisture streaming toward the West Coast on April 5.
Space Science & Engineering Center, University of Wisconsin-Madison

Another Pineapple Express has been bringing massive April showers to California since Thursday. And this one is so big it may set records.

A Pineapple Express is an example of an atmospheric river, a channel in the atmosphere that moves vast amounts of moisture. This kind forms over Hawaii (hence “pineapple”) and directs moisture toward the West Coast. Daniel Swain, a University of California Los Angeles climate scientist, told Vox last month that it channels “more water than the Mississippi River.”

The last Pineapple Express hit the West Coast in late March, bringing torrents of rain that caused flooding and mudslides and triggered evacuations in Southern California.

Now, the National Weather Service is expecting up to 8 inches of rain through Saturday further north — in the Santa Clara Valley and parts of the San Francisco Bay Area. It’s also calling for 6 to 8 inches snow parts of the Sierra Nevada. The ongoing downpour has already caused accidents and spin-outs on freeways.

For California, which has suffered years of drought and is still reeling from its worst fire season ever, rain is a welcome relief.

But getting so much at once can be disastrous. The fire-charred landscape, denuded of vegetation, does little to retain water and is vulnerable to deadly mudslides. California has already seen 21 deaths from mudslides this year.

“The main concerns we have with this atmospheric river include moderate to heavy rain across the region, urban flooding and ponding of water on roads, flooding of creeks or streams, and also the possibility of mudslides and debris flows for recent burn areas,” said Scott Rowe, a meteorologist at the National Weather Service’s San Francisco Bay Area office, during a briefing.