California braces for back-to-back atmospheric river storms

[January 31, 2024]  By Steve Gorman

LOS ANGELES (Reuters) - California braced for a pair of approaching Pacific storms expected to drench much of the state in heavy rains that could trigger widespread flooding while also helping bolster longer-term fresh water supplies.

The first of the back-to-back storms, both products of vast airborne currents of dense moisture called atmospheric rivers, was expected to hit Northern California on Wednesday and sweep over the Southern California coast on Thursday.

The two systems also fit the definition of a "Pineapple Express," Pacific storms originating from the warm, subtropical waters around the Hawaiian islands, according to Daniel Swain, a climate scientists and meteorologist for the University of California, Los Angeles.

The first storm will mark an abrupt change in the weather for California, which like much of the West has been basking in record, mid-winter warmth since the weekend.

The initial storm will probably hit hardest in the San Francisco Bay area on Wednesday evening, Swain told reporters on Tuesday. The precipitation will fall mostly as rain, with snow in high-elevation mountains.

The National Weather Service posted a flood watch for the Bay area and California's Central Coast, along with high-wind advisories for the region.

Some roadways and streams could flood in Southern California on Thursday, though major bouts of inundation are less likely, according to Swain. Heavy to locally very heavy rains from the system could linger over parts of Northern California for six to 12 hours, he added.

The second, potentially more powerful storm is forecast to hit California on Sunday, bringing strong, gusty winds to the north and much heavier downpours in the south, while dumping yet more snow in the mountains.

City workers begin clearing debris along a drainage canal that overflowed causing flooding in adjacent neighborhoods after a heavy rain storm in San Diego, California, U.S. January 26, 2024. REUTERS/Mike Blake/File Photo

While much about the second storm's trajectory remains uncertain, Swain said it appeared to be packing a denser plume of moisture capable of unleashing heavier, more sustained rainfall.

"Suffice it to say there will be some flooding in Southern California," Swain said. "The question is whether it is the unremarkable street flooding we see in any big rainstorm or something considerably more significant than that."

San Diego County endured record-breaking rainfall and severe flash flooding from a more localized storm last week, and parts of Ventura County were evacuated after a month's worth of rain fell in just one hour in December.

A series of about a dozen atmospheric river storms lashed California in rapid succession last winter, causing mass evacuations, power outages, levee breaches and road closures in a state long preoccupied with drought and wildfires. At least 20 people perished in those storms, which carried the upside of helping break the grip of years-long drought in California.

The latest storms are expected to help improve the state's water supply picture, which is lagging at below-average levels so far this winter.

(Reporting by Steve Gorman in Los Angeles; Editing by Lincoln Feast.)

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