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California seeing a ‘rainfall convoy’ thanks to atmospheric river. But how long will it last?

Los Angeles skyline shrouded in clouds on Wednesday during steady rain
Los Angeles skyline shrouded in clouds on Wednesday during steady rain.
(Irfan Khan/Los Angeles Times)

After a brief respite, soggy weather returned to Southern California on Wednesday thanks to an atmospheric river that unleashed heavy rain across the region.

Former Jet Propulsion Laboratory climatologist Bill Patzert described the storm activity in recent weeks as a “rainfall convoy” after eight dry months. But what does the rest of the winter hold?

Does all this rain mean we’ll have a wet winter?

That’s up for debate.

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Long-term models suggest the southern section of the state should expect overall warmer and drier conditions than average through March, according to a report from the National Interagency Fire Center. Officials with the National Oceanic and Atmospheric Administration also predict drier-than-normal weather from December through February across much of the state, save for a small portion of Northern California near the Oregon border.

Patzert, however, said the long-term models should be taken with a grain of salt. It’s a little too early to tell with much certainty what Mother Nature has in store for California this winter, he said.

“This atmospheric river is not necessarily a guaranteed precursor to rain this season. What we know for sure is November started out like a lamb and went out like a lion in the West.”

la-me-g-atmospheric-rivers-web
(Paul Duginski / Los Angeles Times)

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What is the forecast for the latest storm?

The storm now over the area, fed by a plume of subtropical water vapor at the lower and middle levels of the atmosphere, had already dumped plenty of precipitation across the Southland before sunrise. Rainfall rates are expected to pick up slightly through the morning before tapering off by the afternoon, said Andrew Rorke, a senior forecaster with the National Weather Service in Oxnard.

Rainfall totals of 1 to 2 inches are expected for much of the region, but the San Gabriel Mountains could see up to 3 inches of precipitation. Peak rainfall rates are expected to range from half an inch to nearly an inch per hour across Los Angeles County. There’s also a chance of thunderstorms, which could bring even higher rainfall rates.

How big were the Thanksgiving weekend storms?

The late-November storms dumped 2.12 inches of rain on downtown Los Angeles — more than the area had seen in that month since 2011, according to the National Weather Service. Long Beach got 2.86 inches of precipitation last month, while the Hollywood Burbank Airport received about 1.85 inches. Those areas also hadn’t seen as much rain in November since 2011, data show.

The Southland saw a series of storms in November 2011, including a cold front similar to the one that slammed the state on Thanksgiving Day, pouring rain across the state and blanketing the mountains in snow. It marked a promising start to the 2011-12 water year — which runs from October to September — but L.A. County saw below-average precipitation from January through the rest of the season.

la-me-atmospheric-rivers-11292019-01.jpg
(Paul Duginski / Los Angeles Times)

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What are atmospheric rivers?

Atmospheric rivers are concentrated streams of water vapor in the middle and lower levels of the atmosphere. They’re like a continuous conga line of moisture streaming across the ocean without interruption until they encounter an obstacle such as the coastal mountain ranges of California. These obstacles force the atmospheric rivers into higher altitudes, where they cool and start shedding their burden of moisture.

Some atmospheric rivers are weak and produce beneficial rain, and some are larger and more powerful, causing extreme rain, floods and mudslides.

On average, 30% to 50% of the West Coast’s annual precipitation comes from a few atmospheric rivers each year.

Atmospheric rivers are roughly 250 to 375 miles wide, and a strong one can transport as much as 7.5 times to 15 times the average amount of water that flows through the mouth of the Mississippi River.

Times staff writer Paul Duginski contributed to this report.


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