Pineapple express: An atmospheric river is set to drench Southern California Thursday


Carlos Barria

Two atmospheric rivers are lashing California this week.



CNN
 — 

The first of two atmospheric rivers expected to lash Southern California will strike the region Thursday, bringing gusty winds and heavy rainfall that could lead to flooding in some areas.

More than 20 million people across California are under flood alerts Thursday as storms threaten flash flooding in cities including San Diego, Sacramento, San Francisco, San Jose and Oakland. The threat will last into Friday morning for central and Southern California.

And while there may be a brief respite after Thursday’s storm drenches Southern California, another atmospheric river – likely stronger – is poised to move across the region beginning Sunday.

Rainy conditions are expected to continue well into next month as a more typical El Niño pattern kicks into gear.

El Niño – a natural phenomenon in the tropical Pacific that influences weather around the globe – causes changes in the jet stream that can point storms directly at California. Storms can also tap into an extra-potent supply of moisture from the tropics called an atmospheric river.


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On Wednesday, the first of two atmospheric river storms slammed into Northern California. As the storm began to shift south, central California saw steady rain begin Wednesday afternoon. This will continue into Thursday.

Areas north of San Luis Obispo to the California-Oregon state line, which includes the Bay Area, are expected to see the heaviest rainfall through the overnight hours.

By Thursday morning, steady rainfall and periods of stronger winds are expected to begin impacting Southern California. Much cooler air will also begin to overspread the state.

Rainfall of 1 to 4 inches is possible in the southern part of the state, falling at rates that could exceed 1 inch per hour.

A Level 2 out of 4 risk of excessive rainfall is in place Thursday for Southern California as the rain moves south, according to the Weather Prediction Center. Roads and low-lying areas are at the most risk for flooding, and rises on some waterways are also possible.

Flood watches across central California are expected to remain in effect through Thursday evening. Rainfall totals of 1 to 3 inches are possible, with isolated totals exceeding 4 inches possible.

Meanwhile, the western half of California could see a few thunderstorms that may produce bursts of heavier rain. This would come after a series of torrential thunderstorms wreaked havoc on San Diego last week.

To respond to potential impacts, California officials are readying hundreds of crew members from multiple agencies to respond to potential calls for rescue, according to a Tuesday news release from the governor’s office.

“The state is working around the clock with our local partners to deploy life-saving equipment and resources statewide,” California Gov. Gavin Newsom said.

California’s Office of Emergency Services said it has prepared swift water personnel and equipment in 12 counties to respond when needed.

More than 400 personnel have been prepositioned across 16 California counties, according to the state’s office of emergency services.

Farther north, officials are also preparing to respond to a bout of wintery weather conditions.

More snow is expected to accumulate at lower elevations across parts of Northern California and the Sierra Nevada on Thursday as cooler temperatures move in. Feet of snow could bury the highest peaks of the Sierras, with at least 6 inches of snow possible for some lower-elevation mountain roadways.

Winter storm warnings remain in effect for the Sierra Nevada, where up to 4 feet of snow are possible in some higher elevations.

The snow is welcomed for California’s snowpack, which has been beleaguered by warmth and storms that have brought more rain than snow. This winter’s snowpack is just 52% of average for this time of year, according to the latest survey conducted by the state’s Department of Water Resources Tuesday. Snowpack is a vital water source, and the survey helps California to forecast how much water will be available for the remainder of the year.

Friday will bring showery weather that’s expected to linger over much of California as moisture slowly pushes out and across the Southwest.

Then on Sunday, another more potent atmospheric river-fueled storm is poised to lash Southern California. That could become the “largest storm of the season,” the National Weather Service in Los Angeles warned.

“It is very likely that this will be a serious 2 to 3 day storm system,” meteorologists there said.

While more details on this storm are still coming into focus, forecast models show a more widespread and prolonged flood threat, especially for Southern California. At the very least, another few days of rain and snow are likely across the rest of the state.

The second storm could potentially stall over the region and unload several inches of rain from Sunday to about midweek.

Temperatures are also likely to start out much cooler with this storm than with the first storm, with more snow possible down to mountain pass levels or potentially even lower elevations.





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