An atmospheric river is drenching Southern California today before another sweeps in over the weekend

CNNThe first of two atmospheric rivers is unloading heavy snow at high elevations and gusty winds and soaking rainfall Thursday across a large part of California.

More than 20 million people are under flood alerts as storms threaten flash flooding in cities including Los Angeles, 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 February 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.

On Wednesday, the first of two atmospheric river storms slammed into Northern California.

As the storm shifted south through the afternoon and overnight hours, much of central California saw steady rain. Rain will continue Thursday across central California, with the greatest chances for heavy rainfall shifting into Southern California early Thursday morning.

Steady rainfall and periods of stronger winds will impact Southern California into at least early Friday. Much cooler air will also begin to overspread the state during this time.

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. 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 will remain in effect through Thursday night. An additional inch of rain is possible across the region Thursday. The coastal ranges picked up 2 to 4 inches of rain through Wednesday night, while valley areas recorded 0.5 to 1.5 inches.

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.

San Diego officials warned of potential evacuations in flood prone areas, including some that flooded last week.

“This warning is voluntary. It is designed to encourage residents in these flood-prone communities to prepare if, or when, an evacuation order does become necessary,” San Diego Mayor Todd Gloria said in a Tuesday news release.

Swift water personnel and equipment are ready to respond in 12 California counties, with 400 personnel prepped across 16 counties, the state Office of Emergency Services said.

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

Snow is also a threat

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. Up to 4 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.

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 Tuesday by the state’s Department of Water Resources. 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.

‘Largest storm of the season’ to begin this weekend

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.