CNN
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A second and more powerful atmospheric river is set to bring potentially life-threatening flooding across Southern California on Sunday after an earlier storm already unleashed record rainfall in some parts of the region.

Round two of atmospheric rivers hitting California within days could begin lashing the central and southern regions as early as late Saturday night and linger for several days.

Forecasts show Southern California, including the greater Los Angeles area, is expected to bear the brunt of this storm, with the worst flooding expected to begin Sunday.

“Chances are increasing that a lengthy period of heavy rain will develop late Saturday night and continue through most of Sunday and Monday,” the National Weather Service in Los Angeles warned. “This storm could end up generating unprecedented amounts of rain across a widespread area!”

More than a month’s worth of rain could fall across much of Southern California, with widespread rainfall totals of 3 to 6 inches expected from late Saturday to Wednesday, the weather service said. That would be in addition to the torrential rain the region saw Thursday, when the first strong atmospheric river unloaded record rainfall in several areas and triggered water rescues.

This weekend’s atmospheric river – a long, narrow moisture band that carries saturated air thousands of miles then discharges it like a fire hose – is projected to be even more powerful and stall over land, soaking many of the same areas that are already saturated from Thursday’s potent storm.

Periods of moderate to heavy rainfall could be nearly continuous for up to 48 hours, with Sunday and Monday dumping the most dangerous rainfall.

The projected deluge threatens urban flooding, mud and rock slides, flash and river flooding. Making matters worse is that soils are already saturated from Thursday’s storm, and another round of heavy rainfall elevates the risk of debris flows in Southern California.

“With the ground already saturated from today’s rain, onset of dangerous flash flooding will be much quicker with this next event and everyone, especially those near or in south facing mountains, needs to start preparing now for possible evacuations during or even before the storm hits,” the National Weather Service warned Thursday.

Beginning Sunday, a Level 3 of 4 risk of excessive rainfall is in place for coastal sections of Central and Southern California, including the Santa Barbara Area, and a Level 2 of 4 of the same risk exists along the coast from north of San Francisco to Los Angeles, according to the Weather Prediction Center.

The threat of excessive rain will continue into Monday for Santa Barbara and will expand to also include Los Angeles.

Overall, 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.

For Southern California, the second atmospheric river is expected to impact the region from late Saturday through Wednesday with thunderstorms potentially bringing heavy downpours of rain, the National Weather Service warned.

“Heavy rain expected with potential for damaging, life-threatening flooding,” the National Weather Service in Los Angeles said Thursday, urging people to prepare.

Southern Californians should be ready for potential evacuations near rivers and creeks and use sandbags to protect vulnerable areas, the weather service said.

Meanwhile, mountain and foothill areas in Southern California could see up to a foot of rain from the atmospheric river, according to the National Weather Service.

Mountain travel is also discouraged as heavy snow is expected, with the Sierra Nevada forecast to see up to 4 feet of snowfall.

“Residents might be stranded for several days, get extra supplies & gas,” the weather service warned.

The ominous forecast comes as Southern California reels from the round of heavy rain it endured Thursday from an earlier atmospheric river. Parts of the Pacific Coast Highway and the 710 Freeway closed as first responders carried out water rescues.

Long Beach – south of downtown Los Angeles – was heavily flooded, with social media videos showing roadways entirely underwater. Fire crews rescued some people from a few submerged vehicles, officials said, though no injuries were reported.

One person was hospitalized in stable condition after he was rescued in nearby Orange County where he became trapped in a storm channel that was inundated by heavy rainfall, fire officials said.

Isolated storms through the overnight hours brought isolated hail rain and even pea sized hail to Long Beach and the Central Valley.

Thursday’s deluge also dumped 2.37 inches of rain in Los Angeles Airport, breaking the daily record of 1.55 inches set in 1960. The airport’s average rainfall for February is 2.99 inches, meaning the station received nearly its monthly rainfall for February on the first day of the month.

Meanwhile, Santa Barbara Airport saw 2.93 inches of rainfall, exceeding the daily record of 2.02 inches set in 1960. Long Beach Airport saw 2.43 inches of rainfall, surpassing its 1960 daily record of 1.45 inches.