It could rain for days in Southern California starting on Saturday afternoon, potentially in record amounts, creating the risk of what the National Weather Service described as “life-threatening flooding” Sunday into Monday.
The storm system is also expected to bring several feet of snow to the Sierra Nevada, and powerful onshore winds and intense, damaging surf along the California coast.
Here is what to know.
The effects of this system will begin to be felt in California on Saturday afternoon, and will last through Tuesday.
The heaviest rainfall is likely south of the Bay Area, and excessive rainfall is most likely from Santa Barbara to Los Angeles.
There could be dangerous flash flooding in places. The Weather Service warned that, starting on Sunday, the Los Angeles River “will fill quickly and become a raging river and a very dangerous place to be.”
There is an extremely high chance — over 90 percent — of at least two feet of snow, especially above 6,000 feet in the Sierra Nevada, causing difficult to impossible driving conditions Sunday and Monday.
This atmospheric river will be stronger than the last two.
This storm will connect to an atmospheric river, a stream of moisture in the sky that is typically a couple of hundred miles wide and can be seen on satellite imagery.
It will carry an abundant amount of moisture from the tropics near Hawaii and as it makes landfall, the mountains will wring the moisture out of the air like a sopping wet towel, causing precipitation to fall in record amounts.
The angle and orientation of the mountains can have a big effect on how much precipitation can be created from an atmospheric river, and this latest system will have a southwest to southerly orientation, which will hit the Transverse Ranges from Santa Barbara to Los Angeles at the perfect angle to create a worst-case outcome.
Making matters worse are the higher ocean temperatures just off the coast.
“Anytime you have these rainfall amounts intersecting with terrain and the built environment, you’re going to have trouble,” said Greg Carbin, the branch chief of forecast operations for the National Weather Service.
Some of the heaviest rainfall may reach one- and two-day daily records, Mr. Carbin said.
Up to a foot of rain could fall.
This storm could generate “unprecedented” amounts of rain across a widespread area — up to a foot in some locations in less than two days, the Weather Service said.
It has the potential, forecasters said, to rival or exceed the atmospheric river event that led to people being rescued from flash flooding in Los Angeles last winter.
The office of Gov. Gavin Newsom of California said late on Friday that it was mobilizing 8,300 emergency responders and 21 swift water rescue teams in preparation for the storm.
Also on Friday, Santa Barbara County issued an evacuation warning for some residents living near waterways and areas scarred by wildfires.
The Weather Service in Los Angeles advised people to avoid driving from Sunday through Tuesday.
This storm is expected to be even stronger than the atmospheric river that hit the same region earlier this week, and considerably stronger than the very weak storm that sent a torrent of water through San Diego last week. That storm was enhanced because the winds swept over higher-than-average ocean temperatures.
In issuing its stark warning about the Los Angeles River, the Weather Service said that flooding was likely in urban areas and other places where poor drainage is prevalent.
The higher-than-usual temperatures in the waters of the North Pacific will continue to be a factor with this atmospheric river.
The warmer ocean means more evaporation can take place in the lower atmosphere, which could translate into more rainfall onshore, Daniel Swain, a climate scientist at the University of California, Los Angeles, said on social media.
Martin Ralph, the director of the Center for Western Weather and Water Extremes, said that “this atmospheric river will hit the area with winds nearly straight into the mountains from Santa Barbara to L.A., and it will cross exceptionally warm coastal waters, adding moisture and heat that can increase rainfall on land.”
The atmospheric river alone would be problematic, but add the higher ocean temperatures, the right angle to the mountains, and the timing, on the heels of another storm system, it causes meteorologists to be concerned and use stronger than usual language, like “a very dangerous situation.”
Mr. Carbin said he believed that before the weekend is out, there will be reports of mudslides, landslides and road closures.
Not everything, including exact locations, is known for sure.
As confident as forecasters are that this could become a major storm, there is still some hesitation in pinpointing exactly where the heaviest rain will fall.
“It’s still possible for some shifts as the forecast comes into focus,” said Alex Lamers, a warning coordination meteorologist at the Weather Service.
There is one situation in which the main plume of heavy rainfall could stay over Santa Barbara and Ventura Counties, increasing the risk of serious flooding in those areas and limiting the amount of rain that falls in Los Angeles County.
Rainfall totals will likely average three to six inches in most coastal and valley areas, and six to 12 inches in the foothills and mountains.
“Even if urban areas don’t necessarily see the higher totals,” Mr. Lamers said, “intense bursts of rainfall can generate significant flooding impacts, given the amount of paved ground.”
Raymond Zhong and Emily Schmall contributed reporting.