Biggest storm of winter brewing for Southern California

A potent storm will drop southward along the Pacific coast and hit California hard with torrential rain, heavy snow and widespread travel disruptions late this week, AccuWeather meteorologists warn. Impacts will be felt statewide, but the most significant impacts are forecast for Southern California.

The leftovers of the massive cross-country storm that began in the West on Monday will continue to affect California into Wednesday evening with sporadic low-elevation rain showers and areas of mountain snow.

The new storm will spread heavy rain southward as snow levels plunge from north to south at the same time, beginning on Wednesday in Northern California and in Southern California on Thursday.

"Several inches of rain are likely to fall along the south- and west-facing lower elevations of the Coast Ranges in Southern California during the second half of this week, with the heaviest rains [expected] from Thursday night through Friday night," AccuWeather Senior Meteorologist Heather Zehr said.

A winter storm makes a dramatic approach to the Santa Clarita Valley in Southern California Wednesday, Feb. 22, 2023. (Photo by David Crane/MediaNews Group/Los Angeles Daily News via Getty Images)

A vehicle is crushed by a tree downed by high winds in San Francisco, Wednesday, Feb. 22, 2023. (AP Photo/Godofredo A. Vásquez)

An AccuWeather StormMax™ rainfall of 12 inches is possible with the storm from Thursday to Saturday in the wettest locations of coastal Southern California. That amount of rain is not likely to fall on Los Angeles or San Diego, but this single storm could potentially deliver a month's worth of rain and possibly two times that amount or more in some locations, AccuWeather forecasters say. The historical average for February in downtown Los Angeles during February is 3.66 inches, while San Diego's average is 2.20 inches.

Not only will the intense rain lead to travel slowdowns, but it can also trigger significant urban and flash flooding and set into motion mudslides and other debris flows.

Temperatures will drop significantly with this storm, and accumulating snow is likely to fall in elevations as low as 2,000 feet on average from Thursday to Saturday, according to Zehr.

Since the snow around the 2,000 to 3,500-foot level will tend to be wet and clinging in nature, it could weigh down trees and potentially trigger widespread power outages in Southern California. Above 4,000 feet in elevation, 1-2 feet of snow may pile up. The highest ridges and peaks in the region may pick up 2-4 feet of snow.

An extremely rare blizzard warning has been issued by the National Weather Service for the mountains north of Los Angeles from Friday to Saturday. AccuWeather meteorologists believe the worst conditions over the passes will be from Friday night to Saturday as snow to start Thursday night may change to rain for a time during Friday midday.

"Over a foot of snow may pile up over the Grapevine, and that is likely to lead to the closure of the passes in Southern California at times from Thursday to Saturday, AccuWeather Chief On-Air Meteorologist Bernie Rayno said. Tejon Pass is along Interstate 5 and Cajon Pass is along I-15. Motorists that choose to venture over the passes during the height of the storm will run the risk of becoming stranded.

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The worst conditions from heavy snow and rain in Southern California are likely to be from Friday to Saturday, when the storm will be at its peak and closest proximity

"It is conceivable that travel may be impossible in coastal areas and in the mountains of Southern California from Friday to Saturday," Rayno said. "This would be due to areas of high water at low elevations around Los Angeles and San Diego and from road-clogging snow in the mountains and the upper deserts."

Airline passengers should be prepared for lengthy delays and the possibility of flight cancellations to and from airports in the region, which can have ripple-effect impacts across the nation.

Farther north, the San Francisco Bay Area will be thoroughly soaked by the storm from Thursday to Friday but not to the extent of areas from the San Gabriel Mountains and areas farther south. However, forecasters say some travel delays are likely on area roads and at the airports in the region.

As chilly air rotates in with the storm, snow levels can dip to around 1,000 feet in Central California. At this elevation and above is where accumulating snow is most likely to begin. Snow levels will be substantially lower and may dip to a few hundred feet above sea level north of San Francisco. There is a risk that enough clinging snow falls between 1,000 and 2,000 feet to cause tree limbs to break and power lines to come down. Some observant people may notice wet snowflakes mixed in with the rain from Thursday evening to early Friday over the hilly locations around the Bay Area.

Despite the storm focusing most of its efforts on Southern California, it will still manage to unload heavy snow throughout much of the Sierra Nevada. A general 1-3 feet of snow will fall with the likelihood of major delays and the possibility of road closures along stretches of Interstate 80 through the high country into Saturday.

Soaking rain is in store for the deserts in the Southwest from the late-week storm. Enough rain may fall around Las Vegas to lead to urban flooding from Thursday to Friday night and perhaps Saturday. Even Phoenix could experience minor flooding problems this weekend.

The desert rain will follow a vast storm that will produce snow along a 2,600-mile-long swath from the coastal Northwest to Down East Maine.

Widespread travel problems will extend across the West from the Cascades and the Sierra Nevada through the Wasatch Range and the Rockies into Wednesday night. Motorists should be prepared for road closures over the mountains and through the passes, experts say.

AccuWeather meteorologists are already eyeing the next batch of storms forecast to arrive from the Pacific Ocean.

"Yet another storm, or series of storms, will begin to move southward along the Pacific coast early next week with the first rounds of rain and mountain snow to spread into Northern California beginning on Monday or Tuesday of next week," Rayno said. The storms are likely to push southward as next week progresses.

"The storm poised to impact California in earnest from Thursday to Saturday, and others to follow into early March, have the potential to wipe out existing drought conditions in the southern part of the state," AccuWeather Senior Meteorologist Joe Lundberg said.

Drought conditions have substantially improved over California thanks to substantial and frequent storms from December through January. However, even though areas of extreme to exceptional drought have been wiped out, abnormally dry to severe long-term drought conditions remain, according to the United States Drought Monitor.

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