Major Southern California storm threatens flooding, but may eliminate fire risk

Evacuation warnings are set to take effect across Los Angeles County Thursday evening as an atmospheric river approaches Southern California, potentially ending fire season early and bringing new risks of flooding and mudslides.

According to the National Weather Service, downtown Los Angeles could see 2.62 inches of rain Thursday through Sunday under what forecasters consider the most likely storm scenario. However, forecasts remain unstable and highly variable.

There is about a quarter chance of a lower but still significant precipitation amount of about 1.39 inches, but an equal chance of a more intense 4.81 inches of precipitation during the same time period.

Regardless, when it comes to fire season, “it’s definitely going to help,” said Ryan Kittle, a meteorologist with the National Weather Service office in Oxnard.

“If we do end up getting the expected rainfall, it will certainly bring us closer to the end of the fire season,” and give Southern California a real buffer against potential Santa Ana wind events, Kittle said.

That’s a far cry from last year, when Southern California suffered a record drought in the fall and winter that left vegetation wilted and ready to burn. These “extraordinary” conditions, coupled with unusually erratic Santa Ana winds, helped fuel the rapid spread of the Eaton and Palisades fires, which were among the deadliest and most destructive fires in California history.

Meteorologists say widespread rainfall of 3 to 4 inches is needed in lower elevations of Southern California to end the high fire season. Downtown Los Angeles has received 1.41 inches of rain, almost all from one stormy day in October.

Last year, only 0.07 inches of rain fell in downtown Los Angeles from early October to mid-November. By Jan. 7—the day the Eaton and Palisade fires raged—the cumulative thickness downtown had reached just 0.16 inches. It wasn’t until February, after fires had devastated Altadena, Pacific Palisades and Malibu, that the region received enough rain to bring the fire season to an end.

But the silver lining to the coming storm also comes with a hint of grey. Such heavy rainfall could trigger damaging flows of mud and debris.

An evacuation warning is in effect for areas near recent burn scars from 6 p.m. Thursday to 11 a.m. Sunday due to the risk of mudslides. This includes areas near the Palisades, Eaton, Kenneth, Sunset and Hurst fires that burned during the January storm.

There are many reasons why California has faced unusual fire risks in recent years. Extreme heat, exacerbated by climate change, causes shrubs and grasses to wilt. Residential development has increased in fire-prone wildlands. Electrical infrastructure and a failure to extinguish fires caused by arson are suspected to be to blame for recent devastating fires.

All of this adds to California’s already delicate year-end dance between Santa Ana winds, whose intensity and heat have sparked many fires, and the arrival of the rainy season.

With the return of La Niña conditions in the central and eastern Pacific, expectations for this fall are considered grim. Californians generally believe that La Niña, a natural weather pattern involving cooler ocean surface temperatures, is a harbinger of drought, while El Niño is synonymous with wet winters.

There is a reason for this. In 1982-83, California experienced heavy rains during a severe El Niño. The other occurred in 1997-98, which saw massive flooding across the West Coast and the wettest February on record for Los Angeles. This was followed by a strong La Niña in 1998-99, which was very dry.

Most recently, 2020-21 and 2021-22 were both La Niña seasons, with downtown Los Angeles being drier than normal.

But using La Niña as the only crystal ball can be very disappointing. As meteorologist Jan Null points out, California’s two costliest flood seasons — 2016-17 and 2022-23 — both occurred during La Niña conditions.

This week’s rain is not a clear indication that the remainder of this fall and winter will be particularly wet. Kittel said some La Niñas start out wet, but then the storm doors suddenly close.

Uncertainty remains over the intensity of the storm in Southern California, although rainfall totals are expected to have been rising as the storm approaches.

Forecasters expect two peaks in the Los Angeles area — Thursday night into early Friday, and Saturday, Kittle said. But there is still a chance of rain during the day and night on Friday.

The best chance of rain in Ventura County is Thursday night through Saturday night, and the best chance of rain in Los Angeles County is Friday morning into Saturday night. There is also a 10% to 20% chance of thunderstorms Friday night and Saturday daytime, with a small risk of locally damaging winds or even tornadoes, the weather service said.

“If we get these higher-end conditions on Saturday, we wouldn’t be surprised if we hear a report of a tornado or two,” Kittle said.

In December, a five-minute tornado struck Santa Cruz County, injuring three people, knocking down trees and power poles, snapping branches, overturning vehicles and damaging street signs.

For Orange, Inland Empire and San Diego counties, moderate rainfall is expected on Friday, with the heaviest rainfall possible on Saturday. There is a 10 to 20 percent chance of thunderstorms across the region on Friday and a 20 to 30 percent chance on Saturday.

By Sunday, Long Beach is most likely to see 2.38 inches of rain; Redondo Beach, 2.48; Oxnard, 2.49; Thousand Oaks, 2.63; Santa Clarita, 2.77; Covina, 2.89; and Santa Barbara, 3.59.

San Diego could see 1.5 to 2 inches of rain; Riverside, San Bernardino and Escondido, 2 to 2.5 inches; San Clemente, 2.5 to 3 inches; and Anaheim and Irvine, according to the weather service, could see 3 to 4 inches.

Even deserts can bring impressive amounts of rainfall. Palm Springs could see 1 to 1.5 inches of rain, and Joshua Tree National Park could see 1.5 to 2 inches of rain.

Saturday is particularly concerning because of the high degree of uncertainty in the forecast. Therefore, the risk of major impacts, including dangerous flooding, is highest on this day.

“Uncertainty is unusually high,” said Dave Munyan, a forecaster at the weather service’s San Diego office.

One worrisome scenario, Kittel said, is a storm off the California coast, which would produce “a couple of hours of fairly stable, moist southeastern flow, but with quite a bit of instability.”

On Saturday, “there is the potential for heavy rains that could cause flash flooding and/or mudslides,” Kittle said.

There’s also a chance of gusty winds coming from the south, southeast and east, which could cause delays at LAX.

However, Saturday could prove to be a bit of a failure. Because the system will be part of what’s known as a “low-pressure cutoff,” in which a low-pressure system is cut off from the jet stream, “it’s just going to spin like a top and go where it wants to go — it’s hard to predict,” Kittel said.

One scenario, he said, involves a large amount of low pressure spinning away from the coast and “actually producing almost no rainfall in our area.”

The cutoff low system is “very unstable and finicky, and any slight fluctuation in the path of the low and where it tracks will determine how much rainfall we can get and how long we can see that precipitation,” Munyan said.

Kittle said road flooding and significant traffic delays are expected on Saturday. If rainfall totals are higher than forecast, rapid-water rescues may be needed when people become trapped in flood control channels or streams, Kittle said.

Meteorologists expect “at least minor mudslides, canyon rocks and debris flows,” especially in recently burned areas, Kittel said. Peak rainfall rates are expected to be between 0.25 and 0.5 inches per hour, with up to 1 inch per hour.

Mudslides and debris flows, a type of landslide in which rainwater can pick up dirt and other objects as it flows rapidly down a hillside, can be triggered by as little as half an inch of rain per hour, Kittle said.

Kittel said the risk of damaging mudslides is low on Thursday and Friday, “but on Saturday, the risk is there,” especially if rainfall totals are higher than expected. He added that there was about a 20 to 30 percent chance of a serious mudslide on Saturday.

Topanga Canyon Drive between Pacific Coast Highway and Grand View Drive will be closed Thursday at 10 p.m. due to the high possibility of severe mudslides, according to the California Department of Transportation. The road is expected to remain closed during the Friday morning commute and possibly through the weekend.

The storm is not expected to produce widespread snow accumulation in the Southern California mountains. In the San Bernardino Mountains, most snowfall occurs on the highest peaks, which are about 10,000 feet or higher.

In the Sierra Nevada, snowfall levels around Tahoe and Mono County are expected to drop to around 8,000 feet in elevation Thursday night into Friday morning. Mammoth Mountain pushed its opening date to Saturday because of the storm, but Tahoe-area ski resorts Heavenly and Northstar are scheduled to open Nov. 21; and Palisades Tahoe, Nov. 26.

For those traveling through the Sierra Pass in Mono County, forecasters warn that about 6 inches of snow is expected and travelers should prepare for delays or road closures. In Yosemite National Park, up to 6 inches of snow could accumulate Thursday at elevations as high as 8,000 feet.

Forecasters said the storm was expected to hit the San Francisco Bay Area and Sacramento Valley starting Wednesday night. The most severe impacts are expected to be strong winds, with gusts potentially reaching 50 to 60 mph. Some tree damage may occur and strong winds may make driving difficult for tall vehicles. Minor urban and small stream flooding is possible.

San Francisco and San Jose could see 1 to 1.5 inches of rain Wednesday through Friday, and Sacramento, Santa Cruz and wine country could see 1.5 to 2 inches of rain.

Looking ahead to next week, there is a chance of rain in Southern California on Monday and Thursday. But for now, meteorologists expect the storms to have relatively little impact.