Evacuation warnings were issued across Los Angeles County on Thursday evening as an atmospheric river approached Southern California, bringing with it the potential to put an early end to fire season while also bringing fresh risks of flooding and mudslides.
Under the storm scenario deemed most likely by forecasters, downtown L.A. would see 2.62 inches of rain Friday morning through Sunday, according to the National Weather Service. That could be enough to flood roads, swell rivers and creeks and cause minor shallow debris flows.
Rain of that extent would also make this L.A.’s wettest November in 40 years. And there’s a 30% chance of even higher precipitation — 4.81 inches over the same time period for downtown L.A.
That would raise the possibility of significant mud and debris flows, as well as localized flooding of homes and businesses.
There’s also a 10% chance that just about 1.4 inches of rain could fall downtown, which would result in only minor problems.
The day of highest risk is Saturday. For that day, “the main message is that there’s still a pretty substantial potential for heavy rain over parts of our area, with a reasonable risk for flash flooding and debris flows,” said Ryan Kittell, a meteorologist with the National Weather Service office in Oxnard. Debris flow are a type of landslide in which rainfall combines with mud and debris to move with destructive potential off hillsides.
However, as far as fire season is concerned, “this will certainly help,” Kittell said.
“If we do end up getting the rainfall that we expect, this will certainly get us close to the end of the fire season,” he said, and give Southern California a real buffer against a potential Santa Ana wind event.
That would be a far cry from last year, when Southern California grappled with a record dry streak over the fall and winter that left vegetation withered and primed to burn. Those “off the charts” conditions, combined with exceptionally erratic Santa Ana winds, fueled the rapid spread of the Eaton and Palisades fires, which rank among the deadliest and most destructive in California history.
Meteorologists say that Southern California needs 3 to 4 inches of widespread rain in the lower elevations for the high fire season to end. Downtown L.A. has already received 1.41 inches, almost all of it from just one stormy day in October.
Last year, downtown L.A. saw just 0.07 inches of rain from the start of October through mid-November. By Jan. 7 — the day the Eaton and Palisades fires roared to life — downtown had received only a cumulative 0.16 inches. It wouldn’t be until February, well after the blazes had devastated Altadena, Pacific Palisades and Malibu, that the region saw enough rain to bring the fire season to a close.
But the silver lining of the upcoming storm also carries a touch of gray. Rain of this magnitude could trigger damaging flows of mud and debris.
Evacuation warnings are through 11 a.m. Sunday in areas near recent burn scars, due to the risk of mud and debris flows. This includes areas near the Palisades, Eaton, Kenneth, Sunset and Hurst fires that burned in January.
There are many reasons California has faced extraordinary fire risk in recent years. Climate change has fueled increasingly extreme heat, withering shrubs and grasses. Residential development has increased in fire-prone wildlands. Power infrastructure and the failure to extinguish an arson-caused fire have been the suspected culprits behind recent devastating blazes.
All this heightens the stakes of California’s already delicate year-end dance between the Santa Ana winds — the strength and heat of which have turbocharged many a fire — and the arrival of the rainy season.
The expectation for this autumn was considered grim with the return of La Niña in the central and eastern Pacific Ocean. Californians commonly think of La Niña, a natural climate pattern involving cooler sea surface temperatures, as a herald of drought, and the sibling El Niño pattern as synonymous with wet winters.
There’s a reason for that. California saw epic rains during a significant El Niño in 1982-83. Another in 1997-98 coincided with massive flooding all over the West Coast, as well as L.A.’s wettest February on record. That was followed by a strong La Niña in 1998-99, which was very dry.
More recently, 2020-21 and 2021-22 were La Niña seasons, and were drier than normal for downtown Los Angeles.
But using La Niña as your only crystal ball can prove spectacularly disappointing. As noted by meteorologist Jan Null, two of California’s costliest flood seasons — 2016-17 and 2022-23 — were during La Niñas.
This week’s rains are not a definitive sign that the rest of autumn and the winter will be especially soggy. A number of La Niñas have gotten off to wet starts only for the storm door to suddenly shut, according to Kittell.
There continues to be uncertainty regarding how intense this storm will be for Southern California, though expected rainfall totals have been rising as the system approaches. A review of more than 100 different computer projections show most suggest a moderate-to-heavier rain scenario. But because the Saturday storm will come from a “cut-off low,” which is cut off from the jet stream, the storm is expected to spin like a top — making its path inherently unpredictable.
We “can’t completely discount this thing pulling off the coast a little bit and resulting in less rain,” Kittell said.
What is more certain is that forecasters expect two peaks of the storm. In L.A. County, the first peak is expected Friday during the day, but will probably bring mostly beneficial rain. Downtown L.A., for instance, is expected to receive 0.72 inches on Friday.
The more concerning peak is Friday night through Saturday. Saturday is the “period of particular concern,” Kittell said. “There’s a lot of indicators that have spinning thunderstorms, which can produce either damaging rain or tornadoes.”
As a result, Kittell said Southern California would be under a severe weather threat Saturday, with the period of most concern between midnight and 9 p.m.
“While 99% of the area will not see such conditions, any portion of our area, especially in the coastal and valley areas, could see it,” Kittell said. “Consider changing any plans that you might have for Saturday. Stay home and indoors, and it’ll protect you from all these hazards.”
“If you hear thunder or see lightning or rapidly changing winds, no need to panic. Just go inside. Stay away from the windows, and you should be fine,” Kittell said. “And then, likewise, with the flood situation, if you are out on the roads and experience a flooded route — find a different route.”
For Orange County, the Inland Empire and San Diego County, the heaviest rainfall is also expected late Friday through Saturday.
A flood watch will be in effect across a huge swath of Southern California on Saturday between 4 a.m. and 10 p.m.
Through Sunday, the most likely scenario is for Long Beach to receive 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, 4.21.
San Diego could get 2 to 2.5 inches of rain; Riverside, San Bernardino, Escondido, and San Clemente, 2.5 to 3 inches; and Anaheim and Irvine, 3 to 4 inches, according to the weather service.
Even the deserts could tally impressive rainfall. Palm Springs may get 1 to 1.5 inches of rain, and Joshua Tree National Park could receive 1.5 to 2 inches.
Peak rainfall rates on Saturday for Southern California could be between 0.75 and 1.25 inches per hour.
“It’s just not certain where that swath, that line, that band is going to center itself. If you look at all of our projections, it’s really all over the map between L.A. County, Ventura County, Santa Barbara County,” Kittell said.
“Wherever that line focuses in, we’re looking for around an inch-per-hour rates over pretty wide, larger-than-normal area,” while areas farther away from that line will get more like half an inch of rain per hour, Kittell said.
Mudflows and debris flows can be triggered in rains of half an inch per hour, Kittell said.
Topanga Canyon Boulevard between Pacific Coast Highway and Grand View Drive will close at 10 p.m. Thursday due to the high potential for heavy debris flows, according to the California Department of Transportation. The roadway is expected to remain closed through the Friday morning commute and potentially through the weekend.
Major snow accumulation is not expected in Southern California’s mountains with this storm. In the San Bernardino Mountains, most snowfall will be just on the highest peaks, around 10,000 feet above sea level or higher.
In Sierra Nevada, snow levels are expected to fall to around 8,000 feet above sea level around Tahoe and in Mono County from Thursday night into Friday morning. Mammoth Mountain is moving up its opening date to Saturday because of the storm, but Tahoe-area ski resorts Heavenly and Northstar are set to open Nov. 21; and Palisades Tahoe, Nov. 26.
For those traveling through Sierra passes in Mono County, forecasters warned that about 6 inches of snow is expected to fall, and travelers should be prepared for delays or road closures. At Yosemite National Park, up to 6 inches of snow could accumulate at elevations 8,000 feet above sea level on Thursday.
The storm began hitting the San Francisco Bay Area and the Sacramento Valley Thursday. The most significant impact was expected to be strong winds, with possible gusts in the range of 50 to 60 mph. Some tree damage is possible, and strong winds may make driving difficult for high-profile vehicles. Minor urban and small stream flooding is possible.
San Francisco and San José could get 1 to 1.5 inches of rain Wednesday through Friday, and Sacramento, Santa Cruz and wine country could see 1.5 to 2 inches.
Looking to next week, there is the possibility of rain Monday as well as Thursday in Southern California. But at this point, meteorologists expect those storms will have relatively minor effects.