The strongest storm in a year is set to pound Southern California this week, with forecasters warning of potential widespread roadway flooding and the chance of landslides throughout the region — especially in areas recently charred in the devastating Los Angeles County firestorms.
The rain will be nothing like the relatively modest storms that have brought largely beneficial rain to Southern California the last two weeks. Forecasters suggest avoiding travel if possible on Thursday.
“Thursday is just not a great day to be on the road,” said Ryan Kittell, a meteorologist with the National Weather Service in Oxnard. “If nothing else, [expect] lots of slick roads, lots of traffic accidents. There will be some roadway flooding. Not all areas … but certainly more than we’ve seen this winter so far.
“If you have the option of canceling, delaying [or] rescheduling any kind of events — on Thursday in particular — that would be a good idea,” Kittell said.
Those living in or around recently burned areas should plan to leave their homes before the storm or, if they choose to remain at home, hunker down and stay off the roads. The National Weather Service warned of “potentially life-threatening and damaging flooding and debris flows,” and said the next few days could end up being the wettest storm of the entire winter.
Even if significant debris flows don’t happen, roads could be mucked with mud, “and there will probably be a lot of road closures in and around those burn scars,” Kittell said. A debris flow is a type of landslide in which water rapidly flows downhill — moving as fast as 35 mph and picking up not only mud but also rocks, branches, and sometimes even massive boulders and cars.
There’s a chance light rain will arrive Wednesday, but the greatest risk of flooding and debris flows is on Thursday. Downtown Los Angeles could see 2.17 inches of rain Wednesday through Friday.
The last time there was more rain than that was about a year ago, when 8.51 inches pelted L.A. over a three-day period, triggering damaging mudflows in Beverly Glen, Studio City, Tarzana, Baldwin Hills and Hacienda Heights.
A flood watch or flash flood watch is expected to be in effect for much of Southern California and the Sierra foothills east of the San Joaquin Valley.
This storm is arriving through an atmospheric river. Atmospheric river storms are long plumes of water vapor that can pour over from the Pacific Ocean into California. They carry so much water that they’re said to be like a river in the sky. Just a few atmospheric river events can bring California from one-third to one-half of its annual precipitation.
Much of Southern California, and the Sierra foothills along the San Joaquin Valley, are expected to be under a flood watch or flash flood watch later this week.
(National Weather Service)
Here’s what you need to know:
Timing
Showers could begin Wednesday — but they’ll be light, if they materialize at all.
Heavy rain is expected in Santa Barbara and San Luis Obispo counties on Thursday, and in Los Angeles and Ventura counties from Thursday afternoon through Friday morning.
Heavier rain could arrive in L.A. County around 2 or 3 p.m. Thursday, then build into the night, Kittell said.
“The morning could start out dry or damp, he said, “but certainly it’ll be ramping up through the afternoon hours.”
The heaviest rainfall is expected in Los Angeles and Ventura counties Thursday night into Friday morning, and in Santa Barbara and San Luis Obispo counties on Thursday.
(National Weather Service)
In the Inland Empire and San Diego and Orange counties, the heaviest rain is expected Thursday afternoon and evening. The storm isn’t expected to be as potent there, but it’s “still a significant event for this winter,” said Adam Roser, a meteorologist with the weather service’s San Diego office.
Thursday is also shaping up to be the strongest day of the storm in the San Francisco Bay Area and the Sierra Nevada.
Shallow landslides are likely, large river flooding and localized flash flooding are possible, and urban and small stream flooding is expected in the Bay Area, as well as Santa Cruz and Monterey counties, according to the weather service office in Monterey.
Rainfall amounts
Forecasters expect 1.5 to 3 inches of rain across much of Los Angeles, Ventura and Santa Barbara counties, with 3 to 6 inches expected in the mountains. San Luis Obispo County is expected to see more rain: 3 to 5 inches, with 5 to 10 inches in the mountains.
Lancaster could get 1.14 inches of rain; Redondo Beach, 2.04 inches; downtown Los Angeles and Long Beach, 2.17 inches; Thousand Oaks, 2.19 inches; Canoga Park, 2.32 inches; Santa Clarita, 2.38 inches; Covina, 3.07 inches; Santa Barbara, 3.25 inches; San Luis Obispo, 3.75 inches; and Cambria, 3.97 inches.
(National Weather Service)
During the lighter round of rain expected Wednesday, Riverside could get up to three-tenths of an inch; Anaheim and Irvine, up to two-fifths of an inch; Ontario, Lake Elsinore and San Clemente, up to half an inch; and San Diego, Escondido and Oceanside, up to seven-tenths of an inch.
Map of forecasted rain in San Diego and Orange counties, and the Inland Empire, on Wednesday.
(National Weather Service)
Getting about 2 inches of rain in a single storm is not particularly unusual for downtown L.A., but such a storm can cause problems including localized flooding.
“That is the way it tends to go in Southern California: We don’t have all that many storms with just light rain. We tend to have more of an ‘It rains, it pours,’ scenario here,” said Rose Schoenfeld, a weather service meteorologist.
For the heavier rain expected Thursday through Friday, San Diego and Escondido could get 1 to 1.5 inches of rain; Anaheim, Irvine, San Clemente, Riverside, Lake Elsinore and Oceanside, 1.5 to 2 inches; and San Bernardino, Ontario and Temecula, 2 to 2.5 inches.
Although it’s not routine for places such as Orange County to get 1.5 to 2 inches of rain in a single storm, Roser said, “it definitely happens in winter storms like these.”
Map of the rain totals forecast in Orange and San Diego counties, and the Inland Empire, on Thursday and Friday.
(National Weather Service)
Farther north, San José, Concord and Livermore could get 1.5 to 2 inches of rain; San Francisco, Napa, Monterey and Santa Rosa, 2 to 3 inches; and Big Sur, 4 to 6 inches.
Bakersfield could receive from 0.73 to 1.67 inches of rain; Fresno, 0.82 to 1.78 inches; and Merced, 0.6 to 1.36 inches. The Sacramento Valley has a 50% to 80% chance of receiving 1 inch or more of rain over a two-day period; the neighboring Sierra foothills and Shasta County have a 50% to 85% chance of 2 inches or more of rain.
Burn areas on alert
Southern California has endured one of its driest starts to the rainy season in recorded history, and one of the most destructive fire seasons ever. As a result, there are many burned areas now on high alert for potential landslides, given that vegetation has burned away and can no longer hold soil in place.
Experts say the risk of mud and debris sliding off burned hillsides rises once rain starts falling at a rate of half an inch per hour. That could happen this week, as rainfall rates are expected to peak between half an inch and 1 inch per hour in Los Angeles, Ventura, Santa Barbara and San Luis Obispo counties.
“We do expect quite a few enhanced cells” of precipitation, Kittell said, with “some of them maybe even organizing into some very narrow bands of fairly intense rainfall.”
These cells may end up behaving very much like a thunderstorm, Kittell said, “with brief, heavy downpours and possibly even strong, gusty winds.”
Animated infographic shows a debris flow works
There will be “a moderate risk for significant flooding, as well as recent burn scar debris flows,” Kittell said.
A flood watch will be in effect for recent burn scars, likely from midday Thursday through Friday morning.
Most concerning are the Eaton fire scar in the Altadena area, the Palisades and Franklin fire scars in Pacific Palisades and Malibu, and the Bridge fire scar in the San Gabriel Mountains west and southwest of Wrightwood.
A flood watch will be in effect later this week for recent burns areas in Los Angeles and Ventura counties.
(National Weather Service)
Residents of burn areas who have returned to their homes may want to consider relocating temporarily, Kittell said.
“If you have an option to go somewhere else, that’s great,” the meteorologist said, “especially on Thursday.”
Otherwise, he advised, avoid leaving, and try not to drive in recently burned areas through Thursday.
In San Diego and Orange counties and the Inland Empire, rain may fall at rates of more than half an inch per hour on Thursday in higher-elevation areas.
Flood risk
Expect roadway flooding, including on freeways, as well as at onramps and offramps. A few roads could be closed due to floods, and creeks and rivers will swell and strengthen. Swift-water rescues could be necessary if people become trapped by rising water in rivers and other waterways.
There’s a moderate risk for small-stream flooding in Santa Barbara and San Luis Obispo counties. The highest risk in San Luis Obispo County is near Cambria and other communities downslope of the Santa Lucia Range, the mountains that tower over the coast.
Strong winds
The storm is also expected to bring the strongest winds from a rainstorm this winter, with peak gusts of 40 to 60 mph in the mountains, deserts and Central Coast and 20 to 40 mph elsewhere.
Downtown L.A. and Santa Barbara could see peak gusts of 23 mph; Long Beach, 26 mph; Redondo Beach, 30 mph; Santa Clarita and Canoga Park, 32 mph; Thousand Oaks, 35 mph; Pyramid Lake, 37 mph; San Luis Obispo, 44 mph; and Lancaster, 52 mph.
Strong winds could down trees and power lines, causing electrical outages.
Winds are expected out of the east at Los Angeles International Airport, which could alter flight patterns and cause delays.
There is also a very low risk of water spouts and tornadoes, Kittell said. “I wouldn’t be surprised if we get a report or two of a funnel cloud or even a waterspout,” he said.
(National Weather Service)
After the storm leaves
The storm will likely move out of the region sometime Friday — kicking off what could be a bit of a dry spell. Through most of next week, no strong storms are expected in Southern California.
“Certainly, by Saturday, we’ll by dry,” Kittell said. And temperatures are expected to rise next week.
This story originally appeared on LA Times
