The storm — a vast atmospheric river of tropical moisture known as a "pineapple express" — made landfall Tuesday night and is predicted to last through Thursday.
"It's going to be steady, light rain with periods of heavy rain," said Stuart Seto of the National Weather Service in Oxnard. Heavier bouts of rain will occur Wednesday evening and the following day, he said.
By Wednesday afternoon, rain-triggered slides forced Amtrak's Pacific Surfliner train to stop south of Carpinteria as Union Pacific maintenance crews removed debris from the tracks. The railway was cleared after an approximately 45-minute delay, Amtrak said.forced
The rain comes just months after the Thomas fire scorched more than 440 square miles of land, reducing thick forest and chaparral to ash and making steep hillsides susceptible to mud and debris flows. Residents who live below the Thomas fire burn area, as well as the Sherpa and Whittier fires were told to evacuate their homes by noon Tuesday.
Flash flood watches were issued in Santa Barbara and Ventura counties, where scorched hillsides will start to dissolve into mud flows if it rains at a rate of more than half an inch an hour, according to the U.S. Geological Survey.
In Montecito, where at least 21 people died in massive mud and debris flows on Jan. 9, crews were keeping close watch on debris basins, creeks and roadways for potential mudflow and debris.
"Right now, we are seeing a light, steady rain, and the water in the creeks is chocolate-milk color," said Scott McGolpin, director of public works for Santa Barbara County. "There isn't a lot of debris. This is what we want."
Robert Lewin, director of the county's Office of Emergency Management, said boulders have been reported moving along San Ysidro Creek but that they have so far remained within the channel.
"Our hope is that the debris basins can capture the material that comes down from the mountain," he said.
Wednesday's rain also triggered road closures near Ojai and along the coast. Highway 33 was closed between Fairview Road and Lockwood Valley Road in the Los Padres National Forest because of mud and debris on the roadway, Caltrans said. A portion of Pacific Coast Highway also was closed at Ragged Point — more than 130 miles north of Santa Barbara — after a mudslide blocked the road.
Rainfall rates up to 0.6 inches per hour were possible Wednesday morning, the National Weather Service said, with rates likely increasing to 0.75 inches per hour or higher in the afternoon. There is a slight chance of thunderstorms, which likely would produce rainfall rates in excess of 1 inch per hour, according to the weather service.
"Thunderstorm areas could produce really heavier rains," Seto said. "They're keeping an eye on that at this point."
Experts predict that by Friday, the storm will have dumped 3 to 6 inches of rain along the coast and up to 10 inches in the mountains and foothills above Montecito, Carpinteria and Ojai.
The atmospheric river is expected to deliver nonstop rain across much of the state and provide some relief to areas that have seen a resurgence in drought conditions.
In Los Angeles County, authorities lifted most evacuation orders near fire-scarred mountains. Residents who live in the 8300 to 9000 blocks of La Tuna Canyon Road were still under a mandatory evacuation order because of debris flows.
Deputies will ask residents who refuse to evacuate to sign a waiver and list their next of kin.
"The key is being proactive in preparing for the worst and hoping for the best," Los Angeles County Sheriff Jim McDonnell said at a news conference Wednesday.
Residents of Kagel Canyon, Lopez Canyon and Little Tujunga were allowed to return home, though shelters will remain open for residents who feel unsafe returning home, said Helen Chavez, a spokeswoman for the multi-agency response to the storm.
More than 100 homes on or near recently burned slopes in Corona were also under mandatory evacuation orders, which authorities said would remain in place overnight.
Los Angeles County Supervisor Kathryn Barger, who represents the affected neighborhoods, urged residents to heed officials' warnings even though they've been asked to evacuate multiple times in the last year and may have "evacuation fatigue."
Authorities say residents there may be trapped if mud flows cut off access to local roads, or if everyone tries to leave at once.
"If you are told to evacuate, it's because it's been evaluated that you live in a high-risk area. Please, please follow these instructions," Barger said. "It is imperative that you leave before the flow of the mud begins. It moves at a fast pace, and you may think you'll be able to outrun it but you can't."
No one wants a repeat of Montecito, officials said. In January, thousands of residents there were caught flat-footed after a once-in-200-year storm cell drenched the town in the middle of the night and sent a deadly river of mud and rock from the foothills to the ocean. In addition to the 21 people who were killed, two remain missing.
Only a fraction of residents in that community followed voluntary and mandatory evacuation orders before the storm arrived.
"When we see the power of Mother Nature and the power these rains have to bring down giant boulders, I would certainly ask that people take a look at what happened there and realize you are in harm's way," McDonnell said.
Staff writer Melissa Etehad contributed to this report.
sarah.parvini@latimes.com
Twitter: @sarahparvini
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