A moisture-rich atmospheric river dumped rain across Southern California on Wednesday, flooding roads, sending cars and trucks spinning out and closing some theme parks in the region.
The storm, fed by a plume of subtropical water vapor at the lower and middle levels of the atmosphere, had already poured up to a quarter of an inch of precipitation across the Southland before sunrise. Rainfall rates picked up slightly through the morning before tapering off by the afternoon, said Andrew Rorke, a senior forecaster with the National Weather Service in Oxnard.
“Rainfall rates have honestly been a little less than we were expecting, which is a good thing,” Rorke said. “It’s been just enough to make the roads bad.”
Shortly after 4 a.m., a semi truck skidded off the transition road from the northbound 710 Freeway to the 60 Freeway and dangled precariously over the elevated roadway, forcing the California Highway Patrol to close the ramp. Dozens of crashes were reported during the morning commute and traffic was snarled along most major freeways in Los Angeles County.
The persistent rain sent rocks tumbling onto a stretch of Topanga Canyon Boulevard in Malibu, while flooding from the raging Escondido Creek closed a road in San Diego. The soggy weather also kept Knott’s Berry Farm in Buena Park and Six Flags Magic Mountain in Valencia closed for the day.
The latest storm comes on the heels of the wettest November in years in parts of L.A. County. Historically, atmospheric rivers has bolstered California’s water supply and helped pull the state out of persistent droughts. However, the systems can also wreak havoc on the state’s infrastructure, flooding communities and causing mudslides in areas ravaged by wildfires.
In a study published Wednesday in Science Advances, researchers found that from 1978 to 2017, atmospheric rivers accounted for $42.6 billion in flood damage in 11 Western states — 84% of the estimated total water-related damage of $50.8 billion. That’s roughly $1.1 billion in damage done by atmospheric rivers every year.
Wednesday’s steady precipitation prompted the weather service to issue flash flood watches through noon for burn areas across Santa Barbara and Ventura counties, including the Cave, Maria and Easy fire burn scars. A flash flood watch also is in effect for the Saddleridge, Tick and Getty burn areas in Los Angeles County and portions of Orange, San Bernardino, Riverside and San Diego counties through 3 p.m.
Rainfall totals of 1 to 2 inches are expected for much of the region, but the San Gabriel Mountains could see up to 3 inches of precipitation. Rainfall rates in Los Angeles County by mid-morning ranged from a tenth of an inch to just over half inch per hour. There’s also a chance of thunderstorms in the afternoon, which could bring even higher rainfall rates.
The weather service warns that heavy rain could cause mud and debris flows in recently burned areas, which may result in road closures and property damage.
“That’s the big worry now,” Rorke said. “Hopefully if one forms, it won’t go over a burn area.”
Heavy rain in the Bay Area on Tuesday night caused a landslide that eroded about 30 feet of hillside in San Bruno. No property damage or injuries were reported. However, the situation prompted the San Bruno city manager to declare a local emergency. Rain is expected to linger across large swaths of Northern California throughout Wednesday.
A winter weather advisory is also in effect for the Sierra Nevada through Wednesday night, with warnings of slippery roads, reduced visibility and travel delays. Between 3 and 7 inches of snow is expected to fall at elevations above 6,000 feet.
The storm is expected to taper off overnight, making way for dry conditions on Thursday and Friday before another bout of rain, expected to be significantly weaker, arrives in the region over the weekend.
Times staff writer Colleen Shalby contributed to this report