California Storm Threatens Southern Coast With Flooding and Tornadoes

A powerful October storm is bringing widespread rain, possible thunderstorms and localized flooding to Central and Southern California on Wednesday, October 21, with hazardous conditions expected to persist into Thursday. The National Weather Service has identified the Santa Monica Mountains and nearby coastal communities as particularly vulnerable to heavy rainfall, flash flooding and debris flows, while forecasters also warn of dangerous surf and a small chance of tornadoes.

The National Weather Service's Los Angeles-Oxnard office forecasts a 30% to 40% chance of thunderstorms in Los Angeles and Ventura counties. Orange, Riverside, San Bernardino and San Diego counties face a 15% to 25% chance. Most communities are expected to experience minor impacts, although intense downpours could cause more serious problems in vulnerable locations.

Rainfall across much of Los Angeles County could average approximately 1.5 inches, with localized totals reaching 2 to 3 inches in mountainous areas and locations affected by thunderstorms. Forecasts cited by the Los Angeles Times project 1.73 inches in Thousand Oaks, 1.53 inches in Oxnard, 1.47 inches in Santa Clarita, 1.31 inches at Los Angeles International Airport, 1.29 inches in Covina and slightly less than an inch in Long Beach.

The National Weather Service described the storm's eastward movement and timing as increasingly consistent with earlier forecasts, with rain expected to reach Los Angeles County during the afternoon.

"The system is starting to turn the corner and head east and seems to be on track for a relatively on time arrival … by afternoon in LA County," the National Weather Service said.

A flood watch covering Malibu, Malibu Bowl, Pacific Palisades, Topanga and the eastern and western Santa Monica Mountains was scheduled to take effect Wednesday afternoon and continue through Thursday afternoon. Forecasters identified these areas as especially susceptible to intense rainfall rates.

"A targeted Flood Watch has been issued for … the Santa Monica Mountains and adjacent coastline as models have been indicating the highest rain rates in those areas, around .75″ per hour," the National Weather Service said.

Wildfire burn scars remain another concern because heavy rain can carry mud, rocks and other debris downhill. Malibu officials said excessive rainfall over local burn scars was not expected, although the city planned to prepare for possible impacts and distribute emergency alerts through its website and social media.

"but we are prepared," said Matt Meyerhoff, a spokesperson for the city of Malibu.

Bryan Lewis, a meteorologist with the National Weather Service in Los Angeles, said some debris movement remained possible around Malibu. Burn scars left by the Palisades and Eaton fires were also being monitored, although they were not considered fresh enough to pose the same level of risk as more recently burned terrain.

"I think it’s fairly safe to say that we could expect at least some minor debris flow in the area," Lewis said.

The agency also warned of a low tornado risk as the storm moves inland. The forecast includes a 5% or lower chance of a weak tornado late Wednesday night into Thursday morning. Along the coast, thunderstorms could generate damaging wind gusts of up to 60 mph, lightning, small hail and waterspouts.

"there is a small (5% or less) chance of a weak tornado as the upper low moves inland late Wednesday night into Thursday morning," the National Weather Service said.

The Los Angeles and Oxnard coastal region faces another hazard from elevated surf and dangerous rip currents. Waves of 3 to 6 feet are expected on south- and southwest-facing beaches through Thursday night, while tides could reach 5.5 to 6.2 feet. A beach hazards statement covers portions of the Los Angeles County coastline.

The National Weather Service's San Diego office also issued a beach hazard warning covering Orange, Riverside, San Bernardino and San Diego counties. Officials cautioned that powerful waves could sweep people off beaches and rocks, while rip currents could pull swimmers and surfers away from shore.

"remain out of the water to avoid hazardous swimming conditions," the National Weather Service advised.

The agency identified recently burned terrain as particularly vulnerable to rain-related debris movement, including areas affected by the Palisades and Eaton fires.

"Some of the more vulnerable areas include first-year burn scars," a National Weather Service spokesperson said.

Central California also faces heavy rain, particularly around Big Sur and the Santa Lucia Range, where forecasts call for approximately 1 to 1.5 inches. The Timber and Plaskett fire scars are among the areas being monitored. The U.S. Geological Survey has warned that just a quarter-inch of rain in 15 minutes could trigger debris flows across Highway 1 in vulnerable locations.

"We are seeing rain totals trending upward," Lewis said.

The storm is expected to produce the heaviest coastal rainfall from late Wednesday morning through late Thursday morning, according to Lewis. Downtown Los Angeles could receive 1 to 1.5 inches, potentially exceeding twice its normal rainfall for the entire month of October.

"definitely concerns for debris flows," forecasters at the National Weather Service's Bay Area office said of the burn scars near Big Sur.

Higher elevations in the Sierra Nevada are expected to receive significant snowfall. A winter weather advisory covers the central and southern Sierra, where at least 6 inches of snow could accumulate above 8,000 feet, with totals reaching 10 inches at high elevations in Mono County, including Mammoth Mountain. Tioga Pass was scheduled to close temporarily at 5 p.m. Wednesday ahead of the storm.

Farther north, the Bay Area is expected to receive less rain than Central and Southern California, although Wednesday could bring the wettest conditions in San Francisco, Oakland and San Jose since May. After the storm clears, dry weather is forecast to return Friday, followed by another round of Santa Ana winds over the weekend, with gusts of 30 to 50 mph possible, strongest in mountainous areas.