Southern California will face severe weather threats on Saturday, with the most powerful atmospheric river storm set to peak in Los Angeles County over the weekend, bringing with it the risk of mudslides, mudslides and even tornadoes.
If the rains go as predicted, the storm could lead to the wettest November in downtown Los Angeles since 1985. Heavy rains could trigger damaging flooding and landslides, and hillsides burned by the Eaton and Palisades fires are at risk of fast-moving mud and debris.
The threat of severe weather is expected for much of Saturday from midnight to 9 p.m. A flood watch will be in effect for large areas of Southern California from 4 a.m. to 10 p.m. Saturday. Evacuation warnings for areas near recent burn scars are in effect until 11 a.m. Sunday due to the risk of mudslides. The warnings cover areas near the Palisades, Eaton, Kenneth, Sunset and Hurst fires that occurred in January.
But as of Thursday evening, it was unclear which areas would be hardest hit by the storm. Peak rainfall rates on Saturday are expected to be 0.75 to 1.25 inches per hour along a relatively narrow line about the width of a Southern California county. That’s enough to trigger landslides, which occur when rainfall reaches half an inch or more per hour.
Forecasters don’t yet know where the peak rainfall will be concentrated.
“The problem is, we just don’t know which counties” will be hardest hit, said Ryan Kittle, a meteorologist with the National Weather Service’s Oxnard office. “If you look at all of our forecasts, some of them are for Los Angeles, some of them are for Ventura, some of them are for Santa Barbara County. So, unfortunately, we can’t pinpoint which county is the bullseye in the time frame on Saturday.”
Kittle said if the heaviest rain bands line up over Los Angeles County, rainfall amounts of about 1 inch per hour are expected. If the band were concentrated elsewhere, Los Angeles could still see speeds of half an inch per hour, and landslides would still be possible.
Kittle said areas with the worst weather could see rotating thunderstorms that could produce damaging winds or a tornado or two.
“While this won’t happen in 99 percent of areas, it could happen in any part of our region, especially along the coast and in the valleys,” Kittel said. “Consider changing any plans for Saturday. Stay home, stay indoors.”
He noted that if lightning occurs, it’s best to stay indoors and away from windows. Those who must travel should never attempt to drive through flooded roads.
There’s still a chance Saturday’s storm won’t be as severe as expected. It’s driven by “low cutoffs,” which are notoriously difficult to predict, hence the nickname “the weatherman’s disaster.” Because the low-pressure system powering the storm isn’t driven by the jet stream, “it just spins like a top and goes where it wants to go — it’s hard to predict,” Kittel said.
Still, Kittle said most of the more than 100 different computer forecasts showed moderate to heavy rain. Under the most likely scenario, downtown Los Angeles could see 2.62 inches of rain Thursday night through Sunday, which would cause road flooding and minor shallow mudslides.
(National Weather Service)
Kittle said 2.62 inches of rain fell over the weekend, making this month the wettest November since 1985. Downtown Los Angeles would need more than 2.43 inches of rain in November to break the 40-year record.
The worst-case scenario has a 30% chance of 4.81 inches of rain falling in downtown Los Angeles, causing mudslides. As mudslides occur, fast-moving landslides tumble down hillsides, kicking up not just dirt but other debris that can move cars and slam into homes with deadly force. A total of 4.81 inches of rainfall would be one-third of downtown’s annual rainfall.
Rainfall rates as low as half an inch per hour can trigger mudslides and mudslides. But it depends on the burn scar, Kittle said. He said it would take twice as fast a rainfall – one inch per hour – to trigger some burn scar water flow.
The National Weather Service office in Oxnard said there is about a 70% chance of rainfall of 0.5 inch per hour or more in the Eaton and Palisades fire burn areas on Saturday. There is a 38% chance of rainfall of 1 inch or more per hour in these areas.
Rainfall is expected to begin Friday morning in Los Angeles, Orange, Riverside, San Bernardino and San Diego counties. Rainfall is expected to begin Thursday in Ventura, Santa Barbara and San Luis Obispo counties.
The heaviest rainfall is expected to hit Southern California Friday night into Saturday.
(National Weather Service)
Although tornadoes are not typically associated with California, they do occur. In most cases, “they’re weak, brief, and usually don’t cause a lot of problems,” Kittel said. “But we do get a lot of them.” Sometimes they form on land, or they start as waterspouts (tornadoes over the ocean) and then move over land.
“They’re not the types that you typically hear in the Midwest that last 15, 30 minutes, even an hour or two, a mile or two wide and cause devastating damage,” Kittle said. “We just don’t have that environment,” but they still pose a threat.
In December, a five-minute tornado struck Santa Cruz County, injuring three people, knocking down trees and power poles, snapping branches, overturning vehicles and damaging street signs.
Storms driven by atmospheric rivers this weekend created a long band of rain that reached San Francisco across the Pacific on Thursday. It will move south and east toward Southern California.
The storm tore down trees and flooded low-lying streets in the San Francisco Bay Area on Thursday. A tree broke and fell near San Francisco’s West Addition, hitting a car, local news outlets reported. A tree also fell on a fence in Santa Rosa. Firefighters said rising water flooded a stretch of road west of the Charles Schultz-Sonoma County Airport.
The California Highway Patrol said a single-vehicle crash occurred on Highway 1 in Santa Cruz County. All lanes of Highway 17 connecting Santa Cruz to San Jose were closed for a period of time Thursday night after a pickup truck overturned on Highway 152 between Watsonville and Gilroy and a CHP cruiser crashed; a CHP officer suffered minor injuries.
Rainfall totals in the region were impressive, with 1.28 inches falling in San Francisco, more than half of the city’s average November rainfall. Napa received 1.45 inches; San Francisco International Airport, 1.5; and San Rafael, 2.3 inches.
As of Sunday, rainfall forecasts in Long Beach are 2.38 inches; 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 see 2 to 2.5 inches of rain; Riverside, San Bernardino, Escondido and San Clemente, 2.5 to 3 inches; and Anaheim and Irvine, according to the weather service, could see 3 to 4 inches.
Even deserts can bring impressive amounts of rainfall. Palm Springs could see 1 to 1.5 inches of rain, and Joshua Tree National Park could see 1.5 to 2 inches of rain.
The storm won’t have much of a snowmaking impact in Southern California’s mountains. Snow levels are expected to remain around 10,000 feet for much of the storm’s duration, said Dave Munyan, a forecaster with the National Weather Service’s San Diego office. Snow levels will drop to about 7,000 to 7,500 feet by Sunday morning, but by then, there won’t be much moisture left in the storm. Big Bear is expected to receive about an inch of snow, Monjan said, while Idyllwild is expected to remain snow-free.
“On the highest peaks of the mountains, there’s going to be accumulation – lots of snow,” Munyan said.
Southeasterly and easterly winds are expected to cause delays at Los Angeles International Airport on Friday and Saturday.
Looking ahead to next week, one storm could hit Southern California again on Monday and Tuesday, with another storm on Thursday and Friday. Both storms may have minor impacts. But Kittel said forecasters are keeping a close eye on the second of the two storms, which could develop into a more severe storm.
