Pair of atmospheric river storms headed to California. SoCal could be in for a soaking

After an extraordinarily dry start to the year, two atmospheric river storms are expected to hit California — potentially bringing much-needed moisture to the still-withered Southland while packing a more powerful and prolonged punch up north.

Forecasters currently believe there’s little risk that the rains expected next week will trigger destructive debris flows and mudslides in Southern California’s recent burn areas. But the risk is still there, and there is a range of possibilities for how much rain will ultimately fall.

There’s a chance of moderate to heavy rainfall in Los Angeles and Ventura counties from Tuesday through Thursday, according to the National Weather Service office in Oxnard. It could rain for many hours each day, with the highest chance of precipitation between Tuesday afternoon and Wednesday afternoon.

There’s a 50% chance of moderate amounts of rain in L.A. and Ventura counties — 1 to 2 inches along the coast and in the valleys, and 2 to 4 inches in the mountains and foothills. There’s a 10% chance of even higher amounts of rain, but a 40% chance that less will fall.

(National Weather Service)

All that pencils out to a “low but non-zero risk for flooding and debris flows” for Los Angeles and Ventura counties, at least for the moment, the weather service said.

Under a best-guess scenario, for the three-day storm that starts Tuesday, Long Beach could get three-fifths of an inch of rain; Redondo Beach and Covina, two-thirds of an inch; downtown L.A., Santa Clarita and Pyramid Lake, three-quarters of an inch; Thousand Oaks and Canoga Park, five-sixths of an inch; Oxnard, about 1 inch; San Luis Obispo, 1.4 inches; Santa Barbara, 1.55 inches; and Cambria, 1.78 inches.

A soaking of that magnitude, though beneficial for vegetation left parched by a historically dry start to the water year, likely wouldn’t be enough to put an end to one of California’s most destructive wildfire seasons. It would take 2 to 4 inches of rain over the region to comfortably consider the wildfire season over, said Rose Schoenfeld, meteorologist with the weather service office in Oxnard.

L.A. County’s first major winter storm, between Saturday and Monday, dropped generally half an inch to 1½ inches across the L.A. Basin. Downtown Los Angeles received a total of 0.54 of an inch of rain in January.

(National Weather Service)

The chance of rain is lower in the Inland Empire and San Diego and Orange counties. The National Weather Service said there was a slight chance of rain in that region on Tuesday and a chance of rain — with minor impacts expected — on Wednesday.

There are chances of light rain in Santa Barbara and San Luis Obispo counties from Friday night through Monday. From Tuesday through Thursday, those counties have a 30% chance of high amounts of rain, and a 50% chance of moderate rainfall.

Areas burned in the recent L.A. County fires have been on edge over the possibility of damaging debris flows and mudslides that rain can trigger. The county’s first winter storm, however, did not result in that worst-case scenario, as the heaviest rainfall missed recent burn areas. Even so, mud and debris prompted some road closures and trapped some cars in the Santa Monica Mountains.

Wide swaths of Southern California have seen only one significant rainstorm in more than eight months — producing for many areas a record dry start to the water year, which began Oct. 1. In the lead-up to the Palisades and Eaton fires this month, the intense dryness kept vegetation tinder dry — a recipe for disaster when combined with strong Santa Ana winds and an ignition.

Northern California did have a wet start to the rainy season but also experienced a dry January. After the area enjoyed robust rains in November and December, when precipitation in the Sierra Nevada was above average, precipitation there is now merely average.

The northern Sierra, for instance, has had 26.5 inches of cumulative precipitation. That’s 98% of the average for this date, but it’s the first time in months the region has dipped below 100%. The northern Sierra is a key piece of the state’s water supply, which relies on a natural reservoir of snow and ice in the mountains that slowly melts to refill liquid reservoirs in the spring, summer and fall.

January is not quite over yet, but so far, downtown San Francisco has endured its third-driest January on record, with just 0.19 of an inch of rain so far this month, according to Nicole Sarment, meteorologist for the weather service office in Monterey. That’s just 4% of the city’s average January rainfall of 4.4 inches.

Two big plumes of moisture are headed toward California with precipitation expected in much of the state.

(National Weather Service)

But that dry spell looks set to end — and not with a whimper.

Northern California is primed to get two atmospheric river storms, according to Courtney Carpenter, a meteorologist with the weather service’s Sacramento office. Atmospheric river storms are long plumes of water vapor that can pour over from the Pacific Ocean into California. They carry so much water, 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.

The first storm for Northern California is expected between Friday and Sunday, with a second arriving Monday and persisting through the middle of next week, Carpenter said.

The first storm will likely bring accumulating snowfall to just the higher Sierra peaks, according to the weather service office in Reno. The second is expected to be colder, potentially bringing heavier mountain snow and more widely affecting travel, Carpenter said.

Both storms are currently in the eastern Pacific Ocean, with the second one extending into the area around Hawaii.

In the Sacramento Valley, given the dry weather, “we’re not expecting widespread major flooding impacts across our area, but do look for extended periods of moderate, heavy rain, significant rises on area waterways ... localized flooding in our usual trouble spots, and the potential for mud and rockslides in the mountains and foothills below the snow line,” Carpenter said.

Sacramento could get 2 to 3 inches of rain during this weekend’s storm, and Donner Peak could see 6 to 8 inches of snow. Sacramento could see another 2 to 3 inches of rain during next week’s storm, with potentially another 30 to 36 inches of snow falling at Donner Peak.

The area of the 429,603-acre Park fire, which burned through Butte and Tehama counties north of Chico, is not expected to see intense enough rainfall to trigger debris flows, Carpenter said, but “you can expect increased muddy and woody runoff,” which occurred in northern Chico neighborhoods in November.

At least six days of rain are expected for the San Francisco Bay Area and surrounding areas, and there could be moderate impacts in San Francisco, the North Bay and San Mateo County, as well as in Santa Cruz and San Benito counties.

The Bay Area has a better-than-even chance of 1 inch of rain for the first atmospheric river storm, hitting Friday through Sunday. More significant rainfall is expected next week.

The “main question is timing and extent of moderate-to-heavy rainfall,” the weather service office in Monterey said, which could cause elevated levels of water in creeks and streams and minor street flooding.

The first atmospheric river storm expected to hit Northern California starting Friday won’t reach far enough south to bring significant rain to Los Angeles County.

Next week’s weather system is considered a “possible atmospheric river” storm for Los Angeles County, the weather service office in Oxnard said.

Times staff writer Grace Toohey contributed to this report.