Atmospheric river storm set to hit SoCal Tuesday. What to know about the rain and the risks

A stronger atmospheric river is set to hit Northern California on Monday and then hit Los Angeles County on Tuesday, aiding hopes of a more definitive end to a devastating fire weather season for Southern California.

The atmospheric river is expected to pack a more powerful punch in Northern California. A flood watch is in effect over a broad swath of Northern California between Monday afternoon through Wednesday morning, and heavy snow is expected for the Sierra Nevada.

For Los Angeles County, what initially appeared to be one storm is now separating into two distinct storm systems, said Rose Schoenfeld, a meteorologist with the National Weather Service office in Oxnard. There will still be chances of rain from Tuesday through Friday across L.A. and Ventura counties.

The peak of the atmospheric river storm in L.A. County will be between Tuesday night and last through Wednesday midday. A second storm — a cold frontal system — is expected to hit L.A. County on Thursday into Friday, and that one “will have more impact, particularly for L.A. County, compared to the first system.”

Generally, forecasters are expecting light-to-moderate rain this week for Southern California, with “some amount of mudslides and road erosion, especially for sensitive canyon roads,” increased flows of water on creeks and swollen ponds on roads, Schoenfeld said.

There is a 5% chance that rainfall rates this week will reach an intensity over recently burned areas that could pose a risk for significant mudslides or debris flow, Andrew Rorke, a weather service meteorologist, said early Monday.

Winds will be coming from the south and southwest along the coast, peaking Tuesday afternoon and night, the weather service said. Los Angeles County could see gusts of 10 mph to 20 mph across highly populated areas, and 20 mph to 40 mph in the mountains and deserts.

Here’s what to know:

Timing of the first storm of the week

Los Angeles County

The peak timing for the first storm will be between Tuesday at 10 p.m. through Wednesday at noon for Los Angeles County. L.A. County is expected to get anywhere from one-quarter of an inch of rain to 1 inch.

A likely scenario involves Covina seeing one-third of an inch of rain through Wednesday; Long Beach, two-fifths of an inch; downtown L.A., Santa Clarita, Redondo Beach, about half an inch; and Canoga Park, about three-fifths of an inch.

(National Weather Service)

Rain is expected to be light in L.A. County — anywhere between one-tenth of an inch per hour to one-third of an inch per hour. Rainfall rates generally need to get to one-half of an inch per hour to pose significant threat of mudslides and debris flow in recently burned areas.

Rain could be slightly enhanced around mountain slopes, particularly around the Eaton fire and in some areas in western L.A. County, “but, overall, this storm looks much more focused to the north of L.A. County,” Schoenfeld said.

This will be the first atmospheric river storm of the winter for Los Angeles County.

Ventura, Santa Barbara and San Luis Obispo counties

The storm’s peak will hit the three counties between Tuesday at noon through Wednesday at 8 a.m.

One likely scenario could bring seven-tenths of an inch of rain to Thousand Oaks through Wednesday; four-fifths of an inch to Fillmore and Santa Maria; nine-tenths of an inch to Lompoc; about an inch to Oxnard, Ojai and Santa Ynez; 1.26 inches to Santa Barbara; 1.39 inches to San Luis Obispo; and 1.9 inches to Cambria.

Rainfall is expected to fall at a moderate intensity in these three counties, generally from one-quarter of an inch per hour to one-half of an inch per hour. However, there could be certain areas that see rain fall at a rate of three-quarters of an inch per hour.

Northern California

Northern California was already hit by an atmospheric river over the weekend, and a second round is expected to begin Monday.

While L.A. County is getting a weak atmospheric river early this week, the atmospheric river over Northern California is considered strong.

San Francisco could get 3 to 4 inches of rain through Friday night; San Jose, 2 to 3 inches; and Santa Cruz, 4 to 6 inches. Heavy snow is expected for the Sierra Nevada and could cause near white-out conditions at times, with winds gusting as high as 60 mph to 70 mph.

“Our biggest concerns are flooding of creeks and streams and an increased risk for downed trees due to the combination of wind and moist soils,” said the weather service office in Monterey, which also issues forecasts for the San Francisco Bay Area.

Timing of the second storm of the week

Los Angeles and Ventura counties and the Central Coast

The second storm of the week is expected to be stronger for Los Angeles County. Peak timing for this storm is expected to be between Thursday night through Friday morning, and there is a 10% chance of thunderstorms, with the highest risk on Thursday.

Rain could fall at a moderate pace, at a rate of between one-quarter of an inch per hour to one-half of an inch per hour. There is a small chance that the area around the Eaton fire could get rainfall rates that could run the risk of concerning mudslides and debris flow, according to Schoenfeld.

“However, with this storm, we’re mostly expecting pretty beneficial rain that’s going to be fairly light and nicely widespread, without too many pockets of enhancement,” Schoenfeld said.

Downtown L.A., Long Beach, Redondo Beach, Covina and Thousand Oaks could see about a half inch of rain; Canoga Park, three-fifths of an inch; Oxnard and San Luis Obispo, two-thirds of an inch; and Santa Barbara, nearly nine-tenths of an inch of rain.

The storm could be more wet for mountain slopes facing the south, Schoenfeld said, but the details are uncertain for now.

Northern California

An atmospheric river will hit Northern California between Thursday and Friday — the third for that region since Friday.

The risk levels are expected to be moderate in the San Francisco Bay Area and major in the Sacramento Valley and Sierra.

Snow

For L.A. County, the storm is relatively warm. “It does not have a super cold core to the system, and so only the highest peaks of the mountains will see snow,” Schoenfeld said. Snow is not expected to fall on the Grapevine section of Interstate 5, which traverses the Tejon Pass and connects L.A. County with the Central Valley.

In the Greater Lake Tahoe Area, there could be heavy snow accumulating between 1 to 3 feet at elevations of 7,000 feet of sea level and above. Below that elevation, there could be half a foot to a foot of snow accumulating between Monday afternoon through Wednesday morning.

In Mono County — home to Mammoth Mountain — at elevations of 8,000 feet and higher, there could be accumulations of heavy snow of between 1 to 3 feet. There could be up to 6 inches of snow below 8,000 feet.

San Diego and Orange counties and the Inland Empire

Rainfall is expected to be fairly light east and south of Los Angeles County.

(National Weather Service)

Between Tuesday night through Friday, Anaheim and Ontario could get up to seven-tenths of an inch of rain; Riverside, San Bernardino and San Clemente, up to two-fifths of an inch; and Irvine, up to half an inch.