Chance of strong atmospheric river storm grows, as does risk of mudslides in L.A. burn scars

The risk is rising for a strong atmospheric river to arrive next week, possibly bringing rain that could trigger debris flows and mudslides in Los Angeles County’s burn areas.

There is now a 30% chance that “high amounts” of rain could fall between Feb. 12 and 15 in Los Angeles, Ventura, Santa Barbara and San Luis Obispo counties, the National Weather Service said, up from the 10% chance forecast Tuesday.

The chance of moderate or low amounts of rain now stands at 30%, with a 10% chance of no rain.

“The range of outcomes is still very uncertain, but that potential for a strong, very wet system is still there,” said Ryan Kittell, meteorologist with the National Weather Service office in Oxnard.

One reason these forecasts are concerning is that the chance of high amounts of rainfall is typically low. “This risk is definitely higher than most of our storms,” Kittell said.

(National Weather Service)

The scenario for high amounts of precipitation could mean intense rainfall rates of half an inch to an inch per hour or more. Rates of half an inch per hour or more are capable of triggering debris flows and mudslides in recently burned areas.

Under this scenario, rain could arrive in a slow-moving storm that lasts for 12 to 24 hours or beyond and dump 2 to 4 inches or more on the coast and in the valleys, and 4 to 8 inches or more in the mountains and foothills.

That would result in a “high risk for flooding and debris flows,” Kittell said. “Again, it’s not the most likely outcome, but it’s a pretty high risk relative to our normal storms.”

Recently burned areas are at risk for landslides in heavy rains, with soil no longer anchored by healthy vegetation. Worsening matters is that the heat from fire makes it harder for soil to absorb water, and ash tends to clog the soil.

As a result, water can start flowing on the soil’s surface downhill. That can trigger mudflows, where water rushes down only with mud, and more serious debris flows, where water can also pick up rocks, branches and sometimes massive boulders, traveling at speeds exceeding 35 mph. Both mudflows and debris flows are considered types of landslides.

×

Animated infographic shows a debris flow works

With intense rain comes the chance of “a bad, significant debris flow that would cause some deeper flows that would impact roads and maybe even structures,” Kittell said.

One of the worst debris flows in modern California history occurred when a river of mud and rock flowed through Montecito in January 2018, killing 23 people and destroying at least 130 homes. The area had been burned less than a month earlier in the Thomas fire. That blaze, one of California’s most destructive on record, burned 282,000 acres in Ventura and Santa Barbara counties and destroyed more than 1,000 structures.

Contributing to the severity of the Montecito debris flow was the failure of government officials to build bigger basins that could have made mudslides far less catastrophic, a Times investigation found. The report also found that Santa Barbara County failed to thoroughly empty the existing basins before the disaster, drastically reducing their capacity to trap debris.

The Times investigation found that the county’s flood control officials and Army Corps of Engineers had known for half a century that there were too few catchments and that the ones they had were far too small to stop the enormous debris flows that the Santa Ynez Mountains were prone to produce.

The Times also found that Santa Barbara County released conflicting evacuation instructions, leaving the hardest-hit neighborhoods out of mandatory evacuation zones. It did not send out Amber Alert-style bulletins to cellphones until the mudslides had begun. By then, it was too late for residents to flee.

The potential for a severe storm before or around Valentine’s Day comes as a weak atmospheric river storm is exiting Southern California. The storm was expected to peak in Los Angeles County between Thursday evening and midday Friday.

Additional rainfall totals expected Thursday and Friday.

(National Weather Service)

From the current storm, Long Beach and Redondo Beach could get one-third of an inch of rain; Thousand Oaks and Oxnard, two-fifths of an inch; Santa Clarita and Canoga Park, about half an inch; San Luis Obispo, two-thirds of an inch; downtown Los Angeles, Covina and Fillmore, six-sevenths of an inch; Santa Barbara, about 1 inch; and Ojai, about 1½ inches.

That storm is likely to be “generally beneficial,” with only minor roadway flooding expected and possibly mud and rocks falling on some canyon roads, Kittell said. There’s only a 5% to 10% chance of a thunderstorm.

Rain in the Palisades and Eaton burn areas could fall at a rate of one-quarter to one-half an inch per hour. That’s just under the threshold at which rainfall can be expected to spur significant debris flows and mudslides.

“What we’re going to see is probably a little bit of mud and dirty water flowing onto some of the roads, but ... very minor impacts,” Kittell said. “There’ll likely be something flowing off on those burn scars that might impact some local roads.”

The risk of a significant debris flow through Friday is low, under 10%, Kittell said. This could happen if an enhanced storm cell — such as a thunderstorm — happens to go directly over a burn scar. “It’s kind of like throwing a coin in a big pond and trying to hit a fish,” Kittell said. “It’s not zero, but it could happen.”

The current storm was expected to also bring gusts from the south at around 20 mph to downtown Los Angeles and Long Beach, rising to about 30 mph in Thousand Oaks, Santa Clarita and Santa Barbara, and more than 40 mph in L.A. County’s high desert and in San Luis Obispo County.

Starting Friday night, dry, gusty wind is expected to arrive from the northwest and northeast and last through Monday. Downtown L.A. and Long Beach could see gusts of about 14 mph; Thousand Oaks and Canoga Park, 21 mph; Santa Clarita, 26 mph; Lancaster, 37 mph; and Pyramid Lake, 45 mph. Fire weather conditions are expected to be minimal because of the recent rains.

L.A., however, was not expected to get enough rain this week to definitively declare an end to the fire season. A return of several weeks of dry weather might be enough to tip the region back into the risk of dangerous fire weather conditions, especially if Santa Ana winds return. Severe Santa Ana winds are most common from October through March and can occur as late as May.

Downtown L.A. has received just 1.02 inches of rain since the water year began Oct. 1; the average for this time of year — a little more than four months into the water year — is 7.8 inches. The annual average is 14.25 inches.

Downtown’s driest water year on record ended Sept. 30, 2007, when only 3.73 inches of rain fell. A bad fire season followed — with the largest blazes being the Zaca fire, which burned 240,000 acres in Santa Barbara County, and the Witch fire in San Diego County, which burned nearly 198,000 acres, destroyed 1,650 structures and killed two people.

Despite the rain, drought conditions in Southern California worsened for yet another week, with much of the region now considered to be in “extreme drought” conditions, only the second worse category tracked by the U.S. Drought Mitigation Center.

The center’s latest map, released Thursday, shows nearly all of coastal Southern California, including most of Los Angeles and Ventura counties and all of Orange and San Diego counties, in “extreme drought.” Much of the rest of the region is considered to be in severe drought.

The rain has not brought enough moisture to stop drought conditions, said Lindsay Johnson, a climatologist with the center who authored the latest map. She said the center’s analysis takes into account rainfall but also soil moisture, snowpack, fire danger, and other hydrological metrics.

“The longer it’s been dry, the more rain it takes to come back to normal, generally speaking,” Johnson said. “There’s a lot of deficit to make up.”

Heavy rain across the northern section of the state prompted officials to begin releasing water from the Shasta Dam into the Sacramento River. Two other reservoirs are also releasing water, the Bureau of Reclamation announced Thursday.

Shasta Lake, California’s largest reservoir, has risen more than 22 feet since Jan. 31, according to the agency.

“Concurrently, the federal pumps in the Sacramento-San Joaquin Bay-Delta are currently operating at maximum capacity and storing water in San Luis Reservoir to support south-of-Delta delivery to farms, communities and wildlife refuges when needed later this year,” the agency wrote in a news release.

Shasta Lake is at 134% of its historical average for this time of year.

Lake Oroville — California’s second-largest reservoir and the keystone to the State Water Project, which supplies water to Southern California, the San Joaquin Valley and the San Francisco Bay Area — is at 137% of its historical average.

San Luis Reservoir, the fifth-largest reservoir in California, is at 121% of its historical average. The reservoir, in Merced County, stores water from the Sacramento-San Joaquin rivers delta to be delivered later to Southern California, the San Joaquin Valley, the Central Coast and Silicon Valley.

The northern Sierra has accumulated 37.4 inches of precipitation since the water year began on Oct. 1. That’s 128% of average for this point in the season — an improvement after a dry January, when accumulated precipitation dipped below the average for the first time in months.

Times staff writer Ian James contributed to this report.