Chance of a strong atmospheric river, mudslide grows for la

The risk rises for a strong atmospheric river to arrive next week, which may bring rain that can trigger waste streams and mud lids in Los Angeles County’s combustion areas.

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

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

“The range of results is still very uncertain, but this potential for a strong, very wet system is still there,” said Ryan Kittell, meteorologist at the National Weather Service Office in Oxnard.

One of the reasons these forecasts relate is that the chance of large amounts of rainfall is typically low. “This risk is certainly higher than most of our storms,” ​​Kittell said.

The scenario of high amounts of rainfall can mean an intense rainfall speed of half an inch to an inch per hour or more. Prices of half an inch per hour or more are able to trigger waste streams and mud lids in recently burned areas.

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

It would result in a “high risk of flooding and waste streams,” Kittell said. “Again, it’s not the most likely result, but it’s a pretty high risk compared to our normal storms.”

Recently burned areas are at risk of landslide in heavy rain, with soil no longer rooted in healthy vegetation. Aggravation of questions 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 begin to float on the earth’s surface downhill. It can trigger mud currents where water only rushes down with mud and more severe waste streams, where water can also fetch rocks, branches and sometimes massive stones traveling with speeds exceeding 35 km / h. Both mud currents and garbage flows are considered types landslide.

With intense rain comes the chance of “a bad, significant waste stream that would cause some deeper streams that would affect roads and maybe even structures,” Kittell said.

One of the worst waste streams in modern California’s history took place when a river of mud and rock flowed through Montecito in January 2018, killed 23 people and destroyed at least 130 homes. The area had been burned less than a month earlier in Thomas Fire. This blaze, one of California’s most destructive on the record, burned 282,000 hectares in Ventura and Santa Barbara counties and destroyed more than 1,000 structures.

Contributing to the severity of the Montecito waste stream was the failure of government officials to build larger basins that could have made mud glids far less disastrous, found an hourly examination. The report also found that Santa Barbara County could not empty the existing basins thoroughly before the disaster, which drastically reduced their capacity to catch waste.

The Times study found that the county’s flood control control officials and Army Corps of Engineers had known for half a century that there were too few drainage and that they had they were too small to stop the huge waste stream that the Santa Ynez Mountains were inclined 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 Amber Alert-Stil Bulletins to mobile phones until the mud sliders had begun. At the time, it was too late for the residents to escape.

The potential of a serious storm before or around Valentine’s Day comes as a weak atmospheric river storm leaves southern California. The storm was expected to peak in Los Angeles County between Thursday night and noon Friday.

From the current storm, Long Beach and Redondo Beach could get a third of an empty rain; A 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-seven of an inch; Santa Barbara, approx. 1 inch; and Ojai, approx. 1½ inches.

This storm is likely to be “generally beneficial”, with only less expectation of flooding of roads and possibly mud and rocks falling on some canyon roads, Kittell said. There is only 5% to 10% chance of thunderstorms.

Rain in Palisades and Eaton Burn areas could fall with a rate of a quarter to half an inch per hour. It is just below the threshold where precipitation can be expected to spur significant waste streams and mud lids.

“What we need to see is probably a little bit of mud and dirty water flowing on some of the roads, but … much less influences,” Kittell said. “There is likely to be something that flows on the combustion arrests that can affect some local roads.”

The risk of a significant waste current through Friday is low, less than 10%, Kittell said. This can happen if an improved storm cell – such as thunderstorms – happened to go directly over a combustion. “It’s a bit 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 also expected to bring wind throw from the south about 20 km / h to the center of Los Angeles and Long Beach, rising to approx. 30 km / hi a thousand oaks, Santa Clarita and Santa Barbara and more than 40 km / Hi La County’s High Desert and in San Luis Obispo County.

As of Friday night, dry, Gusty Wind is expected to arrive from the northwest and northeast and last for Monday. Downtown La and Long Beach could see gusts of approx. 14 km / h; Thousand Oaks and Canoga Park, 21 km / h; Santa Clarita, 26 km / h; Lancaster, 37 km / h; and Pyramid Lake, 45 km / h. Fire weather conditions are expected to be minimal due to the recent rain.

However, LA was not expected to get enough rain this week to finally declare a cessation of the fire season. A return of several weeks of dry weather may be enough to tilt the region back into the risk of dangerous fire conditions, especially if the Santa Ana wind returns. Serious Santa Ana -Vinde is most common from October to March and can occur as late as May.

Downtown LA has received only 1.02 inch rain since the water year began October 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 the record ended September 30, 2007, when only 3.73 inches of rain fell. A bad fire season followed – with the biggest blower that was the Zaca fire, burning 240,000 hectares of Santa Barbara County, and the witch fire in San Diego County, which burned nearly 198,000 hectares, destroyed 1,650 structures and killed two people.

Despite the rain, drought conditions in southern California worsened for another week, with large US drought -limiting center.

The center’s latest map, which was released on Thursday, shows almost all coastal southern California, including most of the Los Angeles and the Ventura comments and all 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 the drought conditions, said Lindsay Johnson, a climatologist with the center who writes the latest card. She said the center’s analysis takes into account rainfall, but also soil moisture, snowpack, fire hazard and other hydrological measurements.

“The longer it has been dry, the more rain it takes to return to normal, in general,” Johnson said. “There is a lot of deficit to put together.”

Heavy rain over the northern part of the state caused officials to start releaseing water from the Shasta dam into the Sacramento River. Two other reservoirs also release water, Bureau of Reclamation announced Thursday.

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

“At the same time, the federal pumps in Sacramento-San Joaquin Bay-Delta are currently operating with maximum capacity and storing water in the San Luis Reservoir to support South-AF-Delta delivery to farms, communities and wildlife when needed later on The year, ”The Agency wrote in a news message.

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

Lake Oroville-California’s second largest reservoir and keystone to State Water ProjectDelivering water to southern California, San Joaquin Valley and the San Francisco Bay area – is at 137% of its historic average.

San Luis Reservoir, the fifth largest reservoir in California, is at 121% of its historic average. The reservoir, in Merced County, stores water from Sacramento-San Joaquin Rivers Delta, which will later be delivered to southern California, San Joaquin Valley, Central Coast and Silicon Valley.

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

Times Staff Writer Ian James contributed to this report.