Floods and landslides are risk as heavy rains hit La Liang

Areas of Los Angeles County burned by recent wildfires are at risk of flooding and landslides as Southern California prepares for the first heavy rains of winter this weekend.

“The threat is sufficient to prepare for the worst,” the National Weather Service office in Oxnard said on social media.

Forecasters say there is now a 10 to 20 percent chance of substantial flash flooding and debris flows capable of damaging roads and homes in the most vulnerable areas that have burned recently, namely, the Palisades and Franklin in Pacific Palisades. Burning areas around Cedars and Malibu, Malibu fire, Eaton fire around Altadena and Pasadena, Lake Castaic ) around the Hughes Fire, and the Bridge Fire in the Angeles National Forest north of Glendora.

In the fall, more than 56,000 acres burned in Los Angeles and San Bernardino counties, destroying 81 structures and injuring eight firefighters.

Weather service meteorologist Ryan Kittell said these burned areas have the greatest debris flow potential, based on the U.S. Geological Survey’s assessment.

“They are the freshest burn scars. They are close to communities and/or vulnerable infrastructure. For those higher totals and rainfall intensities, the orientation of the terrain will favor those areas, especially with higher chances, higher Potential,” Kittell said.

The chance of flooding and debris flows in recently burned areas was released Friday afternoon, increasing to 10 percent from the 5 percent forecast a day earlier. “While disruptive debris flow is not the most likely outcome, there is still a lot of uncertainty surrounding this storm,” the weather service said.

Recently, burned areas pose a risk of flooding and landslides because the heat from the fires makes it difficult for water to absorb through the top layer of soil. The soil becomes expelled to the water, which then begins to flow over the surface downhill, picking up rocks and debris.

This can lead to “mud flows,” in which water starts rushing down a hillside in just mud (usually less than 15 feet), and more destructive and potentially deadly “debris flows,” in which mud flows downhill. The tide also picks up rocks, branches, and sometimes huge boulders. Mud flows and debris flows are types of landslides.

“The most likely outcome is that there isn’t a large debris flow, but there’s a high chance that certainly expands on the information that it’s out there, it’s on the table, and the threat is increasing,” Kittell said. “The chance of a debris flow” remains. It’s a threat that people should at least plan for and take into account.”

“The most likely outcome is that there may be some shallow debris flows that would have a smaller impact,” Kittell added.

opportunity

Forecasters with the weather service have issued a 24-hour highest risk flood watch – from 4pm Sunday to 4pm Monday

Weather service meteorologist Ryan Kittell said Sunday night will be a time of particular concern.

Flood watches are issued when weather conditions are favorable for flooding. “This does not mean flooding will occur, but is possible,” the Bureau of Meteorology said.

Among the weather service’s recommendations: Avoid areas that have recently burned during this period. Use sandbags to protect property. Residents who decide to stay can “stock supplies in case road access is blocked”.

(National Weather Service)

The planned flood watch does not include wildfire burn scars in Ventura County.

Rainfall chances begin to take hold Saturday afternoon in Los Angeles and Ventura counties, with the highest expected rainfall intensity expected between Sunday afternoon and Monday noon in Los Angeles and Ventura counties. This is common for really light rain and will continue throughout the weekend into Monday.

This is “a slow-moving storm, so it’s going to be stubborn. It’s going to linger,” said Alex Tardy, a meteorologist with the National Weather Service office in San Diego. “That’s going to send a wave of moisture through Monday on Monday. So I think that’s going to really add up to heavy rain.”

Forecasters added to forecasts of possible rain. The adjusted forecast is the result of a low-pressure system moving down from Canada that appears to be far more to the west than initially expected, making the storm wetter.

That led to “a focus on some of the burnt scar flow debris,” Kittel said.

(National Weather Service)

The mountains in Los Angeles and Ventura counties could get 1 to 2 inches of rain, while elsewhere a half inch to 1 inch is possible.

Thousand Oaks and Oxnard could get three-fifths of their rain between Saturday and Monday. Redondo Beach, Santa Clarita and Fillmore, one-tenth of an inch; Long Beach, four-fifths of an inch; and downtown Los Angeles and Covina, one-tenth of an inch.

If the storm rains on the high end of the estimate, 1 to 1.5 inches of rain could fall in Orange County, Ontario, Riverside, Lake Elsinore, Temecula and northern San Diego. In San Diego, from 0.7 to 1 inch of rain could fall, and in San Bernardino 1.5 to 2 inches.

Rainfall is expected to be record or near record dry weather for Southern California. Most of the region receives less than 5% of its average rainfall at this point in the water year, which begins on October 1.

Since October 1, downtown Los Angeles has received just 0.16 inches of rain, which is just 2 percent of the water-year average of 6.48 inches. The average annual rainfall in downtown Los Angeles is 14.25 inches.

Southern California is now in an “extreme drought” or “severe drought,” according to the U.S. Drought Monitor.

Chance of thunderstorms and risk of flooding

There is a 15 to 25 percent chance of thunderstorms anywhere in Los Angeles, Ventura, Santa Barbara, and San Luis Obispo counties during this rainfall event. This will be followed by rainfall rates of one-half inch to three-quarters of an inch per hour in isolated areas.

This is important because rainfall rates of half an inch per hour are the starting point for triggering detrital processes in recently burned areas. If the rainfall rate is significantly higher than that number, Kittell said, and it occurs directly over the burned area — “that’s where we could get some more pronounced and pronounced debris flow,” Kittell said.

“The vast majority of areas will not see this type of intense rain, but there are some areas that do expect it,” Kittel said. “Predicting exactly which areas will see these rates is very difficult, if not impossible. ”

Kittle said the most common rainfall rates across the region are expected to be about one-tenth of an inch per hour to a quarter of an inch per hour. This should result in beneficial rainfall.

Tardy said if the rainfall is between a quarter inch an hour and a half an hour, that tends to cause urban flooding and water swells, which could force some streets or a lane of street traffic to be closed, for example.

Kittle said there could be significant gusts to the south. Over the weekend, peak wind gusts of 15 mph to 30 mph may occur in the mountain valley foothills, with gusts possibly reaching 60 mph at times. This could cause delays at airports, including at Los Angeles International Airport, as well as hazardous driving conditions and the possibility of power outages and downed trees.

There is also a risk of small hail.

snowfall

Snow levels could be as low as 3,500 feet above sea level, with 6 to 12 inches possible in the San Gabriel Mountains. Kittle said the Grapevine portion of Interstate 5 along Tejon Pass could get 1 to 2 inches of snow, but there could be less or more.

Tardy said Big Bear Lake and Wrightwood could see 12 to 18 inches of snow.

“Ton. Baldy (which has really been suffering this year), could get a couple feet of snow… maybe up to 3 feet, depending on if this storm slows down as much as expected,” Tardy said.

Kittle said there will be heavy snow for most of the storm, but there may be a period between Sunday afternoon and Monday morning when the snowfall could be more moderate. Kittle said to expect “delays and maybe some localized closures” on icy and snow-covered roads in the mountains.

Tardy said this will be one of the first winter snowstorms of the season, after a January 7 outburst from the north brought only a few inches of snow.

“This will become more common and [get to an] Even lower altitudes. “Tadi said.

Other burning areas forecasters will monitor

Elsewhere in Southern California, meteorologists will also keep a close eye on the burn zone of the 43,978-acre fire in San Bernardino County, which started in Highland City in September and spread into the San Bernardino Mountains, Destroy a structure.

The fire line burned as high as 8,000 feet above sea level, Tardy said.

Also being watched closely is the burning area of ​​the 23,526-acre Airport Fire in Orange and Riverside counties. The airport fire destroyed 160 structures, burned a trail through the Santa Ana Mountains and reached Santiago Peak, the highest point in Orange County at nearly 6,000 feet above sea level.

long term forecast

By February, Northern California could see the return of winter storms. But Southern California will likely stay dry through the beginning of the month, Tardy said.