A storm system that could unleash mud and debris flows over slopes denuded by wildfires slowly moved into the Southland Wednesday, with the heaviest and potentially flood-creating downpours expected Thursday.
Rain began falling around midday in various parts of the area, with sporadic showers continuing into the afternoon.
But National Weather Service forecasters said the more severe downpours were expected overnight and into Thursday, increasing the risk of mudslides and flooding in areas recently affected by wildfires.
A flash flood watch will be in effect Wednesday night through Thursday in Orange County. A separate flash flood watch will take effect Thursday morning and continue through the afternoon in the Los Angeles County coastal areas and mountains, the Santa Monica Mountains Recreational Area and Santa Clarita and San Fernando valleys.
Forecasters noted that rainfall rates could reach an inch per hour in select burn areas, including the Woolsey fire area in Malibu.
“There is the potential for more significant and damaging debris flows,” according to the NWS. “Mudslides and rockslides will be likely across canyon roadways and Highway 1 (Pacific Coast Highway), bringing an increased threat of road closures.”
Forecasters said rain is likely to continue through Thursday night, with the heaviest rainfall predicted for Thursday morning and early afternoon. The highest rainfall rates are anticipated along south- and southwest-facing slopes of the Santa Monica and San Gabriel mountains, according to the NWS.
Forecasters said roadway flooding should be considered likely in low-lying areas, “along with rock and mudslides on canyon roads and below steep terrain.”
In the San Gabriel Mountains, a wind advisory will be in effect until 3 p.m. Thursday. NWS forecasters said southeast winds of 20-30 mph would buffet the mountains Wednesday, gusting to 45 mph, then increase to 20 to 35 mph with 55 mph gusts from Thursday morning through mid-afternoon.
“Winds this strong may down trees and power lines, causing property damage or power outages. Cross winds can make driving difficult, especially for drivers of high profile vehicles and vehicles towing trailers,” the NWS warned in a statement. “When driving, use extra caution. Be prepared for sudden gusty cross winds.”
According to the NWS, the storm system could produce 1-2 inches of rain along the coast, 2-3 inches along coastal slopes and 3-4 inches in the mountains.
The system, which originated in the Central Pacific, will combine with a colder system out of the Gulf of Alaska Friday and Saturday, generating heavy rain through Saturday, forecasters said. High surf and heavy winds are also expected.
NWS meteorologist Curt Kaplan noted that this winter’s rain in the Southland is considerably more extensive than in 2018. He said that since Oct. 1, 2018, Downtown L.A had received 13.29 inches of rain compared to the previous year’s 1.89 inches and the normal, which is 8.54 inches.