Steady rain doused the Southland Thursday amid strong winds and mandatory evacuation orders in parts of Orange and Riverside counties prompted by fears of shallow mudslides, minor debris flows and some flash-flooding in recent wildfire burn areas.
According to a National Weather Service statement, the heaviest rain is expected in Orange County and inland areas late Thursday morning.
“Hourly rainfall rates in stronger showers and isolated thunderstorms could approach one-half to one inch in an hour,” according to the NWS. “This could lead to mudslides and debris flows at recent burn scars. The time window of greatest concern is from late Thursday morning through early Thursday evening.”
At 10 a.m., the NWS issued a flash flood warning for Riverside County and parts of Orange County due to fears of mudslides in the burn scar of August’s 23,000-acre Holy Fire.
Forecasters said radar images showed “very heavy rainfall entering the Holy burn scar,” according to the NWS. “Flash flooding will likely begin shortly and debris flows will likely begin in Trabuco Canyon and possibly in the Indian, Temescal, Coldwater, Horsethief, Rice, McVicker and Leach Canyons which drain towards Lake Elsinore. Debris flows can more quickly and with little warning.
“Some locations that will experience likely flooding include western Lake Elsinore, Temescal Valley and Ortega Highway 74,” according to the NWS.
As a result, a voluntary evacuation order that had been in place in the Trabuco Creek area of Orange County was upgraded to a mandatory order. Residents of Rose Canyon remain under a voluntary evacuation order. Voluntary evacuation orders were put in place Wednesday in the Mystic Oaks and El Cariso areas.
“Hard road closures” were in place on Trabuco Canyon Road at Rose Canyon and Robinson Ranch Road, according to the Orange County Sheriff’s Department.
Forecasters warned earlier that the storm could prompt flooding in areas including Malibu, Agoura Hills, Woodland Hills, Calabasas, Westlake Village, Malibu Creek State Park and Oak Park — many of them impacted by the recent Woolsey Fire.
There were sporadic reports of rocks falling on canyon and mountain roads in the Woolsey Fire area, but no major debris slides had been reported as of mid-morning.
Other areas potentially subject to flooding, according to the NWS, were Long Beach, Acton, Wrightwood, Glendora, Alhambra, West Covina, Mount Wilson and Whittier.
The NWS also issued an urban and small stream flood advisory that will be in effect until 11:45 a.m. in Orange, Riverside and San Bernardino counties. It said locations that will likely experience urban and low-lying flooding include Anaheim, Santa Ana, Riverside, Irvine, San Bernardino, Fontana, Moreno Valley, Huntington Beach, Garden Grove and Ontario.
“Turn around, don’t drown when encountering flooded roads. Most flood deaths occur in vehicles,” an NWS statement urged motorists.
Most areas were expected to receive between a half-inch and two inches of rain, although three inches could fall at higher elevations and foothills.
A flash flood watch will be in effect for the Santa Ana mountains and foothills and inland Orange County throughout the day Thursday and into early Friday morning.
The storm originating in the Gulf of Alaska began moving across California’s Central Coast Wednesday afternoon and into Ventura County, reaching Los Angeles County in the evening strengthened by moisture from the Eastern Pacific.
The storm was generating strong, gusty winds, prompting the NWS to issue a wind advisory on Santa Catalina Island; the San Gabriel and Santa Monica mountains; the Antelope, San Fernando, Santa Clarita, Pomona and San Gabriel valleys; L.A. County Beach cities, metropolitan Los Angeles, including Downtown L.A., and the Hollywood Hills.
In the mountains and the Antelope Valley, the wind was forecast to blow at 20-30 miles per hour, with 50-mph gusts. In the other L.A County areas subject to a wind advisory, winds of 15-30 mph with 40-mph gusts were expected. The advisory, indicating winds or gusts of at least 35 mph, was scheduled to be in effect until midnight.
“Gusty winds will make driving difficult, especially for high profile vehicles,” warned the NWS in a statement.
Ahead of the storm, residents made preparations in Malibu, much of which was devastated by the Woolsey Fire this month. In Orange and Riverside counties, residents near the Holy Fire burn area were being urged to evacuate before the rain began falling. Mandatory evacuation orders were issued late Wednesday afternoon for some areas of Lake Elsinore in Riverside County.
“All Malibu residents are urged to prepare for potential flooding, mudslides, power outages and evacuations,” the city said in a statement Tuesday. But those preparations must not involve removing debris, which contains hazardous materials. No debris removal from burned properties is allowed until inspections by state and county health officials have been completed.
The Los Angeles County fire and sheriff’s departments both deployed additional staffing into the burn area in light of the threat of potentially damaging flooding. Officials with both agencies stressed the need for residents to adhere to whatever evacuation orders are issued.
“Evacuation orders should not be taken lightly and are ordered because there is a threat to life and property,” according to a joint statement from the agencies.
Along the coast, a high surf warning is scheduled to be in force until 10 a.m. Friday. The NWS said surf of 10 to 18 feet with sets of up to 20 feet would batter the shore, and a second, larger swell would generate surf of 20-24 feet later Thursday morning through Friday morning. The surf will lower to 10-15 feet Friday afternoon and evening.
In Orange County, where surf of 5-10 feet was pounding the shore this morning, a high surf advisory will be in effect until 10 p.m. Friday, after which the surf will gradually subside, forecasters said.