Helicopter drops water on flames from the Woolsey Fire
This was the disaster scene just days ago as a Los Angeles County Fire Department helicopter drops water on flames from the Woolsey Fire. Now these same areas face rain-driven mudslides. Courtesy of the department.

A bout of overnight rain will have the potential to unleash rockslides, mudslides and minor debris flows in areas recently stricken by wildfires, National Weather Service forecasters said.

The NWS said there’s a chance of rain this evening, a virtual certainty of it after midnight amid southwest winds of around 15 miles per hour and a 50 percent chance on Thanksgiving morning, followed by a partly cloudy afternoon.

The rain could make the traditional Thanksgiving-eve traffic nightmare an even greater challenge for Southern California drivers. Officials warned motorists to be especially careful on rain-slicked highways as traffic crawls.

Between seven-tenths of an inch and an inch-and-a-quarter of rain are expected to fall on coastal slopes and in the foothills, which could trigger slides over areas denuded by the Woolsey Fire in L.A. and Ventura counties and the Hill fire in Ventura County, forecasters said. Highway 1 and Santa Monica Mountain canyon roads are particularly vulnerable, they said.

Authorities reminded Southlanders in burn areas of flood safety preparations that should be made before the rain starts.

The Woolsey Fire unified command warned that mud and debris flows “are a very realistic threat to the communities affected by the Hill and Woolsey fires. Due to an increased probability of mud and debris flows in these fire areas, it is important to plan and prepare. Evacuation orders should not be taken lightly, and are ordered because there is a threat to life and property.”

Some fire-damaged areas remain unsafe, the electrical system is “extremely damaged,” and road crews are working to clear rocks as emergency personnel prepare for the impending storm, Department of Public Works Director Mark Pestrella told the Los Angeles County Board of Supervisors.

“We’re going to have rock fall, we’re going to have roads closed,” Pestrella said. “The roads will not be safe to travel beginning Wednesday evening.”

Pestrella said he expected Pacific Coast Highway would be closed at some point.

Up-to-date information on road closures can be found at www.lacounty.gov/woolseyfire/rain-after-fire-resources.

The NWS said there is a 30 percent probability that the upcoming rainfall will be sufficient to trigger debris flows in the Woolsey Fire and Hill Fire areas.

Burn area residents concerned about mudflow can pick up empty sandbags at their local fire stations, and can visit www.lacounty.gov/larain for storm season emergency resources, including Los Angeles County’s “Homeowners Guide to Flood, Debris and Erosion Control.” The sandbags should be used to divert potential flows, not dam them.

The NWS forecast mostly clouds in L.A. County Wednesday and highs of 56 on Mount Wilson; 62 in Lancaster; 64 in Palmdale; 65 in Saugus and Avalon; 67 at LAX and in Burbank; 68 in Pasadena and San Gabriel; 69 in Woodland Hills, Long Beach and Downtown L.A.

Partly cloudy skies were forecast in Orange County, along with highs of 54 on Santiago Peak; 62 on Ortega Highway at 2,600 feet; 66 in Laguna Beach and San Clemente; 67 in Fremont Canyon; 69 in Yorba Linda; 70 in Trabuco Canyon; 71 in Anaheim and Fullerton; 72 in Irvine and Mission Viejo.

Thanksgiving Day temperatures will be marginally lower in L.A. County and up to 9 degrees lower in Orange County.

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