As a series of rainstorms begins moving across the state, officials in areas recently scarred by devastating wildfires are on high alert for potential mudslides and flash flooding.
Riverside County ordered mandatory evacuations in several neighborhoods burned by the Holy fire, including Amorose, Alberhill, Glen Eden, Grace, Matri, Rice and parts of the Glen Ivy, Horsethief, Laguna, McVicker and Withrow neighborhoods.
“People in these zones MUST GO NOW,” the county’s Emergency Management Department said in an alert.
Heavy rains predicted to hit the Big Sur coast closed parts of Highway 1 on Wednesday. The iconic route will remain closed Thursday at its most vulnerable slide points, with a possible reopening Friday morning after the storm has passed, according to Caltrans.
The National Weather Service issued a warning of possible flash flooding in three counties in Northern California in advance of a storm expected to arrive late Wednesday.
Forecasters predicted the second in a series of three storm systems this week could drop more than an inch of rain on the Camp fire burn area in Butte County; the Carr, Delta and Hirz fires burn areas in Shasta County; and the Mendocino Complex fire scar in Lake County.
Because the rain will be spread out over more than 12 hours from Wednesday through Thursday morning, forecasters are not especially concerned about deadly mudslides in the burn areas. However, flash flooding is possible if a heavy rain cell forms directly above one of those areas, said Mike Smith, a weather service meteorologist in Sacramento.
The soil in areas recently burned cannot absorb significant amounts of moisture, so excessive precipitation can lead to fast-moving flows containing mud, debris and even trees and boulders. When rain comes over time, it can be gradually absorbed or dispersed, but when areas see rapid runoff, that’s when entire hillsides can come down without warning and with deadly results.
Smith said the chances of ash and debris flows are more likely in Butte County because it’s freshly burned, but that Shasta and Lake counties should also be on alert.
“Our concern is if we get a heavy shower or if a thundershower goes right over the burn area,” Smith said. “People should be prepared, but a large-scale mudslide isn’t something we’re especially concerned about right now.”
Smith said there is a greater chance the areas could see fast-moving floods because vegetation that typically would slow water was charred during the fires.
The Butte County Emergency Operations Center has prepared for the rain by clearing drainage culverts of debris, said Matt Gates, public information officer for the Paradise Police Department.
The California Conservation Corps also has been working on erosion-control measures for mountain slopes filled with fire-damaged pine and chaparral.
“Ash doesn’t absorb water, which complicates things,” Gates said.
The National Weather Service said people in burn areas should prepare to leave quickly if heavy rain begins, noting in its alert that “this could quickly become a dangerous situation.”
Butte County officials said residents should not wait for a flash flood warning because hazards could develop before an alert is issued.
“If you are in the impacted area or are located near a waterway downstream from the impacted area, you should be prepared to immediately evacuate to high ground in the event of heavy rainfall, an evacuation notice, or any evidence of soil instability in your area,” county officials wrote in a news release.
Rain is also expected to affect burn areas in Southern California.
The system will bring half an inch to 2 inches of rain to Los Angeles and Ventura counties. Light showers could linger around the Grapevine into Friday, said Bonnie Bartling, a weather specialist with the National Weather Service in Oxnard.
Debris flows in areas charred by the Woolsey and Hill fires are unlikely based on the current rainfall forecast, which falls below the U.S. Geological Survey’s threshold of half an inch of rain per hour that could trigger mudslide danger, Bartling said.
“They’d have to get a half-inch or more in a very short period of time,” she said. “We’ll continue to monitor it to see if anything materializes.”
But the city of Malibu was still cautioning residents to prepare for weather problems, including potential flooding. Free sandbags were being offered at fire stations on Carbon Canyon Road, Malibu Road, Decker Road and Pacific Coast Highway.
At least one neighborhood was posting notices on all homeowners’ doors to alert them to the potential danger and urging them to monitor the weather and be prepared to evacuate.
The subdivision, Malibu West, also asked the city to work with the Los Angeles County Flood Control District to clear rocks and trees from the nearby Trancas Creek channel, a move recommended by the National Park Service to reduce the risk of debris flows in the area.
Voluntary evacuation warnings were also issued in Orange County for residents who live in the Holy fire burn area. Forecasters predict that some mountain areas could see more than 4 inches of rain through Thursday, bringing a risk of flash floods and debris flows in the 23,000-acre burn area.
The storm is also bringing a high surf advisory from Wednesday through at least Thursday night: Waves in Los Angeles, Santa Barbara and Ventura counties will range from 6 to 12 feet. Surf in Orange County will be 5 to 8 feet, and San Diego County is expected to see waves up to 12 feet by Thursday.
Times staff writers Mary Forgione, Cindy Chang, Alene Tchekmedyian and Matt Hamilton contributed to this report.
6 p.m.: This article was updated with mandatory evacuation orders in Riverside County.
12:35 p.m.: This article was updated with road closures in Big Sur.
12:15 p.m.: This article was updated with the forecast and flood warnings for Southern California.
This article was originally published at 8:25 a.m.