The first in a new series of winter storms that will hammer Southern California through next week is expected to arrive Friday night, bringing periods of heavy rain to recent burn areas that officials worry could unleash rockslides and debris flows.
The Pacific system is expected to drop half an inch to an inch of rain across much of Los Angeles and Orange counties through Saturday. Ventura County could see a bit more — 1 to 3 inches — before the storm moves out of the area.
The National Weather Service predicts the heaviest period of rain will begin about midnight and last through 7 a.m. Saturday. There is also a chance of thunderstorms. Peak rain rates of half an inch to three-quarters of an inch per hour are expected over the Woolsey and Hill fire burn scars, which could trigger mudslides, said Kristen Stewart, a weather service meteorologist in Oxnard.
The key factor in forecasting debris flows is the intensity of the precipitation as it falls on an area. The amount of rain necessary to start mud and debris flowing varies based on how recently the area burned, the topography and other factors.
“We’re in a pretty wet pattern right now,” Stewart said.
Malibu city officials said they were preparing for possible debris flows at canyon bases and in ravines by clearing storm drains and culverts to prevent flooding. They also have temporarily set up concrete K-rails and placed heavy equipment in areas likely to flood. Residents should be on alert for possible evacuations and take warnings seriously.
“During storm events, flooding, mud and debris flows are a very real and dangerous threat to the communities affected by the Woolsey fire. Due to an increased probability of mud and debris flows in these fire areas, it is important to plan and prepare,” the city wrote in a statement. “Prepare for lack of water, power and natural gas, nonfunctional traffic signals, and roads that may be impassable.”
This isn’t a new situation for Malibu. The first winter storm of the season last week sent mud and debris sliding across Pacific Coast Highway and left vehicles stuck in the muck.
In anticipation of the upcoming storms, the California Department of Transportation will close a portion of State Route 23 in the Woolsey fire burn area starting at 10 p.m., the agency announced Friday morning. The road will be closed from PCH to upper Mulholland Highway in Malibu. If it’s safe enough, residents with identification will be allowed through.
Caltrans also said maintenance crews will be working around the clock to clear debris from drains in the area.
Crews in Orange County were also preparing for the rain this week, setting up equipment and K-rail barriers near Trabuco Canyon Road, below the area where the Holy fire burned over the summer. A strong storm that hit the region in December triggered debris flows in that region.
Santa Barbara County officials don’t anticipate rain heavy enough this weekend to cause debris flows in the Thomas fire footprint. However, that could change if a thunderstorm brings heavier rain to the region. Residents should stay alert to changing conditions, the county said.
The soil in burn areas cannot absorb a lot of moisture, so heavy rainfall can lead to fast-moving runoff containing mud, debris and even trees and boulders. When rain falls over time, it can be gradually absorbed or dispersed, but when areas see rapid runoff, entire hillsides can come down without warning, sometimes with deadly results.
This weekend’s storm follows a somber anniversary for the region. It’s been a year since deadly mudslides tore through Montecito on Jan. 9, 2018.
Forecasters said Southern Californians will have a brief respite from the wet weather during the day on Sunday before a second system hammers the region that night.
That could make for cold, soggy picket lines for Los Angeles Unified School District teachers who are expected to begin striking on Monday. The rain is expected to linger through Thursday.