Year’s first storm packs potential for mudflows
A fierce winter storm that forecasters predict will be similar in size to the early December system that triggered debris flows in recent burn areas is set to bring rain and snow to California this weekend.
The chilly system that originated in the Gulf of Alaska is forecast to drop up to 2 inches of precipitation on the Santa Rosa, Redding and Chico areas beginning Saturday morning. Other Northern California areas, including San Francisco, could see up to an inch, according to the National Weather Service.
Gusts up to 50 mph are projected as the first storm of 2019 moves through Northern California. The strongest winds are expected Saturday afternoon.
Southern Californians will get their own share of wet weather starting Saturday night.
The most significant precipitation will happen overnight into Sunday, according to forecasters.
“This is a fairly large storm that will extend across the entire state,” said Eric Kurth, a meteorologist with the National Weather Service in Sacramento. “It’s going to be wet and cool. We could be seeing snow in elevations down to 3,500 feet in some areas.”
About half an inch to an inch of rain is expected along the coasts and valleys in Ventura and Los Angeles counties, with up to 2 inches of precipitation possible along south-facing foothills and mountains. Orange and
San Diego counties are expected to see a quarter of an inch to 1 inch of precipitation in some cities.
Oxnard will open a shelter Saturday evening in anticipation of the storm, city officials said Friday. The shelter, at the Armory at 351 North K St., will open at 4 p.m. and, if the weather improves, close at 9 a.m. Sunday.
A moderate El Niño weather pattern that is brewing in the tropical eastern Pacific Ocean is bringing more precipitation to some regions of California.
The outlook for the next several weeks indicates above-normal precipitation, and the odds are looking good for that trend to continue through the next few months, according to weather experts.
Thunderstorms also are a possibility with the weekend system, which could bring rain down at a faster rate, raising concerns about possible debris flows in recently burned areas across the state.
The key factor in determining whether a storm will result in debris flows is the intensity of the precipitation that falls on an area.
The amount of rain necessary to send mud and debris flowing varies based on how recently the area burned, the topography and other factors, Kurth said.
Soil in burned areas cannot absorb a lot of moisture, so heavy rainfall can lead to fast-moving flows 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.
“I think that’s always a potential concern,” Kurth said. “It’s something we’ll definitely be watching for.”
Weather service forecasters in Oxnard likened the upcoming storm to the system that hammered Southern California with heavy rain in early December, causing debris flows in Malibu and forcing mandatory evacuations in recent burn zones.
Flood control crews began removing debris this week from McVicker Basin in Riverside County, where the Cranston and Holy fires burned last year, in preparation for the storm.
The basin had captured mud flowing down from the burn areas during last month’s storm.
Forecasters expect the storm to dump 4 to 8 inches of snow on Southern California ski resorts and up to 3 feet in the northern Sierra. News of more snow comes as the first snowpack of the year measured at 25.5 inches Thursday, which is 80% of average for the date.
Fresh snowfall also brings the possibility for significant travel issues for motorists.
Kurth said drivers should be on the lookout for chain control warnings.
“People have a temptation to head up to the mountains when we’re getting snow, but Saturday will be a difficult travel day,” he said. “If people want to head up, I’d encourage them to travel ... very early Saturday. The conditions are going to deteriorate throughout the day.”