A low-pressure system and atmospheric river affecting California early Thursday. (NOAA/NESDIS)
A powerful storm is bearing down on California as an atmospheric river drops through the state. While the fast-moving system will affect nearly the entire state on Thursday, of high concern is Southern California as intense rains fall on the heels of devastating wildfires — downpours that could trigger damaging floods and debris flows.
Flood watches are in effect up and down the California coast and for the many burn scars in Southern California — with evacuation orders or warnings in some hazardous areas. Most coastal and valley areas will see between 1 and 3 inches of rain, with 3 to 6 inches in foothills and mountains. Some spots along the Central Coast and in the Sierra Nevada could see more than 8 inches of total precipitation. The National Weather Service’s Weather Prediction Center has issued a level 3 out 4 risk for flash flooding from Santa Barbara to Riverside — a swath where a number of recent wildfires have burned.
Throughout the state, the moderate-strength atmospheric river (level 2 out of 5) brings risks of abundant moisture, damaging winds and dangerously high surf. The biggest snowstorm so far this year will also deliver multiple feet of snow to the Sierra Nevada. As of early Thursday, the system was bringing heavy rain and localized flooding to San Francisco, with wind gusts of around 40 mph.
Debris flow risk increasing for Southern California
Southern California faces an increasing chance of thunderstorm downpours and debris flows as a cold front moves through Thursday afternoon and overnight. Bursts of rain over burn areas, even as little as a half-inch per hour, can send torrents of mud, rocks and boulders downslope toward communities.
“Total rainfall … is not the worry with this system,” the National Weather Service in Los Angeles wrote. “Rainfall rates are the main concern and due to the increased chance of convective cells there is a higher probability of rainfall rates in excess of 1 inch per hour.”
In the Los Angeles area, the burn scars that carry the highest risk are those from the Eaton and Palisades fires from January and the Bridge fire from the fall. But there were numerous Southern California blazes this past season, and even the past several years, whose burn scars could be problematic.
“The ones we’re worried about the most are the ones that have steep terrain and moderate and high burn severity - where you can see that bare soil,” said Jason Kean, a research hydrologist with the U.S. Geological Survey in Golden, Colo.
Evacuation orders or warnings have already been issued near the sites of the Palisades, Eaton and Bridge fires, as well as the Lake Fire in Santa Barbara County, the Line Fire in the San Bernardino Mountains and the Airport Fire in Orange County.
Kean is most concerned about the Eaton burn scar given the high population at the foot of the rugged San Gabriel Mountains. The mountain front has a long history of debris flows, including deadly and destructive events in 1934 and 1969.
To protect communities at the base of steep mountain ranges, Los Angeles County has a number of “debris basins” designed to capture sediment, rock and vegetation that surge downstream during floods. But not every channel has such a structure.
It’s also unclear how material within burned neighborhoods and urban areas will respond to the rains - only the region’s wildland areas have been assessed for debris flow hazards.
Nearly 100 inches of snow in parts of the Sierra
The storm will also bring high snow potential. Heavy snow with winds gusting to 60 mph — and up to 100 mph along some ridgelines — could severely affect travel in the Sierra Nevada, where winter storm warnings are in place until 10 p.m. Friday.
The National Weather Service in Sacramento is warning of “very difficult travel conditions with chain controls and potential road closures” and snow rates of 1 to 2 inches per hour on Thursday.
Between 2 and 6 feet of snow is forecast for the Sierra, with some rare low-elevation snow expected down to 500 feet in Northern California, including in the city of Redding.
Up to 98 inches of snow could fall in the higher terrain of the southern Sierra, according to the National Weather Service in Hanford.
The drought-plagued southern part of the range has fallen far behind on snowpack — it’s just 53 percent of average for the date - so the storm will provide a much-needed boost during a dry winter.
This may be the last storm for some time, however, as outlooks for California lean warm and dry into the end of February.
