A winter storm is expected to bring much-needed rain and snow to Central and Southern California this week, but also a risk of flash flooding and mudslides, especially in San Luis Obispo and Santa Barbara Counties and areas of Los Angeles County that were burned by wildfires last month.
The robust storm system will swing waves of moisture up and down the California coast, including the already saturated northern part of the state, while also sending moisture inland, adding feet of snow to the Sierra.
The storm will pull in moisture from the subtropics, and the severity of its impacts will depend on where “the bulk of that moisture goes through and how slowly the storm moves,” said Kristan Lund, a meteorologist with the National Weather Service.
“We’re looking at likely seeing the strongest storm of the season and at least so far,” Ms. Lund said.
It’s too early to pin down the forecast with certainty, but as of Monday, the most likely scenario is that rain ahead of the storm will arrive Tuesday night, with rainfall amounts anticipated to be light. After a brief lull of largely dry conditions from Wednesday afternoon, the storm is expected to arrive in San Luis Obispo and Santa Barbara Counties through the early hours of Thursday morning and then head to Ventura and Los Angeles Counties by late morning.
However, behind the storm a cold front will sweep in, bringing intense rainfall that could set off life-threatening debris flows around recent burn areas, particularly in the areas of the Eaton, Palisades, Franklin and Bridge fires, said Andrew Rorke, a meteorologist with the National Weather Service in Los Angeles.
“This will be the worrisome portion of the storm, as rainfall rates near one inch per hour will be possible,” Mr. Rorke said. The most intense period is expected between 4 p.m. Thursday and 4 a.m. Friday: The cold front, the storm’s final surge, moves into San Luis Obispo County late afternoon Thursday, and then pushes through to Santa Barbara, Ventura and Los Angeles Counties into Friday morning before exiting.
The Weather Service issued a flash flood watch for all recent burn scars of southwest California, in effect from early Thursday afternoon through to Friday morning. Additional areas may be added.
Expected rainfall totals will range from one to three inches at lower elevations, and three to six inches across mountains and foothills. Portions of San Luis Obispo county may receive almost twice these totals, with three to five inches around coasts, and six to 10 inches near the Santa Lucia mountains.
Currently, portions of Southern California face a moderate risk of excessive rainfall, embedded within a broader slight risk for much of the Central and Southern California coast.
The projected rain totals for Santa Barbara County are close to the amount of rain that fell across the area in February 2023, when flooding rains turned streets into rivers. But the flooding this week should be less severe if the storm unfolds as expected.
Two years ago, there was a lot of rain leading up to the heavy rain on Feb. 9 and soils were saturated, whereas this year, the winter has started off dry and the ground will be able to absorb more of the rain.
Snow is also expected to fall in the mountains at elevations as low as 5,000 to 7,000 feet. The highest elevations may see several feet of snow.
Southern California has not received as much rain this winter as areas to the north. and it had unusually low precipitation in January. The region has recently received a few rounds of light to moderate rain, and that, combined with the upcoming storm, could significantly reduce the risk of wildfires.
“Recent rains, especially considering next week, may be sufficient enough to switch to low fire season, but conversations do need to be had before we make that official declaration,” Ms. Lund said.
Northern California will probably see significant rain as well.
At the same time, Northern and Central California will be affected by an atmospheric river from Wednesday to Friday, with the heaviest rainfall expected early Thursday in the Bay Area and along the central coast. Atmospheric rivers are corridors of water vapor in the atmosphere that form over the ocean and are transported through the sky by powerful winds. When they reach land, they unleash moisture that can lead to heavy rain.
Most areas are forecast to receive at least two inches of rain, while higher elevations, particularly the Santa Cruz and Santa Lucia Mountains, could see up to eight inches.
The National Weather Service warned that urban and small stream flooding is likely, as the ground, already saturated from recent rain, struggles to absorb more rainfall. There is also the risk of shallow landslides, localized flash flooding and rising river levels.
In the Sierra, the potent winter storm is possibly the strongest of the winter by a wide margin, bringing periods of heavy snow, rain and light snow, and wind impacts to recreation and aviation interests, forecasters in Reno said.
Multiple feet of snow are expected to fall along the Sierra peaks, and at least a foot for lower elevation Sierra communities, like Truckee, South Lake, Mammoth Lakes. Communities in the foothills will likely see several inches to a foot of snow, and snow may even fall into the valleys in western Nevada.
Nazaneen Ghaffar and Judson Jones contributed reporting.
