A strong, wet storm that was moving toward the West Coast on Thursday was set to deliver gusty winds, heavy snow and drenching rains across California. Forecasters are growing increasingly concerned about its potential to bring flash flooding to Southern California in the coming days.
This complex system will bring potentially the most widespread and heaviest precipitation to the state so far this fall, and the heavy soaking is expected to bring a decisive end to the state’s wildfire season.
Here are the key things to know:
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One or more inches of rain is forecast across the state, with two to four inches of precipitation in the foothills and mountains and potentially up to eight inches in the wettest locations.
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While this storm is expected to move quickly across Northern California, models suggested the storm would stall over Southern California, potentially bringing more rain than earlier forecasts advised. Some evacuation warnings were set to begin around Los Angeles on Thursday evening.
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With those models, expectations for the storm intensified quickly on Wednesday among forecasters in Southern California. “There’s a lot of uncertainty in when the rain is going to start and how long it’s going to last and how much we’re going to receive,” said Todd Hall, a meteorologist with the National Weather Service office for the Los Angeles area.
The rain will hit Northern California first.
In the Bay Area, the bulk of the rain reached the North Bay in the early hours of Thursday morning, bringing the potential for minor flooding.
“It’s forecast to move through the Bay Area at a decent pace,” said Roger Gass, a meteorologist with the Weather Service’s office in Monterey, Calif.
Scattered showers are expected through Friday. Most urban locations, including downtown San Francisco, are predicted to receive a half inch to one and a half inches of rain through Friday morning, with the mountains of the North Bay and the Santa Cruz Mountains likely to pick up four to five inches, according to Mr. Gass.
The Weather Service issued a wind advisory for the greater Bay Area, warning of damaging winds Wednesday night into Thursday morning. A more severe wind warning was issued for some areas, including the North Bay and the hills of Marin County, with winds of up to 60 miles per hour possible.
The rain pushed southward into Central California late Wednesday night into early Thursday. This region — the Big Sur coast, where Highway 1 winds along rocky cliffs that plunge into the sea — could see some of the heaviest rains, with three to five inches very likely and up to seven inches possible.
Southern California’s forecast is evolving quickly.
There is still significant uncertainty in the forecast for Southern California, where meteorologists have become increasingly concerned about the storm stalling and dropping a fire hose of rain over the region for several days.
The Weather Service said the rain was likely to arrive in Santa Barbara County mid- to late morning on Thursday before pushing into Ventura County Thursday afternoon, and Los Angeles County by Thursday night.
Because the storm may stall off the coast, the chance for rain continues into Friday and Saturday, potentially even Sunday.
Bryan Lewis, a meteorologist at the Weather Service’s office in Los Angeles, Calif., said Saturday looks to be the most intense day of high rainfall totals.
“Especially during the afternoon and evening,” he said. “It’ll be the most concerning when it comes to things like debris flows for those burn scar areas.”
Downtown Los Angeles could record anywhere from one inch to six inches, Mr. Hall said, but two to four inches was the most likely outcome as of early Wednesday afternoon.
“We’re going to get rain, we just don’t know the amounts,” he said.
The mountains above Santa Barbara and Ventura Counties and the hills of Los Angeles could record up to eight inches of rain through the weekend, raising concerns about landslides.
Wildfires, including the catastrophic fires in Los Angeles in January, have torn through these areas in recent years, and the landscape is particularly susceptible to landslides and debris flows. Los Angeles County officials said an evacuation warning would be in place for some places, including areas that were burned in the Palisades and Eaton fires in January, beginning Thursday evening. A warning means people should be prepared to leave.
“We’re going to get flooding, but we just don’t know whether it’s going to be road flooding or debris flows in wildfire areas,” Mr. Hall said.
In the southernmost area of the state, San Diego is expected to see lighter rains for Thursday night into Friday morning, with increasing chances for heavier rainfall on Friday into Saturday. In San Diego County, low-lying areas could record two to two and a half inches of rain with up to four inches in the mountains.
Rain and snow are expected in the mountains.
Farther from the coast, the Sierra Nevada is expected to get two main pushes of precipitation, the first likely occurring Thursday into Friday and the second Saturday into Sunday.
Pulling in moisture from the tropics, this storm is warm. Rain is forecast at lower elevations and snow will fall only at the highest elevations, according to Brian Ochs, a meteorologist with the Weather Service in Hanford, Calif.
The Weather Service issued multiple alerts for the Sierra, including a winter weather advisory for the greater Lake Tahoe area, warning that travel could be difficult over Donner, Carson and Ebbetts Passes from Thursday afternoon into Friday morning.
