With some California mountain towns still buried in snow from weeks of cold-weather storms, a warm and potentially dangerous storm began to move into the state on Thursday, bringing heavy rain, wind and snow and raising concerns about widespread flooding.
The storm, known as an atmospheric river, arrived with rainfall along the coast late on Thursday morning and was expected to become heavier and push inland throughout the day and into Friday. It will melt lower-elevation snow and cause flooding Thursday through Friday, forecasters said. And rain could linger in some parts of the state through the weekend before another atmospheric river approaches next week.
Forecasters also warned of “copious amounts of heavy snow” in the mountains of Northern and Central California through Friday.
By Thursday evening, parts of the state had already recorded close to two inches of rain, David Roth, a meteorologist with the National Weather Service said by phone.
“The rainfall should increase with intensity across much of central California overnight,” Mr. Roth said. “A lot more is to come.” There had already been some reports of cars hydroplaning in several inches of water on roadways, he added.
The fire department in Contra Costa County said on Twitter that one person was in hospital in critical condition after a four-vehicle accident which the authorities said they believed was linked to the weather. Another other person involved in the accident sustained “moderate” injuries, it added.
Officials warned of potentially devastating river flooding, urban flooding and coastal flooding as the storm arrived. Strong winds could also knock down trees and power lines.
“This is an unrivaled, unparalleled weather event not experienced in several decades, perhaps back to 1969,” Kris Mattarochia, a science and operations officer at the National Weather Service office in Hanford, Calif., said on Thursday at a news conference with Fresno County officials, who warned residents to be prepared to evacuate.
“There will be high water in areas that are usually not impacted, so everyone needs to be ready,” Mr. Mattarochia said.
In anticipation of the storm, Gov. Gavin Newsom on Thursday expanded a state of emergency that he had declared last week for 13 counties to include 21 additional counties, many of them in Northern and Central California. The additional counties covered by the emergency declaration included Fresno, Sacramento, San Francisco, San Mateo, Santa Clara and Santa Cruz.
Across the state, close to 45,000 customers were without power on Thursday evening, according to poweroutage.us.
In Humbolt County, local authorities declared a state of emergency because of consecutive “major winter storms” that it said had resulted in a large accumulation of snow, impassible roadways, downed trees, disrupted utility services, damaged and flooded roadways, mudslides, damaged structures and dead livestock. These impacts, the county said, were “exhausting and exceeding” its available resources.
The City of Merced, about 115 mile southeast of Sacramento, issued an evacuation warning for some of its residents on Thursday evening.
Excessive rain was forecast along the Central Coast on Friday, the National Weather Service said on Twitter. The stretch from Salinas to San Luis Obispo, as well as the inland regions surrounding Fresno and Visalia, the Service said, were at “high risk.” The last time this category was used in this region was in 2010.
The atmospheric river that was barreling into California was essentially a giant conveyor belt of moisture, known as a pineapple express because it built up moisture in a tropical area near Hawaii.
The storm will bring heavy rain to communities above about 9,000 feet, where there are already large amounts of snow. In Southern California, the rain may fall as high as 11,000 feet, according to the Weather Prediction Center.
The most significant snowmelt, and overall flooding risk, was expected in areas with a shallow snowpack, typically below 5,000 feet. Creeks and streams in the foothills of the Sierra Nevada mountains were most vulnerable to flooding, the Weather Prediction Center said.
Significant river swelling was expected late Thursday into the weekend in many parts of Northern and Central California because of rainwater runoff and reservoir releases, the California Nevada River Forecast Center said.
State officials said that the California National Guard was moving high-water vehicles into position in preparation for flood-response operations. San Francisco said it was giving away 10 free sandbags per address to residents and businesses.
The Santa Lucia Mountains in coastal Central California could be inundated with up to 15 inches of rain, flooding roads and creeks and producing mudslides. In the Bay Area, one to four inches of rain were expected in lower elevations and as much as 10 inches in higher elevations through Friday.
San Luis Obispo County, which was bracing for four to eight inches of rain in parts, warned some residents of low-lying areas of coastal Oceano, Calif., to prepare to evacuate. Los Angeles County was expecting 0.75 to 1.5 inches of rain.
At higher elevations, the rain is not expected to cause as much flooding because deep snow may absorb the additional moisture.
However, the rain could add more weight to the snow, putting additional stress on buildings and increasing the risk of avalanches. The National Weather Service in Los Angeles warned that “significant avalanches” were possible above 5,000 feet, and that roofs could collapse.
While rain was the main concern, up to two feet of snow could fall in the Coastal Range and Shasta County mountains and up to eight feet could blanket the Sierra Nevada mountains, the National Weather Service said. Wind gusts in the mountains could reach 80 miles per hour. Blinding conditions and road closures were likely.
“Any sort of travel is discouraged, especially up in the mountains,” Katrina Hand, a meteorologist at the National Weather Service in Sacramento, said in an interview on Thursday.
The latest storm arrived as mountain communities continued to dig out from blizzards that occurred last month.
At least 11 people have died in the snow-packed San Bernardino County region since Feb. 23, although it was not clear how many of those deaths were directly related to snowstorms, Shannon Dicus, the county sheriff and coroner, said on Wednesday.
Mr. Newsom said the expanded emergency declaration would allow California to mobilize more equipment and personnel.
The California Department of Transportation already had more than 4,000 employees working statewide, including more than 57 who were operating snowplows, graders, loaders and dump trucks in San Bernardino County, officials said.
“The state is working around the clock with local partners to deploy lifesaving equipment and first responders to communities across California,” Mr. Newsom said in a statement on Thursday. “With more dangerous storms on the horizon, we’ll continue to mobilize every available resource to protect Californians.”
Livia Albeck-Ripka contributed reporting.