"California is deploying every tool we have to protect communities from the relentless and deadly storms battering our state," Newsom said in a statement. "In these dangerous and challenging conditions, it is crucial that Californians remain vigilant and follow all guidance from local emergency responders."
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The emergency declaration enables impacted counties to immediately access Direct Federal Assistance to help protect public safety and property. The request for Direct Federal Assistance includes generators, road clearance equipment and potential sheltering and mass care assistance.
Forecasters warned that widespread heavy rain would raise the threat of flooding in a state still digging out from earlier storms.
Rain spread across the north by early afternoon Thursday. The heaviest downpours were expected to last into early Friday, followed by lesser precipitation.
The flood threat will come from the combination of rain and the melting of lower parts of the huge snowpack built in California's mountains by nine atmospheric rivers early in the winter and later storms fueled by a blast of arctic air.
The new atmospheric river is a type known as a "Pineapple Express" because it is a deep tap of warm subtropical moisture stretching over the Pacific to Hawaii. Its greatest impacts were expected in northern and central California, with much less precipitation in the south.
SoCal to see more rain as 1st of 2 atmospheric rivers arrives Friday
The snowpack at high elevations is so massive it should be able absorb the rain, forecasters said. But elevations below 4,000 feet will see melting and runoff.
The California Department of Water Resources activated its flood operations center, Director Karla Nemeth said.
"All of this could contribute to significant runoff," Nemeth said, urging people to be prepared because conditions can be stronger than predicted and rivers and creeks can rise quickly.
Forecasters warned that mountain travel could be difficult to impossible during the latest storm. At high elevations the storm was predicted to dump heavy snow, as much as 8 feet in some locations.
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California's Sierra Nevada snowpack, which provides about a third of the state's water supply, is more than 180% of the average on April 1, when it is historically at its peak.
Yet another atmospheric river is already in the forecast for early next week and state climatologist Michael Anderson said a third appeared to be taking shape over the Pacific and possibly a fourth.
California appeared to be "well on its way to a fourth year of drought" before the early winter series of storms. "We're in a very different condition now," he said.
So much snow has fallen in the Sierra and other mountain ranges that residents are still struggling to dig out days after earlier storms.
Roofs collapsed, cars were buried and roads were blocked. Newsom declared emergencies in 13 of California's 58 counties beginning March 1.
In the San Bernardino Mountains a late February storm reached blizzard status. Mountain towns like Lake Arrowhead were buried. In nearby Crestline, Don Black watched as a team wielding shovels cleared his neighbor's property.
"This is the worst storm I've seen in 34 winters," Black said.
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Evacuation warnings were issued for foothill and mountain communities in central California that are prone to flooding and mudslides. An evacuation order was in place for a small number of central coast residents who live below a levee near Oceano in San Luis Obispo County.
A 50-mile stretch of coastal Highway 1 south of Big Sur was closed until at least Friday, after multiple rockslides were reported.
Ted Craddock, deputy director of the State Water Project, expressed confidence in the 1960s-era Oroville Dam, where thousands of people had to evacuate in 2017 after heavy runoff collapsed the main spillway and the emergency spillway began to erode.
"The spillway has been reconstructed to modern standards, and we're very confident that it will be able to pass the flows that are coming into Lake Oroville," he said.
On the state's far north coast, Humboldt County authorities have organized an emergency response to feed starving cattle stranded by snow.
Cal Fire and U.S. Coast Guard helicopters began dropping hay bales to cattle in remote mountain fields last weekend and then the California National Guard was called in to expand the effort.
Requests for help came from about 30 ranchers, according to Diana Totten, an area fire chief. The hay is being paid for by the ranchers, who provide information on how many head of cattle need to be fed and where they were expected to be located.
"We won't know until the snow melts how many cattle have died due to these conditions," Humboldt County Sheriff William Honsal said in a statement. "But I know this for certain, if we don't act, there's going to be way more that do die and it will be a catastrophe for our county."
The Associated Press contributed to this report.