California bore the brunt on Sunday of what meteorologists referred to as a “bomb cyclone” and an “atmospheric river,” a convergence of storms that brought more than half a foot of rain to parts of the Bay Area, along with high winds, concerns about flash floods and the potential for heavy snow in the Sierra Nevada.
The National Weather Service warned that the rainfall could cause mudslides, especially in areas scorched by wildfires.
From Marin County to the area just south of Big Sur along the Pacific Coast, flash flood watches were in effect until late Sunday night and, in some areas, early Monday morning, including parts of the San Francisco peninsula.
The threat of flash floods prompted evacuation orders for parts of San Mateo County, south of San Francisco, where officials warned that downed trees, branches and mudslides could impede efforts to leave. In San Francisco, emergency responders ordered evacuations on one block, where they said several structures were threatened by a tree that was leaning.
The convergence of storms comes at a challenging time for California, which has been besieged by wildfires and drought, the result of extreme weather brought on by climate change.
As of 11:30 a.m. local time on Sunday, at least 66,000 utility customers in the Bay Area had lost power, said J.D. Guidi, a spokesman for Pacific Gas and Electric. About two-thirds of those outages were reported in Sonoma and Marin Counties.
The utility said that more than 3,000 workers and contractors were ready to help restore power. It said it also expected to bring in hundreds of workers from areas that were less affected by the storm, adding that equipment had been moved into position before the storm.
At San Francisco International Airport, 139 flights had been delayed and 52 others had been canceled as of 10:10 a.m. local time, said Cathy Morrison, an airport duty manager.
“The atmospheric river is aiming a fire hose, if you will, into our area,” Sean Miller, a meteorologist for the Weather Service in Monterey, Calif., the forecast office for the Bay Area, said on Sunday.
An atmospheric river is a concentrated plume of moisture that extends over the ocean, typically in the troposphere, the lowest layer of the atmosphere, Mr. Miller said, adding that the current trough was angled toward the North Bay.
In the Pacific Northwest, a bomb cyclone, a type of storm known for its falling atmospheric pressure, was expected to push the atmospheric river south, affecting areas south of San Francisco, Mr. Miller said.
“This is more typical of something we tend to see in December or January,” he said, pointing out that the confluence of the two meteorological phenomena was “anomalous.”
The high winds and heavy rain prompted the authorities to close the sidewalks on the Golden Gate Bridge on Sunday. In the East Bay, organizers of the Alameda County Fair closed the event on Sunday because of the storm, while an Ironman triathlon scheduled for Sunday in Sacramento was canceled.
Alcatraz Island canceled all of its tours on Sunday, and the San Francisco and Oakland zoos were also shut.
In San Mateo County, the authorities on Sunday morning ordered people in parts of the county to evacuate. “Please do not wait,” they said in a statement.
Elsewhere in the county, CalFire, the state’s firefighting agency, shared a video on Sunday morning of a small fire on a road that likely had ignited after a pole and a tree fell down. Downed trees have been reported across the northern part of the state.
White it appeared that most of the flooding was reported in the North Bay, landslides and dangerous road conditions were reported in areas across Northern California. Images were shared on social media on Sunday of small and large landslides, including one in Plumas County showing piles of rocks blocking a highway.
The California Highway Patrol in Truckee, north of Lake Tahoe, said on Twitter on Sunday afternoon that rocks and water had fallen down a mountainside, blocking a road.
On Tuesday, Gov. Gavin Newsom issued a proclamation extending California’s drought emergency statewide and asked residents to redouble their water conservation efforts. This has been California’s second driest year on record, with near-record low storage in the state’s largest reservoirs, the governor’s office said.
Severe drought conditions, worsened by climate change, continue to affect much of the Western United States and even the northern part of the Great Plains.
While droughts are not uncommon in the region, scientists say that climate change, in the form of warming temperatures and shifts in precipitation, is making the situation worse.