Updated January 5, 2023 at 5:05 AM ET

Heavy rainstorms barreled into California early Thursday, bringing high winds, heavy rain, snow and power outages as residents braced for possible flooding.

It is the state's third hit from an atmospheric river since Dec. 26.

"We anticipate that this may be one of the most challenging and impactful series of storms to touch down in California in the last five years," said Nancy Ward, the new director of the California Governor's Office of Emergency Services, at a late-morning news conference about the threat.

Gov. Gavin Newsom signed a statewide emergency declaration Wednesday to help agencies send aid and resources where they're needed, Ward said.

As the system arrived, many areas will also be confronted with the effects of a bomb cyclone: The powerful system was seen "undergoing bombogenesis" off California's northern coast, the NWS office in Sacramento said, referring to the rapid intensification of a midlatitude cyclone.

Forecasters have been raising alarms about the storm for days now, saying it could start to affect land as early as Tuesday night.

At about 4 a.m. ET Thursday, the band of rain had touched down in the Los Angeles area and was "moving faster than anticipated," which could be a good thing, as less people will be on the road and rainfall totals could be less than previously forecasted, the NWS Los Angeles office said.

Rainfall is predicted to reach 2 to 4 inches on the coasts, valleys and deserts, while mountains could see up to 10 inches of rain, with heavy snow at high elevations. Forecasters are urging people to show particular care in areas where fires recently burned vegetation, citing the heightened risk of flash flooding and mudslides.

Huge system triggers warnings along the Pacific coast

Weather experts warned people in their coverage areas to prepare for potential power outages, and for travel to be threatened by high winds, debris and felled trees and power lines.

As of 4 a.m. ET Thursday, nearly 196,000 Californians were without power, according to Poweroutage.us.

In an area stretching for hundreds of miles along the coast, National Weather Service offices from Los Angeles to Eureka, Calif., and Medford and Portland, Ore., alerted people to the threat of damaging winds, with peak gusts expected to top 60 and 70 mph in some areas, and even exceed 100 mph in parts of the Bay Area.

"The most intense part of this weather event will occur later this evening and last through noon Thursday," the NWS office in Los Angeles said. "A slow moving cold front will entrain the moisture from a moderate atmospheric river."

Atmospheric rivers carry prodigious amounts of water

Atmospheric rivers are a normal part of the West Coast's weather pattern, and they're often the solution to months of warm-weather drought, bringing sorely needed rain and snowfall that packs water away high in the mountains.

The precipitation can be extreme: A single atmospheric river "can carry more water than the Mississippi River at its mouth," as NPR has reported. Forecasters have long warned that the systems' winds are very dangerous. In 2017, one of the storms toppled the legendary "Pioneer Cabin Tree" sequoia in Calaveras Big Trees State Park.

"It's just a narrow area of high moisture that gets transported away from the tropics towards the higher latitudes," often before a cold front arrives, as NWS senior forecaster Bob Oravec recently told NPR.

For states along the West Coast, atmospheric rivers are "actually responsible for a good majority of the rainfall during the colder season, which is the season when they get most of their rain," Oravec said.

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