California holds its breath as an atmospheric river and bomb cyclone approach

As California is poised to absorb its third hit from an atmospheric river since Dec. 26, the National Weather Service is warning people to prepare for a major storm with high winds, snow, and "heavy to excessive rainfall, flooding with debris flows and landslides."

"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 morning to help agencies send aid and resources where they're needed, Ward said.

As the system arrives, 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 on Wednesday, 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. But they also predicted it would move slowly; as of late Wednesday morning, the system was still approaching the West Coast.

To people asking, "Where's the storm?" the NWS office in the Bay Area said at 9:39 a.m. local time, "It's still coming."

Rainfall is predicted to reach 2 to 4 inches on the coasts and in valleys, 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 on Wednesday to prepare for potential power outages, and for travel to be threatened by high winds, debris and felled trees and power lines.

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.

"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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