Massive ‘atmospheric river’ to bring heavy rains, winds, flooding across California

Palo Alto —  

After a brief lull in precipitation Tuesday, forecasters are warning of another “atmospheric river” expected to bring heavy rains and winds to an already saturated California.

The National Weather Service is warning of “widespread flooding” and strong winds across the state Wednesday and Thursday, with most dangerous conditions expected in Northern California — a region that has already seen significant flooding and power outages this week.

Weather officials are tracking a strengthening cyclone, or “bomb cyclone,” moving across the Pacific toward California’s northern coast, creating a strong band of moisture and significant cold front.

The hazard risks and heavy precipitation will be felt “just about everywhere from the Monterey Bay area northward, with some risks even extending south into Los Angeles County,” UCLA climate expert Daniel Swain said in a briefing Tuesday. The heaviest rainfall is likely to be focused in the north and central coastal ranges and the Sierra foothills north of the Bay Area.

Among the chief concerns are strong winds, high rainfall rates and the potential for landslides and flooding, including urban and creek flooding in areas already soaked. Swain said residents that have seen signs of landslides and debris flow over the last several days should remain alert.

“The Wednesday night storm, at least, is definitely one that could produce new mudflows or exacerbate existing ones,” Swain said. “For tomorrow night, that might be a good night to be vigilant or activate whatever plans you have for storms like that, if you live in that kind of terrain.”

The state has already seen multiple days of significant rainfall, first from a massive New Year’s Eve system and then a smaller storm that moved in Monday night. Northern California fared the worst in the New Year’s storm, with substantial flooding forcing rescues and some evacuations, and thousands of Californians losing power in parts of Sacramento and San Francisco. More than 2,000 in Sacramento remained without power Tuesday, according to the Sacramento Municipal Utility District.

City and state officials were calling Tuesday’s break between storms a chance to prepare for the worsening weather, handing out sandbags in many Northern California cities, working to clear storm drains and downed trees and asking residents to prepare homes and evacuation plans.

“We already have some ongoing flooding in the area ... and we’re expecting more heavy rain,” said Scott Rowe, a meteorologist with the National Weather Service in Sacramento. The Sacramento Valley can expect 2 to 3 inches of rain from Wednesday through Friday, while some areas in the foothills could see up to 6 inches — on top of the multiple inches from days prior, he said. San Franciscans can expect to see at least 2 inches of rain from the midweek storm.

A flood watch has been extended across much of the state, from parts of Orange County up through northwest California, beginning Wednesday and stretching through Thursday evening, with warnings of “excessive rainfall” that will probably cause even more flooding. Earlier rains raised water levels on the Cosumnes River near Sacramento to record heights, breaching three levees and inundating nearby areas.

Certain burn scars in northwest California and into the central Sacramento Valley have beenupgraded to a flash flood watch. While burn scars and other flood-prone regions are particularly vulnerable, there is growing flooding potential in urban areas and around small streams, as well as for some rivers, said Eric Boldt, a National Weather Service meteorologist in Oxnard.

The storm is expected to also bring heavy winds to the state — up to 50 to 60 mph for the Sacramento Valley, Rowe said. A high-wind warning for Wednesday and Thursday was issued for the greater Bay Area and central coast, warning of gusts up to 70 mph.

In the mountainous northeast — from Sequoia National Park through the greater Lake Tahoe area and toward the Oregon border — the National Weather Service has also issued a winter storm warning for Wednesday through Friday morning, with forecasts showing heavy snowfall dropping up to 4 feet of snow and wind gusts up to 55 mph.

Rowe warned that people should expect widespread tree damage and possible power outages.

Southern California also saw heavy rains this week but was spared the widespread disruptions, even with the Los Angeles area recording 2 to 5 inches from the New Year’s storm and then a quarter-inch late Monday from the smaller storm, according to the National Weather Service.

Thompson called Tuesday a “temporary drying out” for the L.A. area before the rain picks back up again Tuesday evening, becoming heavier through Thursday. He said most of Los Angeles County could see an additional 2 to 4 inches of rain during those days, with some areas getting up to 6 inches.

“We’re definitely looking at a lot of rainfall for this,” Thompson said.

With high winds also expected, forecasters predicted rush hour Thursday morning could become a nightmare, with minor flooding, the potential for fallen trees and other debris on the road, and possible power outages. Winds in Southern California are expected to reach 30 to 50 mph in the coasts and valleys, and up to 60 mph in the Antelope Valley and mountains.

“Thursday morning could be quite a challenge for people trying to get to and from work or other places,” Boldt said.

Thompson also warned of hazardous coastal conditions during the storm, with high surf Thursday and Friday that could cause coastal flooding and beach erosion.

But even at the end of the week, weather officials are warning that the moist weather pattern is likely to continue for the foreseeable future, though the timing and intensity of these systems are not yet certain.

“As we get into Sunday and early next week, there’s still potential for some wet events, just not as strong as what we’re expecting Wednesday and Thursday,” Boldt said. “We’ll continue to see rainfall going into next week.”

Officials advise people to sign up for county emergency alerts, fuel up vehicles and charge devices in case of power outages and be prepared to evacuate, especially if homes are in flood-prone regions.