1st of 2 atmospheric river storms barrels into Bay Area; late-night roadway flooding expected

The first of two atmospheric rived-fueled storms moved into the Bay Area Wednesday, bringing heavy rains that triggered flood advisories covering large portions of Sonoma and Marin counties.

Widespread flooding, downed trees, and possible power outages are possible across the Bay Area into Wednesday evening, and the storm is expected to possibly cause some flooding issues into the morning commute Thursday.

The storm also caused part of Highway 9 in Santa Clara County from Sanborn Road to Redwood Gulch Road to be closed because of a mudslide, around 11:30 p.m., and in the North Bay, Caltrans said Highway 121 in Sonoma County was closed from Highway 12 to Arnold Drive due to flooding.

The National Weather Service said Wednesday the storm will be "impactful" with strong, gusty winds and heavy rainfall as the system moves slowly over the region throughout the day.

The heaviest rain and strong winds were expected to arrive by midday, continuing through the afternoon and well into the night. Rainfall totals were expected at 1.3 to 2.5 inches for most of the Bay Area, 2 to 3 inches in the North Bay, and 4 inches and above for coastal Sonoma County and the Santa Cruz Mountains.  

KPIX First Alert Weather: Current conditions, alerts, maps for your area

The Bay Area office of the National Weather Service issued a flood advisory at 4:45 p.m. in an area stretching from north of Novato to Cloverdale, covering a large part Sonoma County. The advisory indicated flooding in low-lying areas with poor drainage from excessive rainfall was likely. The advisory will remain in effect until 10:45 p.m. When the advisory was issued, that part of the region was experiencing extremely heavy precipitation.

A second flood advisory was issued for southern Marin County shortly before 6 p.m. That advisory will remain in effect until 11 p.m. About 20 minutes later, a third advisory was issued for a large portion of San Francisco. That advisory will stay in effect until 11:15 p.m., weather officials said.

A Flood Watch was in effect from 4 a.m. Wednesday through 4 a.m. Friday, even after the heaviest rain moves out of the region by Wednesday night since river and stream flooding could still be a problem.  

One school district in Sonoma County said it its schools would be closed on Thursday for storm-related reasons, and because of a power outage. 

• Kashia School District 

Other school districts will still have classes.

There is also a High Wind Warning in effect for the Santa Cruz mountains with gusts expected to top 50 mph. A Wind Advisory was issued for the rest of the Bay Area, with gusts in the 40 to 50 mph range along the coast, 30 to 40 mph inland.

The weather service said residents should prepare for the likelihood of downed trees and power outages on Wednesday, especially within the higher elevations.     

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In addition, the weather service said there will be hazardous marine and beach conditions through Thursday, with storm-force winds over the open waters and gale-force winds in the bays. Large breaking waves will also impact the coast and create dangerous conditions for surfers, swimmers, and anyone on rocks, jetties, or beaches.

With winds already kicking up Wednesday morning, a large tree was toppled outside a home in Saratoga and trapped a young woman who required rescue.  

According to a Santa Clara County Fire post on X, crews responded to Oak Place in Saratoga at around 8 a.m. after receiving reports about the fallen tree. 

Fire crews were able to extricated the female -- a juvenile -- who was transported to the hospital with non-life-threatening injuries.  

By Thursday, an unsettled pattern of scattered showers and possible isolated thunderstorms should persist through Saturday ahead of another strong storm system expected to arrive on Sunday, the weather service said. 

KPIX meteorologist Paul Heggen said the system would bring more heavy rain to the region along with additional wind impacts.

"The model guidance indicates that wind may be a bigger threat with this system, as the heart of the storm tracks right along the coast," said Heggen. "[There will be] widespread 40 to 50-plus mph gusts capable of significant tree damage, given the saturated soils around the region."

In the Sierra, Wednesday's storm has prompted a Winter Storm Warning in effect through Friday around Lake Tahoe, and Friday night for the western slope. Snow levels will steadily drop from 6,500 feet Wednesday to 3,500 feet by Friday. Some 6 to 12 inches of snow is expected at lake level, and up to 3 feet of snow above 7,000 feet.

The second storm arriving on Sunday will have snow levels hovering around 4,000 feet and will be the larger of the two events, Heggen said.

In:
• Storm
• Sierra Nevada
• High Winds
• Storm Watch
• Atmospheric River
• Lake Tahoe
• Flooding
• San Francisco Bay Area