The second day of an atmospheric river brought heavy rain to Northern California and the Bay Area on Thursday, with the heaviest falls in the North Bay, ahead of the storm moving south on Friday.
The National Weather Service said Marin, Napa and Sonoma counties are under a flood advisory until early Saturday morning. The 24-hour rainfall in the North Bay included more than 7 inches at the Santa Rosa Airport and reports of more than 10 inches in the coastal mountains of Sonoma County, the Weather Service’s Bay Area office said in its daily forecast discussion.
Until 6 a.m. Friday, a high surf advisory was also in effect for the Bay Area coast from Point Reyes south to Big Sur, including southern Monterey Bay. The Weather Service said large breaking waves between 4 and 6 meters would be seen on southwest-facing beaches and between 5 and 6 meters along well-exposed west-facing beaches.
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The storm’s intensity results from a combination of the tropical moisture from the atmospheric river, combined with a deep, rapid drop in pressure when a polar air mass collides with a tropical air mass, known as bomb cyclogenesis, or a bomb cyclone.
Most of the storm impacts occurred north of the Golden Gate, including downed trees and power lines, along with thousands of power outages since Wednesday. Pacific Gas and Electric said more than 2,400 people were without power as of 6 a.m. Wednesday, most of them in the North Bay.
Strong southerly winds hitting the Bay Area are expected to decrease Thursday but will not improve substantially until Friday evening, the Weather Service said.
Precipitation amounts could decrease slightly in the North Bay on Thursday; However, 1 to 7 centimeters of rain is still expected in the valleys and up to another 15 centimeters in the mountains over the next 24 hours.
While the rain will be mostly concentrated north of Interstate Highway 80 on Thursday, the storm will be more evenly distributed across the Bay Area on Friday. The Weather Service said bomb cyclone conditions off the Pacific Northwest coast will change Friday, increasing the intensity of atmospheric river moisture flowing through the region. Heavy rain and strong winds are expected all day on Friday.
Longer-range forecasts predict more rain in the Bay Area even as atmospheric river conditions end, the Weather Service said. Showers are expected on Saturday and subsequent waves of rain Sunday night and possibly Tuesday will also wet most of the region.
The total rainfall forecast from Thursday through next Tuesday ranges from about 2 inches in San Jose to over 6 inches in Santa Rosa, on top of what has already fallen.
