The second day of an atmospheric river brought heavy rain to Northern California and the Bay Area Thursday, with the heaviest downfalls 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 through early Saturday morning. The 24-hour rainfall totals in the North Bay included more than 7 inches at the Santa Rosa Airport and reports of over 10 inches in the coastal mountains of Sonoma County, the Weather Service's Bay Area office said in its daily forecast discussion.
A high surf advisory was also in effect for the Bay Area coast from Point Reyes south to Big Sur including the Southern Monterey Bay until 6 a.m. Friday. The Weather Service said large breaking waves between 14 to 19 feet would be seen on southwest-facing beaches and between 19 to 22 feet along well-exposed west-facing beaches.
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The storm's intensity comes 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 a bomb cyclogenesis, or a bomb cyclone.
Most of the storm impacts were happening north of the Golden Gate, including downed trees and power lines along with thousands of power outages since Wednesday. The strong southerly winds hitting the Bay Area are expected to decrease Thursday, but won't improve substantially until Friday evening, the Weather Service said.
Rainfall amounts Thursday could decrease slightly in the North Bay; however, another 1 to 3 inches of rain are still expected in the valleys and up to another 6 inches in the mountains over the next 24 hours.
While the rain Thursday will mostly be concentrated north of Interstate Highway 80, by Friday the storm will be more evenly distributed across the Bay Area. The Weather Service said the bomb cyclone conditions off the Pacific Northwest will reform by Friday increasing the intensity of the atmospheric river moisture flowing through the region. Heavy rain and strong winds are expected throughout the day Friday.
Longer-term forecasts see more rain for the Bay Area even as the atmospheric river conditions end, the Weather Service said. Showers are expected Saturday and subsequent waves of rain Sunday night and possibly Tuesday will also soak 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 in addition to what has already fallen.