Bomb Cyclone, Atmospheric River To Unleash Rain, Wind, Sierra Snow

SAN FRANCISCO — A new bomb cyclone forming over the Pacific Ocean could play a role in what forecasters already expect to be a stormy and wet Friday across the Bay Area.

Heavy rain from the atmospheric river continued Thursday in the North Bay on the heels of a record-breaking day that brought more than 7 inches of rain to Santa Rosa and over 10 inches in the coastal mountains of Sonoma County over 24 hours.

According to the National Weather Service, Santa Rosa experienced its second-wettest day in 120 years. The Sonoma County Airport recorded 6.92 inches of rain, shattering its all-time rain record by nearly an inch.

While the rest of the region was largely spared from the action, the weather service warns the North Bay, East Bay, South Bay, Peninsula, and San Francisco could all see moderate impacts from heavy rain and flooding Friday.

Forecasters said a cold front due in the wake of the latest cyclone will lend to more widespread precipitation and wind from the ongoing atmospheric river.

"If we zoom out on the current satellite loop, a new surface low is being born in the cold air mass out in the Eastern Pacific," NWS Bay Area said Thursday. "This system will undergo explosive cyclogenesis as it moves towards the Pacific Northwest and occludes by Friday. The associated cold front will come ashore on late Friday or early Saturday, bringing another bout of heavy rain and strong winds in the prefrontal environment through the day Friday."

The weather service predicts another 4 to 6 inches of rain to fall through Friday over the North Bay, with 2-3 inches possible in San Francisco and Half Moon Bay, and up to 1.5 inches near Concord, Livermore, San Jose and Gilroy. A flood advisory will remain in place until Saturday morning in Sonoma, Napa and Marin counties.

A wind advisory will be in effect throughout Friday for much of the region, including the North Bay, East Bay and Santa Clara Hills, with gusts up to 50 mph possible. A slight chance of thunderstorms remains Friday across the region.

Though forecasters expect the atmospheric river to wind down after Friday, moderate rain is likely to persist into next week. The weather service said much of the Bay Area could add another 1 to 2 inches of rainfall between Saturday and Tuesday. Extended outlooks continue to favor wetter, colder weather sticking around over the Thanksgiving holiday.

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In the mountains, several rounds of heavy snow are forecast for the Sierra between Friday night and Tuesday evening. Snow levels are expected to dip below the passes by late Friday, laying the groundwork for up to 4 feet of fresh snow to fall in as many days.