Huge storm hitting Bay Area brings mudslide, downed trees, flash flood watch

An atmospheric river combined with a cold front is expected to bring a deluge of rainfall, gusty winds and thunderstorms to the Bay Area on Wednesday and Thursday, and some of those impacts are already being felt.

The storm caused a Wednesday morning mudslide on Highway 101 in Sausalito, shutting down a southbound lane when rocks, mud and debris slid onto the road.

Public works crews were working to remove downed 50-foot trees after the storm sent them toppling onto streets in Lafayette and Novato, officials said.

The National Weather Service issued a flash flood watch for the Bay Area and Central Coast through 3 a.m. Thursday as forecasters monitor rapid rainfall and rising rivers.

Heavy precipitation and wind gusts up to 50 mph are expected to batter the Bay Area, particularly in the North Bay, said Ryan Walbrun, a meteorologist with the National Weather Service.

As of noon, Sonoma County was reporting a third of an inch of rain an hour, he said.

In total, the storm is expected to bring up to two inches of precipitation in the valleys and up to 4 inches in the North Bay and Santa Cruz Mountains.

An avalanche warning was issued for the Central Sierra between 4 p.m. Wednesday and 7 a.m. Friday. A blizzard warning was already in effect through 4 a.m. Thursday with up to 4 feet of snow expected at the high elevations and close to 2 feet at the lake level.

“The combination of saturated soils and strong winds, that’s the type of setup where trees can come down,” Walbrun said. “That’s another concern.”

Road conditions are expected to worsen as the bulk of the cold front passes through during the Wednesday evening commute, and officials warned drivers to slow down due to increasingly slick roadways.

“We will just see some really heavy rain for a three- to six-hour window,” he said. “And I think people will really notice the winds.”

The storm could also bring thunderstorms across the Bay Area, Walbrun said. Lightning strikes were spotted off the coast Wednesday morning, according to the National Weather Service.

As of 4 a.m. Wednesday, the light rainfall has brought just a little less than an inch to San Francisco, Oakland and the North Bay.

“That’s a good start,” Walbrun said.

If rainfall rates reach a quarter to half an inch of rain an hour for several consecutive hours, officials may upgrade the flash flood watch to an advisory or warning.

“When you get that much rain in that short amount of time it will start to overwhelm, especially a lot of the street drainages,” Walbrun said. “A lot of the streams and creeks in Marin County and Santa Cruz and Sonoma, they will have rapid rises.”

Roadway flooding was already reported around 6:30 a.m. Wednesday in San Jose and in the Marin area, CHP said.

The rainy weather expected Thursday could bring an increased chance of hail, before drying out Friday.

“(Thursday), when there’s a more cold, unstable air mass around, we could see lots of reports of small hail,” Walbrun said.

Road closures heading toward the Sierra are also expected this week with the heavy storm.

Snow might also be seen Thursday on the Bay Area’s highest peaks, with Mount St. Helena possibly receiving a light dusting of snow, Walbrun said.

A high surf warning will go into effect at 5 a.m. Thursday and will last through Friday with waves up to 30 feet in the surf zone.

Sunday could bring more rain before an extended forecast of dry and sunny weather throughout the Bay Area.

Sarah Ravani and Gwendolyn Wu are San Francisco Chronicle staff writers. Email:, Twitter: @SarRavani @gwendolyawu

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