Authorities have already issued warnings to drivers to avoid certain areas due to hazardous road conditions.
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LIVE UPDATES: Several highways closed due to snow, crashes, and fallen trees
Road Closures
Santa Cruz County
The California Highway Patrol says both northbound lanes on Highway 17 are back open. Both southbound lanes are back open from Highway 85 to Big Moody Curve where it moves to one lane up to Summit Road then it's back to two lanes. CHP escorting traffic southbound.
Sonoma County
Westbound CA-128 at North Cloverdale, all lanes are blocked due to hazardous road conditions.
Napa County
Napa County is also dealing with road closures. State Highway 29 has all lanes closed in both directions between Tubbs Lane and Bradford Road due to hazardous roadway conditions.
San Mateo County
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Northbound and Southbound CA-35 from CA-84 to CA-9, all lanes are closed due to downed trees and downed powerlines.
VIDEO: SKY7 flies over Bay Area snow-capped mountains, peaks
SKY7 flies over Bay Area snow-capped mountains, peaks
Santa Clara County
Mt. Hamilton Rd. (CA-130) at Grant Park. All lanes closed in both directions due to weather conditions and downed trees.
Northbound and Southbound CA-9 from Redwood Gulch Rd to CA-35, all lanes are closed due to downed trees and hazardous road conditions.
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VIDEO: Winter storm transforms Northern California, Bay Area into snowy landscape
Winter storm transforms NorCal, Bay Area into snowy landscape
Marin County
Snow on Mount Tamalpais has prompted two road closures, though state Highway 1 remains open. Pantoll Road and Ridgecrest Boulevard were closed as of 11 a.m.
Friday, according to the Marin County Sheriff's Office, and the entire upper mountain is closed until roads can be cleared of snow, county and Mount Tamalpais State Park officials said.
Sleet has impacted the area, making roads dangerous, and park staff said that Panoramic Highway through the Marin hills is "slushy."
The Bay City Newservice contributed to this story.