10 Photos: Atmospheric River Pounds Northern California

NORTHERN CALIFORNIA — Heavy rain from a major storm prompted evacuation warnings for communities near the Russian River, which was flooding Friday. The system also continued to dump heavy snow in mountainous areas where some ski resorts opened for the season.

The system roared ashore on the West Coast Tuesday as a "bomb cyclone," which occurs when a cyclone intensifies rapidly. It unleashed fierce winds that toppled trees onto roads, vehicles and homes.

Forecasters warned about the risk of flash flooding and rockslides in areas north of San Francisco from this season’s strongest atmospheric river — a long plume of moisture that forms over an ocean and flows through the sky over land.

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The National Weather Service said over a foot of rain fell at the Sonoma County Airport over the last three days— more than one-third of the yearly average.

At least 17 schools were closed Friday in Sonoma County because of flooding, downed trees and other storm-related impacts. Also on Friday, a flood warning was issued for Guerneville and several other communities along the Russian River as it was expected to reach flood stage Friday night.

Near Healdsburg, a city along the Russian River in northern Sonoma County, a small mudslide threatened a home in the community of Fitch Mountain. Moderate rain was falling, and officials were concerned the slide could grow and hit several homes downhill.

Tennis Wick, permits and resource director for Sonoma County, said the home has experienced landslides in the past and was currently perched atop a muddy slope.

“Our concern is while this property may be OK, the earth between it and the road below is slipping, and the mudslide is affecting downhill properties,” he said.

Over in Napa County, mudslides closed the Silverado Trail Friday. The California Highway Patrol said the roadway was shut down from Oak Knoll Avenue to Yountville Cross Road. Travelers should instead use state Highway 29 to get north and south in the Napa Valley, the CHP said.

Since Tuesday, the storm system has been centered largely on Sonoma, Marin and Napa counties in the North Bay area, but it started moving south Friday to the San Francisco Peninsula and other parts of the Bay Area.

There were also winter storm warnings Friday for parts of the California Cascades and Sierra Nevada.

Sugar Bowl, northwest of Lake Tahoe, opened three lifts Friday. It is the earliest the ski resort has opened in 20 years, spokesperson Maggie Eshbaugh said.

"With all the new snow coming in, we could get more terrain opening up next week," she said.

Below are nine images showing the impacts of the record rainfall in Sonoma County and snow in the Truckee area of Northern California. The photo at the top of the article is from Thursday in the Sonoma County community of Forestville.

The Associated Press contributed to this report.