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Atmospheric River Brings Heavy Rain, Flooding on Some Roads

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About half a dozen firefighters walk near a flooded road, near a forest.
Parts of Highway 92 near Half Moon Bay, in San Mateo County, were closed due to flooding early Monday, Dec. 13, 2021. The heavy rains are part of an atmospheric river passing through the region. (Courtesy of Cal FIre CZU)

A major storm hitting Northern California with rain and snow was expected to intensify Monday and bring travel headaches and the threat of localized flooding after an especially warm and dry fall.

Light rain and snow that began falling on Sunday got heavier overnight. On Monday morning, the National Weather Service announced a flood advisory for several parts of the Bay Area that ended around noon, but it continues to monitor streams and creeks across the region, including those in areas near fire burn scars, for potential flooding. Parts of southern San Mateo and northern Santa Cruz counties were under flash flood warnings.

Several roads in San Mateo County have been closed off due to flooding, including a mile-long stretch of Highway 92 near Half Moon Bay, between Highway 1 and Route 35, according to Caltrans.

San Mateo, Marin and Santa Cruz counties all have been soaked with more than 5 inches of rain, with more than 9 inches on Mt. Tamalpais. While the NWS predicts some breaks in precipitation in the middle of the day, the rain may resume in the evening or early Tuesday morning.

“This is a pretty widespread event,” said Anna Wanless, an NWS meteorologist in Sacramento. “Most of California, if not all, will see some sort of rain and snow.”

The storm is part of an atmospheric river, a fast-moving storm system that funnels moisture from the tropics, and is expected to dump more than 8 feet of snow on the highest peaks in California and Nevada and drench other parts of the two states before it moves on midweek, forecasters said.

The Bay Area experienced an atmospheric river earlier this year, during the weekend of Oct. 22, which saw evacuation orders in parts of San Mateo County and thousands of residents across the region lose power. The heavy rain could bring much-needed moisture to the broader region, which has been gripped by drought caused by climate change.

Most Western U.S. reservoirs that deliver water to states, cities, tribes, farmers and utilities rely on melted snow in the springtime.

The storm this week is typical for this time of year but notable because it’s the first big snow that's expected to significantly affect travel, with ice and snow on the roads, strong wind and limited visibility, Wanless said.

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Drivers on some mountainous passes on Sunday had to put chains on their tires and were warned of possible road closures in coming days.

About 30 miles away from South Lake Tahoe, Kirkwood Mountain Resort closed Monday, saying on social media that it was not safe to open with 17 inches (43 centimeters) of overnight snow and high winds. A California Highway Patrol car in Truckee nudged a big rig up a snowy hill while smaller vehicles spun out, resulting in minor bumps and bruises but no real injuries, CHP Officer Carlos Perez said.

“It’s just so bad and so thick,” he said of the snowfall. “We’re telling people that if they don’t need to be around this area, they probably shouldn’t travel.”

More snow is expected Monday night.

South of the Bay Area, a 40-mile stretch of Highway 1 in California’s Big Sur area was closed as a precaution until Tuesday. The scenic coastal route frequently experiences damage during wet weather.

Nearby Monterey County residents who live close to burn scars from last year’s Dolan Fire were warned to be prepared to evacuate if rains loosen hillsides and cause debris flows.

In Southern California, the San Bernardino County sheriff’s department issued evacuation warnings for several areas, citing the potential for flooding, and Los Angeles County fire officials urged residents to be aware of the potential for mudflows.

Forecasters said strong winds accompanying the storm could lead to power outages. Karly Hernandez, a spokesperson for PG&E, said crews and equipment are staged across the state to respond quickly if the power goes out.

Rain fell intermittently across California on Sunday. Andy Naja-Riese, chief executive of the Agricultural Institute of Marin, said farmers markets carried on as usual in San Rafael and San Francisco amid light wind.

The markets are especially busy this time of year with farmers making jellies, jams and sauces for the holidays, he said. And, he said, rain is always needed in a parched state.

“In many ways, it really is a blessing,” Naja-Riese said.

A second storm predicted to hit California midweek could deliver almost continuous snow, said Edan Weishahn of the weather service in Reno, which monitors an area straddling the Nevada state line. Donner Pass, one of the highest points on Interstate 80 and a major commerce and commuter route, could have major travel disruptions or road closures, Weishahn said.

The weather follows an unseasonably warm November for California.

Vail Resorts’ three Tahoe-area ski resorts opened with limited offerings over the weekend after crews produced artificial snow. Spokesperson Sara Roston said the resorts are looking forward to more of the real thing.

The Sierra Avalanche Center warned that heavy snow and strong winds on top of a weak snowpack could cause large and destructive avalanches.

One man died Saturday in a backcountry area of the Crystal Mountain ski resort in Washington state when he was caught in an avalanche that temporarily buried five others.

This post includes reporting from KQED's Carlos Cabrera-Lomelí and The Associated Press's Christopher Weber and Amy Taxin.

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