SAN FRANCISCO (CBS SF) — A wave of low pressure systems out in the Pacific edged every closer to Northern California early Tuesday, carrying with it the threat of intense rain and several feet of new snow for the Sierra well into the Christmas holiday weekend.

Showers were expected to begin in the San Francisco Bay Area by the afternoon hours on Tuesday, moving from the North Bay southward, and continuing through Christmas Day.

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“We are going to see waves of rain, so off-and-on rain the next several days,” said KPIX 5 meteorologist Mary Lee. “Bringing 1-3 inches through the holiday weekend.”

The National Weather Service said the jet stream howling across the Pacific was kicking the storm door wide open.

“A conveyor belt of low pressures is moving into the area — this would make for quite a wet ending to 2021, but it`s a little to far out to put too much emphasis on that,” weather service forecasters said. “It will be something to check in on as the week goes on.”

They were predicting 2.3 inches rain through Dec. 25 for San Francisco.

“If verified, it will bring SF back to top the ten wettest water years to date by the end of the week,” the forecasters said.

The coastal mountains in the North Bay could see 3-5 inches with up to 6 inches of rain over the higher peaks. In the Santa Cruz Mountains another 3-6 inches could fall in areas already saturated by rain last week.

“Good news is that rain totals are spread out over many days which should mitigate major impacts,” the weather service tweeted. “But be prepared for minor flooding, slick roads, and potential for debris flows/mudslides.”

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In the Sierra, a winter weather advisory goes into effect at 7 p.m. Tuesday night and transitions into a winter storm warning by Thursday morning running through Sunday.

“Heavy snow expected (beginning Thursday),” the weather service in Reno warned. “Total snow accumulations of 2-to-4 feet up to 4-to-8 feet above 7,000 feet. Wind gusts to 40 mph, with ridge wind gusts exceeding 100 mph likely.”

For the thousands who will be traveling from the Bay Area to Tahoe for the holiday weekend it means it will be a slow go.

“Once the snow starts this evening, travel across the Sierra will become progressively more difficult to near impossible at times through this weekend,” the NWS Reno forecasters warned. “Be prepared for long delays and quickly changing conditions over area passes.”

The mountain range, where ski resorts had struggled to open this fall, is already sporting glistening peaks after recent storms. The snowfall is important because the Sierra’s winter snowpack normally is a significant source of California’s water.

“We had a big storm already recently. We’re just getting over shoveling and all that so now we’re preparing for the next one,” said Karla Brennan, owner of The Cork & More, a longstanding deli, cheese and wine store in South Lake Tahoe. “I think it’s going to be a white Christmas.”

While the current wet trend is positive, it is too early to know if it will last through January and February. The snowpack normally doesn’t reach its maximum until April and last spring there was minimal runoff because much of the water was absorbed by the drought-parched landscape.

Forecasters noted that this week’s storms will also have potential for significant low-elevation snow, including over Interstate 5 north of Redding, the northern region where last week’s storms shut down the vital highway for nearly 24 hours.

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