As Christmas approaches, the Bay Area weather pattern seemed set to deliver a “gift” of extreme conditions to the region — everything from hail to thunder to hurricane-level winds to sheets of rain.
For the Sierra Nevada, add snow to that list and be sure to check your travel plans twice.
The most powerful wave of an atmospheric river will make it all possible, according to the National Weather Service. It was poised to crash Tuesday on the Bay Area and not forecast to recede until Friday.
A smaller system gave the region moderately intense but steady rain that fell all weekend. Even on Monday, showers dotted much of the Bay Area, and cooler air opened the path to more snow in the mountains.
It’s likely to make for a Christmas best spent indoors.
“Everything is pretty well consistent with how we expected it to be,” NWS meteorologist Brian Garcia said Monday afternoon. “As the frontal boundary of the current storm weakens softly and goes north, that allows the door to open for the next one. That door is now open.”
The incoming wave’s intensity could be magnified by the rain that already fell in the Bay Area throughout the weekend — heavily in some areas. The weather service’s 72-hour totals entering Monday showed 10.85 inches of rain on Mount Tamalpais in Marin County; 8.3 inches in northwest Sonoma County; 4 inches in areas of the North Bay Valley and 1½ to 2 inches in the East Bay and Peninsula areas.
Additional cumulative totals were not expected until Tuesday.
Another 2 to 3 inches of rain are expected Tuesday and Wednesday in the North Bay and Peninsula. Some areas of the East Bay and South Bay that often are affected by the rain shadow that comes with the atmospheric rivers — where hills and mountains wring out the moisture from storms before they pass into the “shadow” area — are still expected to get at least an inch and probably more, according to the weather service.
“There’s going to be quite a bit of rain this week,” meteorologist Dalton Behringer said. “Ongoing.”
The rain that fell over the weekend did not cause any major power outages, according to PG&E spokesperson Tamar Sarkissian. An outage Sunday in San Francisco that affected about 130,000 PG&E customers was not caused by the weather but more likely by a fire at a PG&E substation at 8th and Mission streets, the utility said.
In the Sierra Nevada, snow levels were expected to fall to an elevation of 6,500 feet on Monday and stay that way through Tuesday, when the heaviest snow is expected above Donner Pass. On Wednesday, the snow level is expected to fall to 4,500 feet, where it is expected to remain through Christmas.
The weather service issued a winter storm watch for the Sierra and southern Cascades above 5,500 feet beginning Tuesday at 10 p.m. and in effect through 10 a.m. Friday. Snow above that elevation may reach 1 foot, while 2 to 4 feet could fall at the highest peaks.
Chain controls are likely to be in place for those driving through the area.
The winds that are expected with the rain will be strong and potentially destructive, according to the weather service. Gusts may reach as high as 70 mph in the higher areas of the coastal areas, Behringer said. In the Sierra, they are expected to blow as high as 45 mph.
The weather service issued a high wind watch for all areas of the Bay Area and the Central Coast that will go into effect at 7 p.m. Tuesday and run through 4 a.m. Wednesday. The weather service is anticipating that the winds will drop trees, cause widespread power outages and create more difficult travel.
The winds “are going to be as strong as they get,” Berhringer said. “It’s a 12-hour window through the night on Tuesday. It’ll die off on Wednesday. There will be a second push on Wednesday night. But the second push is not gonna be quite as strong.”
A flood watch also began Monday for areas near creeks and rivers from San Francisco north to Cloverdale along the coast and to inland areas such as Concord; at 10 a.m. Tuesday, it will be expanded to include areas south of San Francisco, including San Jose, Santa Cruz, Monterey and Big Sur, and be in effect for all areas of the region until 10 p.m. Friday.
The rain is expected to fall with some regularity until Friday. Behringer said the intensity of the precipitation will start to taper off by then, and that Saturday is expected to begin a drying-out period that will last at least early next week.
