3 Storms Will Bring Much-Needed Rain to Bay Area and Snow in the Sierras

The Bay Area could get “a December’s worth of rain” over the next week, with a rainy Christmas Day and a Sierra Nevada blanketed with white, the National Weather Service said Friday.

Forecasters said they expect a weeklong atmospheric river with three distinct storms to move over Northern California next week.

Joe Merchant, a meteorologist with the weather service’s Bay Area office, said the first wave of precipitation will begin Friday afternoon and last through Saturday, bringing much-needed rain after around six weeks of mostly dry conditions, especially in the North Bay.

“What we’re seeing now is just sort of setting the stage for potential problems later in the latter half of next week, which unfortunately does include Christmas Day,” Merchant said.

The second wave is set to crash over the Bay Area between Saturday and next Tuesday, potentially bringing 1 to 3 inches of rain in the North Bay and the possibility of flash flooding in low-lying areas. San Francisco could receive several inches by Sunday night.

Forecasters wrote in their daily weather discussion that they expect the heaviest rain on Sunday into Monday along the North Bay coast, Santa Cruz mountains and the Big Sur coast, with winds up to 40 mph and a 15% chance of thunderstorms.

The weather service has not yet issued a flood watch, warning or advisory. Flooding depends heavily on where the storm stalls or moves over the region, but meteorologists said urban areas and low-lying areas with poor drainage are most at risk. Flashy streams, such as Mark West Creek in Sonoma County, are also susceptible to flooding, especially as soils become saturated.

Forecasters said they expect the third and potentially most potent wave to make landfall on Tuesday afternoon and last through next Friday. The storm is likely to bring moderate to heavy rain, which could cause larger rivers to flood late next week.

“It’s not going to be raining steadily for seven days straight or anything like that,” Merchant said. “But you will have to pay attention and know when these rounds of more impactful rainfall come through your area.”

While it is unclear where, or even if, the atmospheric river will stall over the Bay Area, these weather systems can act like a fire hose, absolutely drenching wherever they park. In recent history, they’ve caused significant flooding from Guerneville to San Francisco to Watsonville.

Merchant said the first two waves of rain are progressive and don’t look like they’ll stick around, but it is too early to say what the third storm will bring.

“By Thursday, that’s where we’re going to have a little more concern about possibly more widespread impacts like fallen trees and power lines,” Merchant said. “But to be honest, it’s too far out at this point to know exactly the timing of those impacts.”

Because of all the wind and rain, people should allow “more time to travel through our area basically over the next week,” he said.

At the moment, the massive river in the sky headed towards us is building from Hawaii to the Pacific Coast. Weather experts measure the intensity of atmospheric rivers on a scale from 0 to 5. This system could reach a 3 across the North Bay and a 4 across the San Francisco peninsula and South Bay, according to atmospheric river-scale modeling by the Center for Western Weather and Water Extremes at UC San Diego’s Scripps Institution of Oceanography.

As for the Sierra Nevada, forecasters said the first two waves will bring mostly rain at elevations lower than 8,000 feet. But they expect the snowline to drop to around 6,000 feet during the third system, starting Tuesday afternoon. Heavy snow will likely complicate travel on Christmas Eve and Christmas Day.

“We’re talking multiple feet of snow above pass level and even at pass level for the Christmas holiday,” said Scott Rowe, senior service hydrologist and meteorologist with the weather service’s Sacramento office.

The determination of whether mountain residents or visitors will have a white Christmas depends on elevation, Rowe said. People staying in cabins above 6,000 feet in elevation will likely see snow, while those staying below about 5,500 feet will likely experience rain all week.