Bay Area Flood Watch as Atmospheric Rivers Bring Heavy Rain and Rising River Levels

The entire Bay Area — from Santa Rosa to San José — is under flood watch until 10 p.m. Sunday as the first of two atmospheric rivers sweeps through the region, the National Weather Service’s Bay Area office warns.

The good news is that initial flooding concerns remain low for the first few days of rain as the dry month prior will help buffer the impact of the upcoming rain.

Flooding concerns escalate later this weekend and into next week, as consecutive days of heavy rain could bring up to 8 inches to the North Bay, 5 inches to San Francisco and 2 inches to the South Bay.

The weather service is closely monitoring North Bay rivers and streams, where the highest flood risk exists due to heavy rainfall, while the rest of the Bay Area faces potential nuisance flooding.

“We’re not looking much in the way of flash flood conditions, but we are looking at more long-term flooding issues,” said Brayden Murdock, a meteorologist with the weather service’s Bay Area office. “That could be road closures, situations where you need to turn around, don’t drown because of some of these really low-lying spots.”

Friday’s storm originating from the Gulf of Alaska is merging with a Pacific system, intensifying into an atmospheric river. By Sunday, the system may weaken, primarily impacting the North Bay.

“It’s kind of a saving grace that this one is moving around and we can spread those rainfall amounts around the region versus it staying in one spot and accumulating big time for just a few areas,” he said.

Forecasters predict the heaviest showers on Saturday, with a 15% chance of excessive rainfall in the North Bay and San Francisco. As the moisture plume shifts north on Sunday, lighter rain is expected, focusing on the North Bay. A third round of moderate showers arrives on Monday, bringing the potential for excessive rainfall in the northern region as another system moves through.

By Tuesday, the entire region faces a widespread “marginal” risk of excessive rainfall, accompanied by cold nighttime lows and strong winds. The weather service warns this combination could pose issues for residents “without adequate heat or shelter mid to late next week.”

“It’s going to be much more of a marathon than a sprint as far as the rain comes through,” Murdock said. “But over eight days, those rainfall numbers add up. The fact that this will be prolonged could cause additional issues later.”

Paul Lowenthal, division chief fire marshal for the Santa Rosa Fire Department, doesn’t anticipate the extreme flooding seen in November when at least two people died in Sonoma County during a record-breaking storm. As the atmospheric river strikes next week, however, Lowenthal said the greatest flooding risk will emerge then.

“There’s always potential for events like last November when we get back-to-back systems with heavy rainfall,” he said. “Fortunately, the soils aren’t completely saturated like they were during our last storm, but we anticipate a lot of localized nuisance flooding at a minimum.”

Lowenthal said North Bay residents should prepare by clearing gutters of debris, surveying their properties for risks and paying close attention to trees as winds pick up next week.

“A lot of the excessive runoff will end up likely causing rapid rises of our streams and creeks,” he said.

Andrea Rodriguez, communications manager for Sonoma Water, agrees that smaller streams and urban areas are at a higher risk of flooding, but “due to how dry the past month has been, flooding is not expected on the Russian River this weekend.”

While January was unusually dry compared to the past two years, Murdock predicts a wetter February. He advised that it’s still too early to determine if the storm door will remain open for the rest of winter.

But, at least, “it’s open for this week,” he said.