Flooding Hits San Francisco and the Bay Area as Storm Grows More Intense

Updated 4:45 p.m. Friday

Flash flooding was reported in San Francisco and parts of San Mateo County on Friday afternoon as the Bay Area was lashed by a third day of heavy rain and strong winds from the second atmospheric river to hit this week.

The National Weather Service issued a flash flood warning for northwestern San Mateo County — including Daly City, Broadmoor and Pacifica — until 5 p.m. In the East Bay, a flash flood warning including Oakland, Berkeley and Richmond is in effect until 6 p.m. An early warning for San Francisco was reduced to a watch after the rain there backed off around 4 p.m.

The alerts came as a stronger band of the storm moved south over San Francisco and the Bay Area, driving up rainfall rates along the way.

On northbound Highway 101 at Interstate 280, the fourth and fifth lanes were blocked by flooding, the California Highway Patrol said at 2:30 p.m. It was not clear when they would reopen.

By Friday afternoon, nearly 500 flights had been delayed and more than 60 canceled at San Francisco International Airport, the most in the U.S., according to FlightAware.

In the North Bay, multiple days of heavy rain have saturated soils and swelled waterways, raising flood dangers. The CHP reported flooding on Highway 121 in Sonoma County, closing all lanes.

The intense rain over Sonoma County pushed several creeks to their maximum, flooding streets across Santa Rosa. Officials said floodwaters trapped a person in a car in front of a Starbucks, and 150 people sheltered in place at a Santa Rosa medical center and Hampton Inn when floodwaters cut off access to the buildings, said Paul Lowenthal, division chief fire marshal for the Santa Rosa Fire Department.

“We had a report of several vehicles potentially floating away under the volume of water,” Lowenthal said. “They were essentially stranded because of floodwaters. No one was able to leave those facilities.

With more than 20 inches of rain falling in Sonoma County mountains, forecasters are also watching a half dozen streams and the Russian River approach flood stages. A flood advisory is in effect for Sonoma County through Saturday afternoon.

“We weren’t expecting that since it’s November,” said Dylan Flynn, a lead meteorologist with the National Weather Service’s Bay Area office. “It’s rising incredibly fast. If this had happened in February, it would have been catastrophic since the river started so empty. But it could get worse if the rain overperforms today.”

The first storm dropped nearly a foot of rain on Santa Rosa, and Friday’s atmospheric river could break records in the city “by a lot,”  Flynn said. Santa Rosa is nearing an all-time mark for the most rain in three days since record-keeping there began in 1902.

Mountain communities like Occidental, west of Sebastopol, have already received more than 21 inches of rain. Nearly 20 inches of rain has soaked Venado, north of Guerneville.

UCLA climate scientist Daniel Swain said the first major rainfall of the year has already become the Santa Rosa area’s wettest rain event over two days.

“This is not in Venado or the coastal hills where we get these double-digit totals every winter,” he said. “This is in the flatlands of the valley in Santa Rosa proper, having about 12 inches of rain in a 48-hour period, and it’s still pouring at the moment.”

Swain said Friday that the rain totals in Santa Rosa for Thursday looked like the storm could be a 100- to 200-year event. Since rainfall will continue to add up on Friday, he said the two-day totals could veer into the 500-year event territory.

Flynn said the rainfall totals are a first for Sonoma County in over 100 years. “This amount of rain doesn’t compare to the historical record,” he said. “We haven’t seen this much rain focused in the North Bay for a long time.”

The North Bay could get up to 3 more inches of rain on Friday before starting to “thankfully finally dry out,” Flynn said. Two smaller systems are expected to bring scattered showers across the Bay Area into early next week, though.

While atmospheric rivers are normal for California and have “always happened,” Flynn said, human-caused climate change — brought on by the burning of fossil fuels globally — is “raising the benchmark” and adding more “moisture to the atmosphere.”

“I can’t tell you what we would have gotten from the storm without climate change, but just know that on average, on the whole, it kind of sets that floor just a little bit higher for these types of atmospheric rivers,” he said.

As the storm triggers wind advisories stretching from the North Bay to Monterey County, tree companies across Sonoma County are receiving an influx of calls for downed limbs on houses, crushed cars and smashed fences.

William Slater, owner of Slater’s Professional Tree Care, said his crews handled many calls on Thursday. On Friday morning, he was on his way to a Santa Rosa home where a eucalyptus tree snapped and potentially crushed the house.

“A huge 100-year-old oak tree fell across somebody’s driveway, and the client was trapped in his house,” he said. “With this amount of consistent rain, it’s definitely causing damage to homes and the local natural habitat.”

Brad Sherwood, assistant general manager for Sonoma Water, lives in a neighborhood near the flooded area of Santa Rosa.

“The whole cul-de-sac and the parking lot was a pond,” he said. “It was totally unexpected. I have never seen that area flood like that.”

After four hours, people were able to leave the area once the water receded.

At least 30 roads were closed Thursday due to flooding or downed power lines. Only one street was closed as of 9 a.m. Friday, Lowenthal said.

However, with several more inches of rain on the way, Lowenthal said he expects overflowing creeks and nuisance flooding across the city on Friday.

“With us getting roughly 10 to 11 inches of rain just in the last 48 hours, it’s not going to take much for those creeks and streams to rapidly rise once again and lead to a lot of ponding and nuisance flooding,” he said.

From a water supply perspective, Sherwood said the two early season storms benefit the North Bay. Over the last two days, the first storm added more than 16,000 acre-feet of water to Lake Sonoma.

“That’s incredible and pretty much unheard of for this particular time of year,” he said. “Right now, our reservoirs are in prime condition to hold a lot of this water.”

Sherwood is looking forward to a lull in the rain after this weekend but said he is hoping this isn’t a “one and done storm system. We do need the continued precipitation through January, February and March.”

It’s still too early to know if the early-season rain is an indication of a wet winter, and Sherwood said his agency will keep as much water in reservoirs as possible. He noted some past winters that had big storms in November and December, “then the spigot was turned off by Mother Nature, and then a three-year drought started.”