The Bay Area should brace for another soaking Friday and Saturday with a major storm system settling that is raising flooding and landslide concerns given that the region’s soil already absorbed rain from another storm earlier this week.
Both the National Weather Service and U.S. Geological Survey have found that soil saturation is expected to set off a domino effect that leads to an elevated risk of shallow landslides, particularly on Saturday.
“You can’t just keep adding water to the soils when the soils are full,” said Brian Garcia, a meteorologist at the Bay Area’s National Weather Service station in Monterey. “That becomes runoff, putting more water in rivers, creeks and streams and in the watershed. We’re going to see higher rises than what we saw in the last system.”
The region got an early glimpse of the coming rains Thursday when residents woke up to wet and rainy conditions following a short respite Wednesday that saw dry and sunny weather for most of the region.
Still, even after a morning of gray skies and scattered showers, the National Oceanic and Atmospheric Administration’s rainfall survey showed much of the Bay Area got no more than a third of an inch of rain Thursday.
As of 4 p.m., 24-hour precipitation totals included .39 inches in San Francisco, .26 inches in Oakland, .22 inches in Concord and Gilroy, .16 inches in Fremont and .15 inches in Redwood City, according to the National Weather Service.
The stormy conditions are all but certain to carry into the start of 2023, with the weather service’s seven-day forecast showing rain in most of the area’s major population centers.
A flood watch was set to take effect Friday evening and extend through Saturday for the entire Bay Area. The intensity of the storm coming into the region, forecasters say, is fueled by a sub-tropical patch of moisture and warm air coming from Hawaii combined with cold air from the Pacific Northwest.
The predicted shallow landslides will push mud onto a variety of inopportune locations, including roads, farmland and, in some instances, homes, Garcia said. That’s not to mention that trees, now rooted in saturated soil, will be more vulnerable to toppling even in lighter winds far weaker than what usually would bring them down.
Generally, Garcia said, residents can anticipate standing water in low-lying areas and flood plains throughout the Bay Area, from Livermore to San Francisco.
“All those places will be soggy and boggy,” he said.
In San Jose, the incoming consecutive storms evoked memories of the infamous President’s Day weekend flooding of 2017 that saw Coyote Creek spill out of its banks and submerge several neighborhoods in the central part of the city. The city’s worst flood since 1997 caused an estimated $100 million in damage and the emergency evacuation of 14,000 people who lived near Coyote Creek, one of two major bodies of water — along with the Guadalupe River — that flow through San Jose. The neighborhoods of Rock Springs, Naglee Park and several mobile home parks between Old Oakland Road and Coyote Creek were the hardest hit.
That flood occurred after a series of large atmospheric river storms, much like what’s expected this weekend — but Garcia said he does not expect anything approaching that severity.
“If the 2017 storm was a 10 out of 10, we’re probably looking at a 4 out of 10,” he said.
Garcia said rivers will swell, smaller rivers could overflow and that “some storm drains will not be able to keep up and flood some roads” due to the weekend storms, but there are no major flooding threats on waterways such as the Guadalupe and Coyote Creek.
That risk has been alleviated in part because, in the wake of the 2017 flooding, the Santa Clara Valley Water District was ordered by federal regulators to rebuild Anderson Dam and ensure that it holds up in a major earthquake. While the project won’t be finished until 2030, the construction work required that the dam’s water level be significantly lowered.
Floodwalls, berms and new levees along Coyote Creek have also been in the works since the disaster.
But while a major flood event is unlikely, Garcia noted that unhoused populations already vulnerable to weather events face additional risks.
“The area has at least 9,000 unhoused individuals, and a lot of them live in riverbeds,” he said. “The potential need for swift water rescues on New Year’s Eve, that goes up.”
On Thursday, activist Shaunn Cartwright with the Unhoused Response Group was busy warning people in San Jose about the potential for rising water while also passing out tents, tarps and winter clothing to help people stay warm and dry.
She said she is “absolutely” worried about the toll the coming storm will take on encampments along the Guadalupe River and Coyote Creek.
“What if we don’t tell the right camp and they’re the one that gets flooded and somebody dies?” she asked. “You feel an immense amount of pressure.”
While the general advisory from authorities is to stay indoors and wait out the storms, Garcia said those who do have to venture out in the rain should heed simple directives.
“Take a big dose of patience with you through this system if you have to go out,” he said. “If you see a flooded road, it’s not worth it (to drive through).”
Staff writers Marisa Kendall and Jason Green contributed to this report.
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