The Bay Area should brace for another soaking Friday and Saturday with a major storm system settling in, forecasters say, spurring flooding and landslide concerns given that the region's soil already absorbed a 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 introductory glimpse of the coming rains when residents woke up to wet and rainy conditions Thursday, following a short respite Wednesday that saw dry and sunny weather for most of the region.
This time, 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.
The National Oceanic and Atmospheric Administration's rainfall survey showed much of the Bay Area had received a quarter to a third of an inch of rain in the 24 hours before 1 p.m. Thursday. San Jose had 0.03 inches in the same time period.
Accordingly, a flood watch was set to take effect from Friday evening through Saturday for the entire Bay Area, based on concerns that Friday's rains will set the table for heavier rains the following day. 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 on to 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, which 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.
To be clear, Garcia said rivers should swell, smaller rivers could overflow, and that "some storm drains will not be be able to keep up and flood some roads" due to the weekend storms, but there are not major flooding threats on waterways like 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 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."
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 writer Marisa Kendall contributed to this report.
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