Bay Area sea levels are running six inches above normal at the Golden Gate, USGS research oceanographer Sean Vitousek told KQED on Sept. 16.

They could rise further. A coastally trapped Kelvin wave, a planetary-scale pulse of warm water, is traveling north along Central America's coast at about 6 mph. It is expected to reach San Francisco Bay in the first or second week of October, when it could raise water levels by roughly a foot, according to CBS San Francisco and The Weather Channel. Kelvin waves in general can raise sea levels 6 to 12 inches for months, according to the Los Angeles Times and the Mercury News. The wave already raised levels 9 to 14 inches along Central America's coast in late August, the San Francisco Chronicle reported.

The near-term risk is limited. September and October tides are among the lowest of the year, so the Kelvin wave alone is unlikely to cause flooding when it arrives. The bigger concern starts in November, when king tides return. California's highest king tides in 2026 fall on Thanksgiving Day and Christmas Day, with another in late January 2027, according to the Chronicle.

For Walnut Creek, the more direct threat may be less about tidal surge than about what happens when heavy winter rains meet an already swollen watershed. Climate scientist Daniel Swain has said tidal influence from elevated sea levels can extend "all the way inland into the Delta to Stockton," a dynamic that, if it applies to Walnut Creek's own creek system, could mean rising Bay levels reduce how much capacity local channels have to drain storm runoff. The Contra Costa County Flood Control District has not confirmed whether that mechanism applies directly to Walnut Creek's creeks.

Climate scientist Daniel Swain, affiliated with the University of California system and the NSF National Center for Atmospheric Research, told KQED on Sept. 16 that elevated tides will reach far inland. "Anything that's tidally influenced will be affected," Swain said. "You're also generally going to see that all the way inland into the Delta to Stockton."

In an Aug. 10 online talk reported by KQED, Swain warned that record-high sea levels were "almost certainly" coming to California this winter.

The National Oceanic and Atmospheric Administration (NOAA) puts the chance of a strong El Niño persisting through October, November and December at greater than 90%, according to the Mercury News. There is a 75% probability it will rank as the strongest since modern records began in 1950.

Dillon Amaya, an assistant professor of marine science at North Carolina State University, told the Los Angeles Times that the combination of the Kelvin wave, El Niño and an ongoing marine heat wave is adding roughly a foot of sea-level rise on top of long-term climate change.

Swain said some locations could see as much as 5 to 6 feet of temporary inundation above historical high-tide levels when El Niño sea-level rise, storm surge and king tides combine.

Walnut Creek has flooded before.

On Dec. 31, 2022, Walnut Creek Police closed Walnut Boulevard at Sierra Drive and Homestead Avenue after a storm caused localized flooding.

The county's flood infrastructure is aging. A 2017 Contra Costa County Civil Grand Jury report found the system spans 75 miles of channels, 35 detention basins and 25 trash racks. As of that report, the maintenance backlog shortfall was projected to exceed $25 million, with no reserves set aside. Current funding levels have not been confirmed.

European weather forecast data cited by the Mercury News puts the chance of January-through-March 2027 rainfall ranking among the wettest 20% of the historical average at 70%.

No statement from Walnut Creek city officials or the Contra Costa County Flood Control District about specific preparedness steps for this El Niño season was available as of Sept. 16. Residents can monitor flood conditions through the Contra Costa County Flood Control District and sign up for emergency alerts through the county's Community Warning System.