San Mateo County's low-lying bayfront could face elevated flood risk for months after a coastally trapped Kelvin wave reaches the Bay Area, scientists tracking the ocean phenomenon warned.

The wave, a broad bulge in sea-surface height tied to El Niño, was near Puerto Vallarta on Mexico's Pacific coast as of Tuesday, Sept. 15. Dillon Amaya, an assistant professor of marine, earth and atmospheric sciences at North Carolina State University who is tracking the wave, told Patch it could reach the San Francisco Bay Area by the first or second week of October and raise coastal water levels by 6 to 12 inches.

"Six inches may not sound like a lot, but it can absolutely be a big deal," Amaya said.

The concern for San Mateo is timing. Once a Kelvin wave raises the background sea level, it can take three to six months for water levels to return to normal, according to Amaya. That means elevated seas arriving in early October could persist through California's king tide season, when the highest astronomical tides hit from November through January.

The Los Angeles Times reported the peak king tides of the 2026-2027 season are expected around Thanksgiving Day and Christmas Eve, citing Scripps Institution of Oceanography researcher Reinhard Flick. Daniel Swain, a climate scientist at the University of California Agriculture and Natural Resources, told CBS News that every two or three inches of additional sea level "counts in terms of the damage."

San Mateo County residents saw what high water can do in January 2026. King tides topping 7 feet combined with an atmospheric river to cause record flooding across the county, according to the San Mateo County Sustainability Department.

Flooding hit Miramar Beach Road on the Coastside, Pacifica and Half Moon Bay. Trails in Millbrae were inundated, and bayside communities including East Palo Alto and Redwood City saw significant impacts. Photos from the flooding included images from locations such as Coyote Point in San Mateo.

The county is no stranger to flood exposure. A 2024 peer-reviewed study published in Scientific Reports found San Mateo County has the most projected flood exposure of any county in California, with more than 140,000 people and $50 billion in property at risk through this century. The county's bay shoreline includes San Francisco International Airport, two regional bridge touchdowns and an interstate highway.

OneShoreline CEO Len Materman wrote to the San Mateo County Charter Review Committee in an April 2026 letter that "continued sea level rise threatens more people and property here than in any other county in California." OneShoreline, the county's flood and sea level rise resiliency district created by AB 825 in 2019, lists a San Mateo Shoreline Plan among its current workplan items, according to its June 2026 board agenda.

A Kelvin wave is not a visible wall of water. It raises the baseline height of roughly the nearest 12 to 19 miles of ocean along the coast. Normal waves and swells keep rolling on top of that higher surface. Amaya cited a rule of thumb: roughly every four inches of sea-level rise can double the flooding risk for low-lying coastal areas.

The National Oceanic and Atmospheric Administration (NOAA) has said there is a 75% chance the current El Niño will be stronger than any since 1950, according to the Los Angeles Times. Amaya said another Kelvin wave already passed California's coast in May and June, and more are likely.

Residents in flood-prone areas can monitor tide forecasts through NOAA's tide prediction service.