A king tide approaches the Embarcadero shore near Pier 14 in San Francisco in January. More high tides in the coming days will probably send water sloshing over the Embarcadero again, a preview of the extremely high tides expected over the winter.
El Niño-warmed seas are pushing tides unusually high in California, prompting the National Weather Service to issue a coastal flood advisory for San Francisco Bay and Monterey Bay.
The advisory is in effect from 9 a.m. Friday to 3 p.m. Tuesday.
“High tides will result in flooding along coastal trails, cut off access to some low-lying beaches and coastal areas, and bring water and waves farther up the beach than typical,” the weather service said. Low-lying parking lots, parks and roads could experience flooding.
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In San Francisco, tides are forecast to peak at 1.2 feet above the average highest daily tide Friday through Sunday in the late morning and early afternoon.
Waves may slosh over parts of the Embarcadero and other low-lying areas along the bay shoreline, including Marin County. Moderate to strong winds and rough seas — a strong possibility for parts of the weekend — could make the tides come up even farther than forecast, especially on the Pacific coast.
During last week’s high tides, Michael Wara, director of Stanford University’s Climate and Energy Policy Program, posted on social media that parts of Tamalpais Valley were underwater.
“Second day in a row that I have been stuck in blue sky, high tide gridlock in Marin County. Truly a taste of things to come,” he said on Oct. 1. “Highway entrance next to (the) under construction affordable housing project is under several feet of water. Also underwater: the park-and-ride lot, the bike path and the bus stop.”
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Those high tides occurred amid tranquil weather, whereas this weekend’s forecast is unsettled.
Typically, early October tides would be far from coastal flood thresholds, but El Niño is warming seas along the California coast. Water also rises as it warms, a phenomenon known as thermal expansion. A coastally trapped Kelvin wave moving up the West Coast is suppressing cold water from rising to the surface of the ocean. Coastally trapped Kelvin waves are a consequence of El Niño.
Sea surface temperatures were well above normal along the California coast on Oct. 7. Water temperatures set daily records from Santa Barbara to San Diego, running more than 7 degrees above normal in some places.
“As far as I can tell, the wave is solidly along the U.S. West Coast now and possibly through San Francisco at this stage,” Dillon Amaya, an assistant professor of marine science at North Carolina State University and the nation’s leading Kelvin wave observer, told the Chronicle by email.
El Niño effects are adding about 9 inches to the gravitational tide of the moon. The long-term melting of ice sheets and warming of global oceans has added 3 inches or so to the tidal baseline set at the end of the 20th century.
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The tidal anomalies will probably last until at least next spring, meaning 30 king tides, by some definitions, are possible in San Francisco this winter. The city typically only experiences 10.
Scientists fear a “perfect storm” of large waves, strong wind and heavy rain could coincide with king tides around the holidays. It could threaten transportation networks around the Bay Area.
In Southern California, the sea surface temperatures and tides are even more extreme.
A gauge in San Diego broke a same-day sea surface temperature record by 7.2 degrees Wednesday. Tides there are also running 1.4 feet higher than normal.
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Abnormally high seas may worsen coastal impacts from a tropical storm threatening Southern California this weekend. Beach erosion and flooding of low-lying areas is possible. Separate coastal flood advisories are in effect in Southern California.
