The slow pulse of El Niño-driven ocean water that scientists have tracked for weeks is now arriving along California's coast. Over the coming week, the 6-inch to 1-foot surge is expected to move north along the state's shoreline at about 6 mph, or roughly 150 miles a day, before reaching Oregon and Washington, the Associated Press reported Saturday. Experts expect additional waves to keep passing through from October into February.

The wave itself will not flood streets. The concern is what it does to the ocean's starting point. "It's not that this Kelvin wave is going to come and flood anything on its own," said NOAA Fisheries research oceanographer Michael Jacox, who explained that the wave instead raises the baseline that tides and storms build on.

That puts a spotlight on the calendar. NOAA tide predictions show San Francisco Bay's highest tides of the month falling in the last week of October, weeks before the winter king tides most people watch for.

A Hidden Wave With a Foot of Reach

A coastally trapped Kelvin wave is not a breaking wave. It is a wide, shallow bulge of warm water that forms when El Niño weakens the equatorial trade winds and lets warm water slosh east across the Pacific. When it hits the Americas, part of it turns north and hugs the coastline.

North Carolina State University oceanographer Dillon Amaya told the AP the bulge extends 12 to 20 miles offshore and about 200 meters deep. It cannot be seen from the beach and does not lift boats, but tide gauges and satellites record it as higher water.

Amaya said the current wave is not the last. One wave passed in May or June, and he said there is evidence a third is forming. Strong El Niño years typically produce five or six.

The water was already high before this one arrived. UC Agriculture and Natural Resources climate scientist Daniel Swain wrote in late September that tides were running a foot or more above average because of an extreme marine heat wave, and that the largest Kelvin wave of the sequence so far would arrive in October, Gizmodo reported.

Late-October Tides in San Francisco and Beyond

Nature World News reviewed NOAA predictions for the San Francisco tide station at the Golden Gate. The month's highest predicted high tides cluster from Oct. 26 to Oct. 29. The peak is 6.67 feet above mean lower low water at 12:56 p.m. PDT on Oct. 28, with 6.65 feet at 12:15 p.m. PDT on Oct. 27 and 6.49 feet at 11:39 a.m. PDT on Oct. 26.

Add the 6 to 12 inches scientists project, and the actual water at Oct. 28's midday high tide could reach roughly 7.2 to 7.7 feet. Those are simple estimates, not an official forecast. The real number will depend on how strong the wave is when it reaches the Bay, plus wind, air pressure and any swell on that day.

A foot sounds small, but coastal flood risk does not rise in a straight line. "Every 2 to 4 inches of sea level doubles our flood risk," Patrick Barnard of UC Santa Cruz's Center for Coastal Climate Resilience told CNN, because so many West Coast communities are built right at sea level.

In practice, that means water reaching parking lots, low seawalls, beach stairways and shoreline paths that normally stay dry at high tide. It also means storm waves start from a higher platform, so they can push sand off beaches and reach bluffs and homes more easily.

Southern California Beaches Face the Greatest Exposure

Orange County remains the most vulnerable stretch. Beaches there were stripped of sand by swells from former Hurricane Marie in early September, and Alyssa Napuri, deputy chief of staff for Orange County Supervisor Katrina Foley, told the AP that crews are placing emergency sand as quickly as possible.

U.S. Geological Survey research oceanographer Sean Vitousek warned in the same report that higher seas, larger El Niño-driven North Pacific swells and large spring tides from October through February can combine to cause exceptional erosion. He noted that the biggest El Niño winters of 1982-83, 1997-98 and 2015-16 produced the worst West Coast beach erosion on record.

Those most at risk include owners of bluff-top and beachfront homes, properties behind aging seawalls, coastal rail riders in San Clemente, and residents of low-lying neighborhoods such as the Long Beach Peninsula and parts of the Bay Area shoreline. Renters in those areas face the same flood exposure as owners but often have less control over repairs.

State officials are already acting. Gov. Gavin Newsom proclaimed a statewide state of emergency on Sept. 21 to prepare for El Niño. "We are not waiting for this season's storms to be on our doorstep," said Caroline Thomas Jacobs, director of the California Governor's Office of Emergency Services, in a statement quoted by The Weather Network.

Preparing Before the Next High Tide

Several questions remain open. Scientists do not yet know exactly how high each local tide gauge will read as the wave passes, whether it will coincide with a large swell, or how many more waves will follow this winter. Amaya told the AP it is too early to say whether this season will break records.

Nature World News previously reported on how the approaching Kelvin wave raised erosion risk for California beaches already damaged by Hurricane Marie's swells. Since that Sept. 22 report, scientists have confirmed the wave is moving onto the coast this month, and NOAA predictions show the October tide peak arrives earlier than the season's typical king tides.

Residents do not need emergency action for the Kelvin wave alone. Reasonable steps include checking NOAA tide predictions for your nearest station, avoiding parking or driving on low shoreline roads during the Oct. 26 to Oct. 29 midday high tides, and moving valuables out of ground-level garages in flood-prone blocks.

Watch for National Weather Service coastal flood advisories and high surf warnings, especially when they overlap with a high tide. Water over a roadway, waves topping a seawall, or new cracks and sinkholes near a bluff are signs to leave the area and contact local officials. Never walk on eroded sand cliffs or turn your back on the surf.

Check flood coverage now. Standard homeowners and renters policies generally exclude flood damage, and new National Flood Insurance Program policies usually take 30 days to take effect, so coverage bought today would not start until early November.

NOAA's Climate Prediction Center issues its next El Niño update on Oct. 8. The bottom line: the ocean along California is sitting higher than normal and should stay elevated for weeks, the late-October tides are the first clear test, and preparation before then is cheaper than repairs after.

Developing Story Timeline

Oct. 3, 2026: AP reports the Kelvin wave surge will move north along the California coast over the next week, with more waves expected through February.

Sept. 22, 2026: Nature World News reports the approaching wave raised erosion risk for beaches damaged by Hurricane Marie.

Sept. 21, 2026: Gov. Newsom proclaims a statewide El Niño state of emergency.

Sept. 8, 2026: CNN reports California sea levels could rise about a foot by mid-October.

May to June 2026: An earlier coastal Kelvin wave reaches California, according to Amaya.

What Readers Want to Know

Has the Kelvin wave reached California?

The AP reported on Oct. 3 that the surge is moving north along the California coast over the coming week at about 6 mph, with more waves expected from October through February.

How much will sea levels rise?

Scientists project roughly 6 to 12 inches above normal, on top of water that was already elevated by a marine heat wave and warm-water expansion.

When are the highest tides this month?

NOAA predicts San Francisco's highest October high tides from Oct. 26 to Oct. 29, peaking at 6.67 feet at 12:56 p.m. PDT on Oct. 28.

Will the Kelvin wave flood my street?

Not by itself. The risk comes when the higher baseline lines up with a high tide, big swell or storm. Watch for NWS coastal flood advisories.

Who is most at risk?

Bluff-top and beachfront homes, properties behind aging seawalls, low-lying neighborhoods, coastal rail corridors and beaches already stripped of sand in Orange County.

Does insurance cover this kind of flooding?

Standard homeowners and renters policies generally do not cover flooding. Flood insurance is separate, and new NFIP policies usually have a 30-day waiting period.

How long will sea levels stay high?

Experts expect a series of waves to keep levels elevated into February, with the effect fading as El Niño weakens.