There’s a massive wave heading across the ocean straight for the California coast — not a freak tsunami, but a slow-moving surge driven by what may be one of the strongest El Niño oscillations in nearly a millennium.

The phenomenon is called a “Kelvin wave,” an incomprehensibly vast displacement of warm ocean water creeping from the west Pacific toward the Americas. As the Guardian notes, the wave spans thousands of miles of surf, which is currently creeping upward toward the California coast after a slow-motion crash into South and Central America.

When the full force of the wave reaches the Golden State, it could raise coastal sea levels by as much as a foot, ocean scientists told the Guardian. And due to the forces of El Niño, sea levels could remain in that elevated state for months, accelerating coastal erosion and feeding severe storms.

“It’s a wave unlike the ones you’re used to at the beach,” research oceanographer at the Southwest Fisheries Science Center Mike Jacox told the Guardian. “They don’t crash, you can’t really seem them come by. This winter, we’re supposed to have some really high tides around Thanksgiving and Christmas.

If you also have more storms, then you have storm surge. It’s just like you’re raising the floor on all that stuff that happens.”

The wave is already swelling off the coasts of countries near the equator like Columbia and Panama. In Ecuador, data collected by the Escuela Superior Politécnica del Litoral, a public university which plays a key role in global monitoring of El Niño oscillations, found that ambient sea levels were nearly 15 inches higher than average. During periods of high tide, the surf rose to nearly 17 inches above usual levels, El Universo reported.

The Kelvin wave is kicked off by unseasonably warm water temperatures in the Pacific, up to 13.5 degrees Fahrenheit higher than usual. During developing El Niño phases, this glob of warm water creeps much farther East than it typically would, creating to the broader conditions attributed to the weather system.

Those conditions will likely persist well into the new year, with the World Meteorological Organization predicting a weakening phase beginning in early Spring of 2027.