A massive Kelvin wave, thousands of miles wide, is predicted to reach the California coast within weeks, potentially amplifying the effects of anticipated El Niño storms and leading to significant coastal flooding.
Scientists from NASA and the National Oceanic and Atmospheric Administration (NOAA) have identified this Kelvin wave as a planetary-scale phenomenon driven by weakened tropical Pacific winds during El Niño conditions. This weakening allows warm ocean water, typically concentrated in the western Pacific, to shift eastward, raising sea levels along the Americas' west coast by as much as a foot.
While not a tsunami, the wave's arrival is expected to elevate the baseline sea level, making coastal areas more vulnerable. NOAA reported on September 25 that the wave had already impacted South America and was moving northward, with effects anticipated on the U.S. West Coast shortly thereafter.
This elevated sea level, combined with an El Niño expected to be one of the strongest on record and potentially lasting through 2027, creates a heightened risk. NOAA oceanographer Michael McPhaden warned of a potential "quintuple whammy" for California, where the Kelvin wave, rising sea levels from a marine heat wave and climate change, and storms from offshore hurricanes could converge.
"A Kelvin wave raises the ocean along California by about six to 12 inches within days or weeks of its arrival, which is sudden compared with normal long-term sea-level rise that can take up to a century to reach the same level," the report stated. This temporary increase can persist for weeks or months, significantly impacting high tides and storm surges.
El Niño winters are already associated with increased storm activity and larger ocean waves. NOAA oceanographer Mike Jacox noted that California typically experiences high tides around Thanksgiving and Christmas. The addition of a Kelvin wave means that storms coinciding with these high tides could push floodwaters further inland, causing more extensive damage to beaches, cliffs, and low-lying coastal communities.
Kate Huckelbridge of the California Coastal Commission highlighted in a September 2026 memo that "the most damaging storms are often those that coincide with the highest tides." She added that storms peaking during high tides are far more likely to cause damage than if the same storm occurred during low tide.
In anticipation of potential winter storms, flooding, and coastal hazards, California Governor Gavin Newsom declared a state of emergency in late September. While not directly linked to the Kelvin wave's arrival, the declaration was prompted by scientific predictions of a potentially record-breaking El Niño.
California has already faced risks from an active Pacific hurricane season. As of September 29, 17 named storms had formed in the eastern Pacific, with six escalating to hurricane strength. This includes Hurricane Polo, a Category 5 storm that made landfall in Mexico's Baja California peninsula.
With the Pacific hurricane season extending until November 30, the approaching Kelvin wave could influence storm impacts along the California coast for the next two months. NOAA's Nate Mantua told The Guardian that the combination of increased storminess, large ocean waves, and elevated sea levels during El Niño winters in the northern Pacific exposes coastal areas to greater ocean energy and inundation risks.