Powerful swells stemming from an extended stretch of stormy seas crashed along the California coastline on Thursday, catching sightseers off guard.
Breaking waves up to 20 feet high overwhelmed seawalls injured several people, forced rescues, and sent surges of water rushing through coastal streets, according to reports.
Meteorologists say conditions aligned perfectly to produce the monstrous surf and forecast more rounds of intense, potentially life-threatening waves and rip currents to pound the shoreline into the weekend.
Most days on the beach pass without incident, but occasionally, approaching storms or churning seas can transform the coast into a danger zone.
In Thursday’s case, winds blowing steadily for an extended distance over the ocean, known as fetch, enabled long-period swells to develop from multiple distant storms and collect into walls of water.
West and southwest winds then pushed the deep swells toward landfall along the California coast.
Long-period swells relate to the strength and duration of winds far offshore. Much of their energy travels powerfully yet invisibly beneath the surface rather than through choppy whitecaps.
So, upon reaching shallow coastal waters, long-period swells can rear up suddenly into much larger breaking waves than common small surf breakers created by more localized winds.
The incoming long-period swells essentially acted like a mini tidal wave upon hitting the beach. While tidal waves originate from seismic activity displacing the seafloor vertically, Thursday’s big waves arose from horizontal wind energy stirring up the deep oceans.
Real tidal waves may be barely noticeable at sea but can flood miles inland at landfall. Though the towering swells lacked that enormous range, they still sent saltwater surging beyond beaches to flood streets up to a couple of blocks away.
On the other hand, regular breakers formed by short-wavelength local winds generally affect only the shoreline itself.