The National Weather Service (NWS) has put out a hazard alert for Wednesday in anticipation of large, long-period swells.
Long-period swells are fast moving waves that are more resistant to winds and current, causing larger surfs than normal. Wednesdays waves are predicted to reach up to 20 feet in height.
With them comes sneaker waves, which are strong and sudden surges that can catch beach goers unaware, and rip currents, which are a strong undertow.
Steep beaches pose the greatest risk, says the NWS, and people are encouraged to stay out of the water and away from the shore.
San Francisco County beaches are under a hazard warning until 6 p.m. on Dec 21.