A Beach Hazards Statement is in force along Pacific coast beaches in the San Francisco Bay Area through Monday evening, with forecasters warning of powerful sneaker waves, strong rip currents and breaking waves of up to 10ft.
The National Weather Service said two long-period southerly swells generated by distant hurricanes Karina and Marie were expected to reach the coast over the holiday weekend. Their energy could combine at intervals, creating sudden surges far higher up the beach than expected.
The advisory covers the San Francisco coast, the coastal North Bay including Point Reyes National Seashore, the San Francisco Peninsula, northern Monterey Bay, and the southern Monterey Bay and Big Sur coast.
Forecasters said the swells had travelled more than 1,000 miles across the Pacific. Although the waves may appear modest offshore, their long interval allows them to run much further up the shoreline, increasing the risk of people being caught off guard.
The National Weather Service urged the public to remain out of the water and stay away from jetties, piers, rocks and other structures beside the sea. It also warned beachgoers never to turn their backs on the ocean, even during apparently calm periods.
Saturday and Sunday are expected to bring generally settled weather, with temperatures in the low 60s on the coast, upper 60s to low 70s around the bay, and the mid-70s to near 80F inland. Officials warned that clear, pleasant conditions could encourage more people to visit the shoreline while the dangerous surf persists.
Patchy, dense fog is also forecast across inland valleys early on Saturday, with overnight temperatures falling into the 50s and a low of about 48F expected in Santa Rosa.
The Beach Hazards Statement is due to remain in effect until 11pm local time on Monday, September 7.
