SAN FRANCISCO (CBS SF) – A low pressure weather system caused strong northwest swell on northern California’s beaches from Sonoma County to the Monterey coast on Monday, warning beachgoers and fishermen of currents and potentially deadly sneaker waves.

The forecasters from the National Weather Service issued a beach hazard warning that should run until 5 p.m. on Monday.

“Initial swell heights of 1 to 2 feet after 23 seconds are expected to increase the risk of sneaker waves,” forecasters said. “On Monday morning, wave heights of 6 to 9 feet are expected in periods of 18 to 20 seconds, with the most popular beaches facing north-west facing 10 to 12 feet.”

One of the areas most exposed to the swell was Ocean Beach in San Francisco; Montara State Beach in San Mateo County; Marina State Beach and Monastery Beach in Carmel in Monterey County.

“Sneaker waves often catch people unprepared and push them into the ocean,” said NWS forecasters. “Rift currents can pull swimmers and surfers out to sea. Big waves can cause injuries, wash people off beaches and rocks, and capsize small boats near the coast. “

When in a current, swimmers should “swim parallel to the shore to escape the current before attempting to swim ashore”.

They also said fishermen should avoid fishing from rocks or jetties as well.