SAN FRANCISCO (CBS SF) — An low pressure weather system sent a strong northwest swell crashing into Northern California beaches from Sonoma County to the Monterey coastline Monday, triggering warnings for beachgoers and fishermen to be on the alert for rip currents and potentially deadly sneaker waves.

The National Weather Service forecasters issued a beach hazard warning that was set to run until 5 p.m. Monday.

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“Initial swell heights of 1 to 2 feet at 23 seconds is expected increasing the risk of sneaker waves,” forecasters said. “Monday morning wave heights of 6 to 9 feet at periods of 18 to 20 seconds are expected, with most favored northwest facing beaches at 10 to 12 feet.”

Among the areas most exposed to the swell were San Francisco’s Ocean Beach; San Mateo County’s Montara State Beach; Monterey County’s Marina State Beach and Monastery Beach in Carmel.

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“Sneaker waves often catch people off guard and knock them into the ocean,” NWS forecasters said. “Rip currents can pull swimmers and surfers out to sea. Large breaking waves can cause injury, wash people off beaches and rocks, and capsize small boats near shore.”

If you do find yourself in a rip current, officials said swimmers “should swim parallel to the coast to escape the ripurrent before trying to swim for shore.”

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