SAN FRANCISCO (CBS SF) — A weather system in the Pacific will begin sending large waves crashing onto Northern California beaches mid-week with swells of 12-17 feet on Thanksgiving, forecasters said.

The National Weather Service issued a high surf advisory on Tuesday, warning that a northwest swell would begin building on Wednesday morning with waves of 4-7 feet at intervals of 18-21 seconds.

By Thanksgiving, forecasters predicted waves of 12-17 feet at 15-17 second intervals.

Waves at the legendary big-wave surf spot Mavericks off the coast from Half Moon Bay could see waves up to 28 feet, according to the San Francisco Chronicle.

“An approaching storm system will generate a large northwest swell train that will arrive early Wednesday and persist to at least noon on Thanksgiving Day,” NWS forecasters said in their advisory. “Long period forerunner (waves) will arrive as early as Tuesday night and may result in infrequent sneaker waves and stronger rip currents through at least sunrise Wednesday preceeding the high surf.”

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

“These large waves can be erratic and unpredictable and may injure or knock beachgoers into the cold, turbulent ocean,” forecasters warned.

While the swells were good news for surfers, they also have triggered warnings for holiday beachgoers and fishermen to be on the alert for rip currents and potentially deadly sneaker waves.

“These large waves can be erratic and unpredictable and may injure or knock beachgoers into the cold, turbulent ocean,” NWS forecasters warned.

“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 rip current before trying to swim for shore.”

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

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