By Katie Nielsen

SAN FRANCISCO (KPIX) — A potent weather system approaching the Northern California coast was expected to churn up massive waves starting Sunday, some towering as high as 50-feet, prompting warnings to stay away from the beach.

The National Weather Service has issued a high-surf warning starting Sunday morning. The weather service said the surf would be the largest of the year with the biggest waves crashing ashore Sunday night through Monday morning.

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The swells would be 17-to-21 feet at intervals of 19-to-21 seconds, forecasters warned. Large breaking waves of 25-to-40 feet will be possible at west and northwest facing locations, with breakers up to or exceeding 50 feet.

Beth Mathews from Sunnyvale visited San Francisco’s Ocean Beach with her husband and their friends Saturday to see the larger than normal swells.

“We lived in Hawaii for 10 years, saw some pretty big waves, and this is pretty close to what we had,” Mathews said.

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San Francisco Fire Lt. Jonathan Baxter said the crashing surf may delay any needed rescue attempts.

“These waves are going to be so severe,” he said. “That our rescue workers may not be able to enter the water to effect a rescue of you.”

Baxter said the biggest concerns are deadly sneaker waves and strong currents — they have the potential to surprise a beachgoer and sweep them out to sea.

The threat has forced officials to place warning signs all along Ocean Beach trying to keep people away from the water.

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Victor Lim with the San Francisco Department of Emergency Management says, “We are asking the public to avoid the area all together.”