(KRON) — The National Weather Service is warning people that dangerous waves could reach up to 20 feet high Monday in the Bay Area –with sneaker waves up and down the coasts.

KRON4 spoke to the NMW about why the big waves should be a big concern. Few surfers and beachgoers about the sneaker waves spoke with KRON4 — all of them say they weren’t really concerned about it.

That exact reaction is what has meteorologists worried. These waves look calm and then seemingly out of nowhere will get large and sweep someone away.

People in Pacifica spent their Sunday walking along the beach, surfing the waves and playing in the water. However, doing the same Monday could be deadly because of sneaker waves.

Meteorologists are concerned more people could be on the beach for the Halloween holiday.

“The ocean is not going to look that chaotic and that scary, and so these sneaker waves that are coming from the Pacific Ocean long period swells that are slowly coming into the coast line that will sneak up on people every 10-15 minutes,” said NWS meteorologist Roger Gass.

Gass says when there are high surf warnings and large waves, people stay away. But the calm conditions around sneaker waves lure people in.

“It’s these lower threat situations that really catch people off guard and is, unfortunately, the time in which we lose most of our people to the Pacific Ocean,” Gass said. “Even if you can swim well, people go through what we call cold water shock and as soon as you hit that cold water, you breathe in and try to get a breath of air and you end up swallowing water.”

That could lead to someone drowning.

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He says to stay out of the water yourself, and don’t let your kids or pets go in either.

“Those are things that always catch people off guard. You try to go in and save someone else or try to go in and save your pet and you end up becoming victim to the ocean itself,” Gass said.

Gass also says colder temperatures are on the way later in the week. So be prepared for some overnight frost and freezing conditions.