SAN FRANCISCO (CBS SF) — The National Weather Service (NWS) warned Thursday that large waves will create dangerous surf conditions along the Bay Area coastline through Friday afternoon.

The NWS noted that it expects 12-14 foot-tall waves to crash into Bay Area beaches overnight, but will slowly decline overtime. But the long period of swells increases the potential for strong rip currents and sneaker waves.

“Sneaker waves are waves that seem to come out of nowhere and can have a periodicity of minutes to tens of minutes. A deceptively calm ocean typically exist between the sets of sneaker waves,” the NWS reported in its dangerous surf warning. “These consequently may catch those on coastal jetties, rocks, piers, or shorelines off guard and may injure them or knock them into the cold, turbulent ocean.”

The massive swell at San Francisco’s Ocean Beach looked more like sights you’d see at Mavericks in Half Moon Bay or even Pipeline in Hawaii.

Some waves were more than 20-feet tall.

“You can see the faces of these monster waves were triple, even quadruple the height of surfers!” enthused one wave watcher.

“The waves have so much force, you make the wrong move, you could die in an instant,” said one concerned surfer.

The biggest waves we spotted were breaking just south of Noriega Street.

Dozens of people, including professional surf photographer Brian Feulner, were there taking in the spectacle.

Feulner was filming the pros who showed up with jet skis to get towed into waves.

He said waves this big combined with the clean conditions created by offshore winds were a rare sight at Ocean Beach.

The National Weather Service advises against beach-combing during this period of long swells, as a sneaker wave could pull an unsuspecting individual into the water and right into a strong rip current.