» Photo gallery: High surf

City officials and lifeguards were preparing for the largest swell of the winter season, which was expected to reach Oahu’s North Shore overnight and last through today.

The National Weather Service issued a high-surf warning that remains in effect until 6 a.m. Thursday. The warning said surf would reach 25 to 35 feet on the north shores and 15 to 25 feet on the west shores of Niihau, Kauai, Oahu and Maui.

Jim Howe, city Ocean Safety Division operations chief, said the previous largest swell of the season was in December, during the Billabong Pipeline Masters, when surf heights reached about 22 feet.

"This is looking to be a lot cleaner, a lot smoother, a lot better conditions than during that period," Howe said. He said spectators who travel to Oahu’s North Shore will get a good show.

"There’ll be a lot of tow-in surfing activity going on (today), we anticipate," he said. "A lot of, probably, spectating."

Howe said surf was about 3 to 5 feet for most of Tuesday but began building at about 3 p.m. and quickly reached 15 feet at Waimea Bay by about 4:30 p.m.

"The conditions kind of went from being what we would call normal caution to high hazard," he said. "It’s already at the extreme-hazard level now."

Howe urged spectators to stay away from the shore break, pay attention to lifeguards and "enjoy one of the natural wonders of Hawaii’s world."

He said the division is working with the Coast Guard, which will conduct a flyover on the North Shore at about 6 p.m. today to check for any surfers who are having trouble returning to shore.

The city Department of Emergency Management is preparing to activate the city’s Emergency Operations Center if needed, a department spokesman said.

According to the weather service, a high-surf warning indicates dangerous, battering waves will pound the shoreline and result in dangerous swimming conditions and deadly rip currents.

Warning-level surf of 25 to 30 feet reached Kauai Tuesday afternoon and was rising about a foot per hour Tuesday evening at a buoy off Waimea, said Tom Birchard, a senior forecaster with the National Weather Service.

He said surf could get as high as 40 feet on the outer reefs off Oahu Tuesday night, decrease before daybreak today, then hold between 25 and 35 feet for most of the day. Tradewinds were expected to increase overnight, improving surf conditions.

"For the big-wave riders, I think the conditions are going to be pretty good," he said.

The weather service warned of possible run-up along low-lying coastal areas with the greatest threat during high tide last night. Birchard said the washes would be similar to typical high-surf run-up and weren’t expected to damage any homes.

Some businesses on the North Shore said business had been good throughout the holiday season and expected another surge in customers today.

Kua Aina Sandwich Shop in Haleiwa ramped up its staffing in anticipation of today’s crowd, said manager Janite Alfanca.

She said the swell would bring in another boost after the holidays, a time when business typically begins to slow.

The large surf, however, wasn’t expected to kick off the Quiksilver in Memory of Eddie Aikau.

"The likelihood is slim to none," said event organizer Glen Moncata in a statement. He said not enough large waves were expected. The event requires waves larger than 20 feet.

The big-wave event, named after legendary waterman Eddie Aikau, who disappeared at sea, has until Feb. 29 to run at Waimea Bay. It has run only eight times in its 27-year history.

Hawaii News Now video: City warns beachgoers and residents about hazardous surf