With powerful waves rising 30 feet or higher along Oahu’s North Shore — kicking off the October-to-March winter surf season — city lifeguards were busy Friday warning beachgoers of the potential dangers.

“During those months is when we get our biggest swells, so that’s the most dangerous time of the year,” said North Shore lifeguard Jeff Okuyama. “You can pretty much forget about swimming or snorkeling. During our winter months it’s experts only.”

By “experts,” Okuyama means people with proper equipment, experience and knowledge of the breaks. Conditions change dramatically from summer to winter on the North Shore, he said, as well as from day to day and even by the hour, so residents and locals alike need to be vigilant.

A high-surf warning for north- and west-facing shores of Oahu is in effect until 6 p.m. today. “No swimming” signs, as well as warnings for strong current and dangerous shorebreak, are posted at North Shore beaches, including the popular Waimea Bay.

Daily, Okuyama is taking what the city calls preventive actions, stopping people who are unaware or unprepared from going in the ocean. There were 2,800 preventive actions and 13 rescues on the North Shore on Friday, according to city Emergency Services Division spokeswoman Shayne Enright.

At Mokuleia Beach around 4 p.m., two men in their 20s were rescued swimming in 15-foot surf.

North Shore lifeguard Kirk Ziegler said visitors also need to be aware of the possibility of being swept out while taking pictures and walking along the rocks at Shark’s Cove or along the shorelines of the sandy beaches in the wintertime.

A good rule of thumb, Ziegler said, is to pay attention to the rocks. If they are wet, then waves are washing over them and it’s best to stay off. If the sand along a shoreline has no footprints, that’s an indication that waves are coming up as well, with the potential to knock people off their feet.

That goes for young children playing along the shorebreak, too.

In February an 8-year-old boy from South Korea playing along the shoreline at Sunset Beach could have been swept out to sea if several bystanders had not swooped in and pulled him from the powerful surf.

Lifeguards had already gone home for the day.

Footage of the incident caught on cellphone video went viral online, and it remains an example of visitors unaware of the North Shore’s dangerous winter shoreline.

Sixty full- and part-time lifeguards work the North Shore, with about 30 on duty daily from 9 a.m. to 5:30 p.m. during the busy winter months, which bring large swells and crowds of spectators.

Okuyama and Ziegler are also rescue craft operators who deploy on personal watercraft wherever needed along shorelines and oceans stretching from Kaena Point to Kaneohe Bay.

Three popular spots on the North Shore — Shark’s Cove, Three Tables and Ke Iki Beach — have no lifeguard stations. Lifeguards on patrol might pass through but are not stationed at any of those beaches, which they consider “trouble spots.”

Ziegler does not recommend snorkeling at Shark’s Cove during the winter, recalling a treacherous rescue within the lagoon last year of a man who nearly drowned and got cut up by the rocks.

“In the summer, yes, it’s beautiful, you can swim,” he said. “In the winter, waves break from inside and wash all the way through Shark’s Cove so they push people up onto the rocks.”

Just two weeks ago, while on patrol, Lt. Adam Lerner of the city’s Ocean Safety and Lifeguard Services Division rescued a young woman visiting from Shanghai who got into trouble while snorkeling at Three Tables. She had taken a bus out from Waikiki and was snorkeling by herself in an area with strong currents. Although she was not screaming for help, Lerner’s experience and instinct told him she was in trouble.

Lerner grabbed his rescue board and paddled out to her.

In a follow-up thank-you email to Ocean Safety, Bonnie Li, a kindergarten teacher, said she was just about to give up. She said she did not take the warning signs seriously and was happy to share her story with others.

Data from the state Department of Health showed 18 ocean drownings during the 10-year period from 2007 to 2016 on the North Shore, with none some years and three in other years, but not necessarily more during the winter, according to state epidemiologist Daniel Galanis. Lifeguard log sheets, however, show that they are much busier during the winter months.

Lt. Kerry Atwood, who also covers the North Shore, said he likes to refer to the breaks in the wintertime as a “double black diamond.”

“I don’t think there’s any particular place where we have such a big surf where so many people from all over the world are trying to test their skills,” Atwood said. “I think a lot of it, too, is people show up here with a false sense of security because they’ve done some surfing somewhere else.”

Spectators at the popular Vans Triple Crown of Surfing contests from mid-November to mid-December on the North Shore are also advised that waves during the winter months can double in size within an hour. In its list of tips, Vans Triple Crown, which hires its own lifeguards for event coverage, cautions against letting little ones play at the shoreline, especially at Pipeline or Sunset.

“It’s better this time of year to just come to the beach and watch the surfers,” Okuyama said. “This is where all the best surfers in the world are; you get a front-row seat at Pipeline, Waimea Bay. Just come and watch the surfers. Don’t try to be a hero and try to surf the waves yourself.”

Ziegler added, “And come talk to us. We’re here. We see what’s going on from sunup to sundown. If you have any questions, come talk to the lifeguards.”