The National Weather Service said last night that a high-surf warning in effect until 6 a.m. today might well be extended as more big waves head for O'ahu.

Overnight surf as high as 20 to 25 feet was expected last night on the North Shore. An Ocean Safety advisory said the high-surf warning would likely be extended as a larger northwest swell is expected to fill in late tomorrow and into Friday. Forecast models indicate open ocean swell heights in excess of 20 feet.

In addition, the recent high-surf events of early December have caused significant erosion at locations on O'ahu's north and west shores. Considering the potential added erosion from this week's swell event, the result could cause significant coastal wave run-up during the peak of the episode on Friday morning.

Residents and visitors are being encouraged to take the necessary precautions to protect life and property.

"The arrival of these large swell events coincides with the Christmas holiday so Ocean Safety personnel are anticipating there will be much more people at the beach," Bryan Cheplic, Emergency Services Department public information officer, said in the advisory. "With this in mind, city lifeguards are again asking for the public's continued cooperation to help maintain the high level of safety and care provided to all beachgoers."

A high-surf warning means waves can result in dangerous conditions and deadly rip currents.

The Honolulu Ocean Safety and Lifeguard Services Division has issued an advisory reminding all beachgoers to:

• Always swim at a beach with a lifeguard.

• Consult with a lifeguard regarding ocean conditions before entering the water.

• Heed all warning signs.

• Never swim alone.

• Call a lifeguard or 911 if you see someone in distress.

• Know your limits — if in doubt, don't go out.