Surfers and beach-goers should be extremely careful on Oahu's north and west shores today, said Bryan Cheplic, spokesman for the city Emergency Services Department.

The 25- to 35-foot surf forecast for today had not materialized as of 6 a.m., but ocean monitoring buoys far offshore show the huge swell is on its way, Cheplic said.

"Lifeguards are concerned that the surf will build very rapidly, in the period of about half an hour to 45 minutes or maybe an hour," Cheplic said.

Surfers who may be able to handle 3- to 4-foot surf could very quickly find themselves in over their heads with the sudden onset of big waves, Cheplic said.

He estimated that the big surf may begin rolling up on Oahu shores between noon and 1 p.m. today.

"I would expect it to be here by lunch time or early afternoon," Cheplic said.

Nonetheless, surfers should be cautious throughout the morning and be prepared for a rapid rise in surf heights, Cheplic said.