Waves could top 20 feet today on the north shores of O'ahu, Kaua'i and Maui, the National Weather Service said yesterday in issuing a high-surf advisory. But even bigger surf may be on the horizon.

An accompanying surf forecast said there is a "strong potential for warning level surf late this weekend into the middle of next week."

Some surf forecasters and big-wave watchers are suggesting that waves having 40-foot faces are a possibility, with the surf being the biggest it's been in 15 years.

Tim Craig of the National Weather Service's Honolulu forecast office said "it's a bit early" to predict how big the surf is going to get.

"It depends on which (forecasting) models you're looking at," Craig said. "All we're saying right now is that we we could very likely see warning-level surf by the middle of of next week if not before."

However, the potential for "very high surf" with wave faces of 25 feet or much more "is there," Craig said.

"There's a big storm and low-pressure area with gale-force winds about 1,000 miles north of Hawai'i," Craig said.

The big swells that generate monster surf on Hawai'i shores typically come out of an area to the north of Japan or to the south of the Aleutian Islands, where the current mid-Pacific storm is raging, Craig said.

"We're looking at it very closely. If the models are correct, we definitely have some very high-level surf coming our way," Craig said.

The Hawaii Surf News Network isn't hedging any bets. Its forecast calls for surf with face values of 20 to 40 feet for Monday and Tuesday.

John Cummings, spokesman for the city Department of Emergency Management, said the agency has scheduled a meeting for 11 a.m. today to talk about the high-surf forecast and what action might need to be taken.

"Our city, state and nongovernment partners such as the Red Cross will be at the meeting to review our (contingency) plans," Cummings said.

"We may keep some staff on standby over the weekend if the forecast suggests we need to," he said.

The high-surf advisory is in effect until 6 a.m. tomorrow. The weather service expected a large northwest swell to peak early today and diminish below advisory levels by tomorrow morning.

Waves in excess of 20 feet were expected today on north shores of O'ahu, Kaua'i and Maui, and 12 feet or more on west-facing shores of O'ahu and Kaua'i.