What is expected to be the largest swell of the winter surf season began rolling in early this morning, promising waves as high as 35 to 40 feet and the threat of coastal flooding.

The National Weather Service issued a high-surf warning for north- and west-facing shores of Kaua'i, O'ahu, Maui and Moloka'i until 6 p.m. tomorrow. A high-surf advisory will be in effect on the Big Island from noon today until 6 a.m. Saturday.

If the surf reports pan out, the waves should be the highest of the year, said veteran North Shore lifeguard Lt. John Hoogsteden. But increasing northerly winds are expected to create turbulent, unruly surfing conditions that won't even make for good wave-watching, he said.

Hazardous conditions and no-swimming signs will be posted, and even professional and expert surfers will likely avoid the water, he said.

Hoogsteden's best advice for the day: "Leave your board at home and bring your common sense."

On Kaua'i, surf along north- and west-facing shores was expected to begin rising after midnight and reach heights of 30 to 40 feet by daybreak. On O'ahu, surf along west-facing shores was expected to rise to 10 to 15 feet late this morning, building to 18 to 24 feet by evening. On the North Shore, surf was expected to be 15 to 20 feet this morning, building to 30 to 35 feet by afternoon.

Surf will be slightly smaller on Moloka'i and Maui, the weather service said.

The large northwest swell will reach the north-, east- and west-facing shores of the Big Island by this evening. Expect surf to reach 15 to 20 feet on that island's north and east shores and 6 to 10 feet on the west shores.

The weather service advises coastal residents to secure property and be aware that large waves may result in coastal flooding.

Dave Curtis, spokesman for state Civil Defense, said O'ahu Civil Defense will be monitoring the high surf and any possible problems that may arise.

"We get constant updates on the forecast from the National Weather Service, and people on our 24/7 staff would be monitoring that constantly throughout the night and the morning hours," Curtis said. "If there are any problems reported from police through O'ahu Civil Defense, then we would take action at that point."

Henry Lau, a forecaster with the National Weather Service in Honolulu, said the swell was generated by a passing storm north of the islands a few days ago.

"This is common this time of the year when you have passing storms spinning out toward the Islands," Lau said. "We had one episode last week and it didn't get that high. I believe this is the highest, so far, and there are a few more weeks to go (in the season)."

The northwest swell is expected to peak tonight and slowly decline tomorrow. Another large northwest swell is expected Monday night and Tuesday.

Hoogsteden emphasized that today's expected rough conditions mean there is likely to be no recreational activity in the water on the North Shore.

"Use caution, check with lifeguards if you have any questions, and don't expect much spectating of surfers," he said.

"There will be no good 'Hawai'i Five-0' waves."

Reach Loren Moreno at lmoreno@honoluluadvertiser.com and Will Hoover at whoover@honoluluadvertiser.com.