Update: High surf advisory for north, west shores extended

This story was updated at 12:03 p.m. Feb. 20.

Feb. 19 update at 3:47 p.m.: The National Weather Service in Honolulu extended a high surf advisory in place since Monday until 6 p.m. Thursday.

Surf heights of 12 to 16 feet are forecast along north-facing shores and 8 to 12 feet along western shorelines. Strong breaking waves and currents will make swimming and other ocean activities dangerous.

Original post: National Weather Service forecasters in Honolulu extended the high surf advisory for north and west shores of Kaua‘i and Ni‘ihau to 6 p.m. Wednesday as a series of large west-northwest to northwest swells are expected to produce advisory level through at least Wednesday.

Those swells will bring large breaking waves of up to 20 feet along northern shorelines and up to 15 feet for western coasts of both islands.

A high surf advisory was already in effect for the same areas originally through this evening because of the first large swell of the series building tonight and forecast to peak today. One of the buoys west of Kaua‘i early Monday evening indicated a 7 to 8 foot northwest swell with medium to long periods.

Swell period is the time between crests or troughs of waves.

The longer the period, the more energy a swell has, resulting in larger and stronger waves. Often called groundswells, long-period swells are typically generated by powerful storms far from shore.

Shorter periods — frequently referred to as wind swells — cause smaller, choppy waves and are caused by windy conditions.

Another reinforcing swell is expected Wednesday, which should maintain advisory level surf along north and west shores through the middle of the week and possibly into Thursday.

Some other shores also could see some small wraparound during the next few days caused by the series of large west-northwest to northwest swells.

Strong breaking waves and strong currents will make swimming and other ocean activities dangerous along the North Shore and western shorelines. The public is urged to heed all advice from ocean safety officials in the advisory areas.

Remember: When in doubt, don’t go out.

A small craft advisory also continues as hazardous seas are expected in waters exposed to the large swells. A buoy in waters off Hanalei on Monday night was already observing significant wave heights of slightly more than 10 feet.

Because of a lack of strong trade winds, surf along east shores is expected to remain relatively small throughout the week. A slight spike is possible Tuesday through the second half of the week with the return of more established light to moderate east to east-southeast trades.

Surf along southern shorelines, meanwhile, should see a small hike Wednesday with a small south swell filling in and producing some rideable surf during the second half of this week.

Visit the National Weather Service Honolulu forecast office website for additional and up-to-date weather and surf information.