A long-period northwest swell moving through the region should peak this evening before slightly declining overnight into the morning of Feb. 9.

The swell is pumping surf of up to 18 feet that will bombard north shores of Kaua‘i and Ni‘ihau, causing National Weather Service forecasters in Honolulu to issue another high surf advisory for those shorelines, this one in effect until 6 a.m. Sunday.

“Long-period swells accumulate energy, travel faster and can easily cope with local winds and currents, resulting in larger surf when it comes to average wave height,” explains SurferToday, a leading surf-and-skate website.

Swell period refers to the time between two wave crests in the swell. It can also be called the wave interval.

Long-period swells typically have about 10 seconds or more between waves and most often generated far away from the shores where they bring heightened surf.

Impacts from the advisory level surf along north shores on both islands are expected to be moderate.

Strong breaking waves and strong currents will make swimming and likely other ocean activities dangerous.

Forecasters urge the public visiting the shorelines in advisory areas to heed all advice from ocean safety officials.

When in doubt, don’t go out.