A high-surf advisory has been issued for the north and west facing shores of Niihau and Kauai, and north shores of Oahu, Molokai and Maui, effective until 6 a.m. Saturday.

The National Weather Service says surf heights of 12 to 16 feet are expected along these shores due to a long-period, north-northwest swell that is larger than initially expected.

The swell will build down the island chain today, then gradually lower overnight.

Those visiting these shores should expect strong, breaking waves, shore break, and strong rip currents that make swimming difficult and dangerous.

“Beachgoers, swimmers, and surfers should heed all advice given by ocean safety officials and exercise caution,” said NWS in the advisory. “Know your limits and seek ocean recreation areas best suited for your abilities. When in doubt, don’t go out.”

A small craft advisory is also in effect due to seas building up to 8 to 11 feet, and NWS says inexperienced mariners should avoid navigating in these conditions. The advisory covers the northwest, windward and leeward sides of Kauai, the Kauai and Kaiwi channels, and windward Oahu and Maui County waters, through 6 a.m. Saturday.