A strong northwest swell is expected to bring surf up to 20 feet to the north-facing shores of some islands and has prompted weather officials to post a high surf advisory for affected areas through 6 a.m. Saturday.

The north- and west-facing shores of Oahu, Kauai, Molokai and Niihau and the north-facing shores of Maui are all under a high surf advisory, according to officials at the National Weather Service. Surf is expected to reach up to 15 to 20 feet along the north-facing shores of Oahu, Maui, Kauai, Molokai and Niihau. Surf up to 10 to 15 feet is expected along the west-facing shores of Oahu, Kauai, Molokai and Niihau.

Officials advised beachgoers to follow advice given by lifeguards. Boaters should be aware of surfers paddling through harbor channels en route to surfing areas.