Waves reaching up to 20 feet in height are expected on the north shores of most islands by tonight, National Weather Service forecasters predict.

A high surf advisory will go into effect for the north shores of all islands except Lanai and Kahoolawe starting at noon today until 6 p.m. Saturday. The advisory also extends to the west shores of Oahu, Kauai, Niihau, Molokai and Hawaii island, although Kauai’s advisory takes effect at 8 a.m. and also covers their east shores.

A northwest swell is expected to arrive this afternoon.

Forecasters say waves will build to 10 to 15 feet on north shores, then 15 to 20 feet tonight.

On the west side, forecasters expect waves with faces of 8 to 12 feet this afternoon, increasing to 10  to 15 feet tonight. West shores on Hawaii island may see 4- to 6-foot surf in the afternoon and 6- to 8-foot surf tonight.

The National Weather Service warns that the high surf will produce hazardous rip currents and rough ocean conditions.