The National Weather Service has issued a high surf advisory for the north and west shores of most islands, effective from 6 p.m. today until 6 a.m. Thursday.

Forecasters expect a large, northwest swell to build down the island chain today, then peak tonight and Wednesday.

The swell is expected to bring advisory-level surf of 15 to 20 feet to the north shores of Niihau, Kauai, Oahu, Molokai and Maui tonight and Wednesday.

Surf along the west shores of Niihau, Kauai, Oahu and Molokai, meanwhile, is expected to build to 12 to 16 feet tonight and Wednesday.

The public should beware of strong, breaking waves along affected shores, with strong currents that will make swimming dangerous.

“Head all advice from ocean safety officials,” said the advisory. “When in doubt, don’t go out.”

A small craft advisory has also been issued for Maalaea Bay in Maui, the Pailolo and Alenuihaha channels, and Big Isle waters, effective until 6 p.m. Friday. Conditions will be hazardous to small craft, and inexperienced mariners should avoid navigating in these conditions.

A gale warning is in place for Hawaiian offshore waters beyond 40 out to 240 nautical miles, including the portion of Papahanaumokuakea Marine National Monument east of French Frigate Shoals.