A high surf advisory has been issued for the north- and west-facing shores of most isles, effective from 6 a.m. to 6 p.m. today.

The National Weather Service says a moderate-sized, northwest swell is expected to build tonight and peak early today morning, producing advisory-level surf along most north- and west-facing shores.

Surf is expected to get a boost to 12 to 16 feet along the north-facing shores of Niihau, Kauai, Oahu, Molokai, and Maui, and to 8 to 12 feet along the west-facing shores of Niihau, Kauai, Oahu and Molokai.

Forecasters said the advisory may need to be extended through tonight if the swell peaks any higher or later than predicted.

Impacts are moderate, but expected to bring strong, breaking waves and currents, which will make swimming dangerous.

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