A high surf warning for the north- and west-facing shores of most isles begins at 6 a.m. Wednesday as a series of large swells builds down the island chain.

The National Weather Service says these large, northwest-to-north-northwest swells are expected to build overnight and reach warning levels by Wednesday afternoon.

Officials warn of “dangerously large, breaking waves” rising to 18 to 25 feet along the north shores of Niihau, Kauai, Oahu, Molokai and Maui. Surf along the west shores of Niihau, Kauai, Oahu and Molokai, is expected to rise to 12 to 18 feet.

In addition to very strong, breaking waves along affected shores, there will be powerful currents and navigation of channels will be dangerous.

“Stay away from the shoreline along the affected coasts,” said the NWS. “Be prepared for road closures. Postpone entering or leaving channels affected by the high surf until the surf subsides.”

Forecasters expect surf to peak Wednesday night and Thursday, with additional swell pulses expected to keep extra-large surf in place through the weekend.

The high surf warning remains in effect until 6 p.m. Thursday.