A high surf warning for the north and west shores of most Hawaiian islands has been extended to Friday morning, with a second swell on the way.

The National Weather Service says surf of 25 to 35 feet is expected today along the north shores of Niihau, Kauai, Oahu, Molokai and Maui as a significant north-northwest well builds down the island chain. Surf of 18 to 22 feet is expected along the west shores of islands from Niihau to Molokai.

Following a brief decline tonight, a second, extra-large north-northwest swell is expected to fill in Thursday, keeping the warning-level surf in place through 6 a.m. Friday.

The public should stay away from the shoreline along affected coasts, officials warned, and be prepared for road closures, the weather service said. Boaters should postpone entering or leaving affected channels until the surf subsides.

“Expect ocean water surging and sweeping across beaches, coastal benches, and lava flows creating the potential for impacts to coastal properties and infrastructure, including roadways,” the forecasters said in the warning. “Powerful longshore and rip currents will be present at most beaches. Large breaking waves and strong currents may impact harbor entrances and channels causing challenging boat handling.”

A small craft advisory is also in place until 6 a.m. Friday.