Twenty-five-foot waves are expected to pummel Oahu’s North Shore today.

What forecasters call "extra-large" surf started hitting the north and west shores of Oahu, Kauai, Molokai and Niihau and the north shores of Maui Thursday afternoon, prompting a high-surf warning through 6 a.m. Saturday.

Forecasters say the powerful northwest swell is creating hazardous surf with "very strong breaking waves" and rip currents.

Surfing conditions will be good "if you like to surf giant waves," said National Weather Service lead forecaster Ian Morrison. The surf should be peaking today at 25 feet on north shores and will gradually diminish Saturday afternoon.

"Anyone entering the water could face significant injury or death," the weather service warned.

On Thursday morning on Kauai, surf ranged from 20 to 25 feet.

On Oahu’s North Shore on Thursday, surf rose quickly, reaching 8 to 12 feet. Lifeguards issued 1,115 warnings and carried out nine rescues, Emergency Services spokes­woman Shayne Enright said by email.

Lifeguards used their rescue watercraft in six of those rescues. Three visitors who got caught in the shorebreak at Wai­mea Bay were also rescued.

Box jellyfish also arrived on south shores Thursday, with 190 counted at Ala MoanaBeach and 100 at Waikiki, the city Ocean Safety Division reported.

The waves may also make navigating exposed harbor channels dangerous.

The surf was expected to build Thursday night and peak Friday before declining to advisory levels Saturday.

Surf this big is unusual for the spring. It usually hits the islands during the winter.

A storm with hurricane-force winds earlier this week generated the surf. Another storm near Tokyo, with gale-force winds, is expected to bring more large surf — half the size of the current swell — to north and west shores Sunday afternoon, which will diminish to advisory levels Monday.

There was decent-size surf on all shores Thursday. A declining south swell brought 3- to 5-foot waves to south shores Thursday but will drop to 2 to 4 feet today. East shores are expecting a tradewind swell of 2 to 4 feet today.