High surf is expected to affect all eastern shores of the Hawaiian Islands as a result of a weakening Hurricane Olaf.

National Weather Service meteorologist Tony Reynes said Tuesday night that the Category 4 hurricane’s wind speed had weakened to 145 mph from 150 mph, and Olaf was expected to move head north this morning.

The National Weather Service said that on the Big Island a 24-hour high-surf advisory for east-facing shores is in effect starting at 6 a.m. today.

Olaf was 995 miles southeast of Hilo and moving west-northwest at 10 mph late Tuesday afternoon, according to a National Weather Service report. Forecasters expect it to begin moving northeast over the weekend.

High surf is expected to affect the Big Island’s and Maui County’s east-facing shores, with wave heights rising today and hitting warning levels of 15 to 20 feet Thursday, according to the Hawaii Emergency Management Agency.

On Oahu and Kauai shores, surf heights are expected to rise Thursday and could reach advisory levels of 10 to 15 feet starting Friday.

Small-craft advisories remain in effect until at 6 a.m. Thursday on the Big Island’s leeward and southeast waters and channels between Maui and the Big Island as well as between Maui and Molokai.

Olaf is also forecast to take a more northerly path starting Wednesday and, as it moves into cooler waters to the north, should begin to weaken.

Olaf is the 15th tropical cyclone in the Central Pacific in 2015, far surpassing the previous record of 11 in 1992 and 1994. It is also the eighth hurricane in the Central Pacific region this season, topping the previous record of five in 1994.