The northern shores of Oahu, Kauai, Niihau, Molokai and Maui are under a high surf warning from 6 p.m. today through 6 a.m. Wednesday, the National Weather Service said.

The warning also includes western shores of Niihau and Kauai.

Wave heights will rise to 20 to 30 feet tonight along north- and west-facing shores beginning tonight, forecasters said.

In addition, the east- and west-facing shores of Oahu and northern shores of the Big Island are under a high surf advisory, covering the same period. Wave heights are expected to rise to 12 to 18 feet, forecasters said.

The weather service warns that rip currents during the high surf can easily overpower even strong swimmers, and that coastal erosion may occur on shores exposed to the northwest swell.