Two days after a 22-year-old surfer was put in critical condition by a near-drowning incident at Banzai Pipeline, a new large northwest swell is expected to generate surf of 20 to 30 feet along the north shores of five islands.

West shores will see waves with faces of 12 to 20 feet, according to forecasters.

In response to the latest swell, the National Weather Service has issued a high-surf warning for north and west shores of Niihau, Kauai, Oahu and Molokai, effective from 9 a.m. today to 6 p.m. Wednesday. The warning also covers the north shore of Maui.

On Monday large surf from an earlier swell kept Oahu lifeguards busy with four rescues and nearly 800 warnings and other preventive actions on the North Shore and two rescues and more than 300 preventive actions on west shores. The weather service said surf was 6 to 8 feet Monday on the North Shore.

“This surf is extremely dangerous, and only the most experienced surfers with the correct equipment should venture out,” said Shayne Enright, a spokeswoman with the Ocean Safety Division of the Honolulu Emergency Services Department.

On Sunday at about 11:45 a.m., lifeguards at Ehukai Beach responded to a surfer unconscious in the water at the Banzai Pipeline.

The surfer, identified as a 22-year-old man, caught a wave but didn’t resurface, Enright said.

A bodysurfer reached him and held onto him until lifeguards responded from shore.

He regained consciousness, and paramedics took him in critical condition to a hospital.