After reasonably high, though mostly rough-riding North Shore waves on Thanksgiving, and some smaller, slightly friendlier waves yesterday, the National Weather Service was prepared to issue a high surf advisory late this afternoon or early tomorrow.

"There's a swell coming in," said Tim Craig, NWS lead forecaster. "And it's going to be generated by a low that is actually pretty close to the north of us. It will arrive Saturday night and will peak to advisory level on Sunday."

Craig said the surf could approach the 15-foot advisory level on north facing shores, and could reach the 18-foot face range. Waves should reach the 8-foot advisory level along east facing shores.

"A north swell as opposed to the northwest swells that we usually see will affect not only the north shores of the Islands but some of the beaches on the east side," he said.

Craig also said because this particular swell "will be coming straight down, almost exactly from the north," it could cause problems on north facing harbors on all the islands.

He also said the sets would be coming in more frequently. That could present problems for surfers. "When the swells do come in, they'll have a period of about 9 to 13 seconds as opposed to the 13 to 19. The surf may not be huge. But because the sets will be coming in relatively quick succession, if a surfer gets into trouble, they don't have enough time to recover."

NWS forecaster Robert Ballard said the low pressure system is causing unstable conditions throughout the area, making it difficult to predict what sort of surfing conditions will exist in the Islands through the remainder of the week.

Reach Will Hoover at whoover@honoluluadvertiser.com.