The National Weather Service is keeping an eye on ocean conditions near Japan that could generate very large surf in Hawai'i by mid-week next week.

A storm has not developed yet, but conditions exist for a large storm to develop in the western Pacific that would send a northwest swell to Hawai'i, lead forecaster Derek Wroe said.

Such a swell could reach open-ocean heights of 20 feet and create surf reaching warning levels of 25 feet or more by the time it reaches the Islands, Wroe said.

"That's the kind of swell we see once a winter," he said.

Weather forecasters will have a much better idea tomorrow of whether an intense storm will be generated in the western Pacific, he said. If a large swell is generated, it is expected to reach Hawai'i Tuesday night and peak on Wednesday.

Surf for today is expected to be 4 feet to 6 feet along north-facing shores, and 2 feet to 4 feet along east- and west-facing shores. A new advisory-level northwest swell is expected to arrive on Sunday, peak early Monday, then slowly diminish through Tuesday.

A cold front northwest of the state is expected to bring heavy rain and possible thunderstorms beginning late Saturday afternoon and into Sunday. Winds will be from the south on Saturday, shifting to the west on Sunday.