Windy, wet weather is expected through Friday, with heavier showers, even thunder and lightning, possible this weekend. Snow could also fall on Big Island summits.

The National Weather Service says unstable weather conditions are moving over Hawaii and will linger into next week.

“Wet weather is likely through the weekend with flooding rains possible,” forecasters said.

A low pressure system south of the state is moving north and will shift the direction of the winds starting Friday. The blustery tradewinds will turn into light, muggy Kona winds by this weekend.

A high-surf advisory remains in effect until at least 6 p.m. Friday for the east shores of the main Hawaiian Islands. “Rough surf” of 6 to 10 feet is expected, the National Weather Service said.

A small craft advisory also is in effect for Hawaii waters until at least 6 p.m. today, with east to northeast winds topping 30 mph and seas of 9 to 12 feet.

A winter storm watch remains in effect for the Hawaii island summits from this afternoon through Friday afternoon, with 6 inches of snow possible, the weather service said. The island will also be under a flash-flood watch during the same period.

The heavy moisture in the air combined with cold, unstable conditions in the upper atmosphere could also bring thunderstorms this weekend, said John Bravender, a weather service meteorologist. Forecasters expect a 50 percent to 70 percent chance of rain through Saturday and a nearly 100 percent chance of rain Sunday in Honolulu.