The National Weather Service has issued a flash flood watch for Niihau, Kauai and Oahu.

The watch was in effect for Kauai and Niihau through Sunday afternoon and scheduled to take effect Sunday morning on Oahu before expiring Sunday afternoon, the weather service said.

A moist and unstable airmass was creating the threat of heavy showers, and bands of heavy showers and thunderstorms were expected to move over Kauai and Niihau tonight and Sunday, the weather service said. Those bands will bring the potential for torrential rainfall in a short period of time.

Flash flooding will be possible in poor drainage areas, streams, and low-lying areas, the weather service said.

The heavy showers and thunderstorms were expected to move over Oahu early Sunday morning.

Meanwhile, a high surf warning for north- and west-facing shores of Oahu, Kauai, Niihau and Molokai; north-facing shores of Maui; and west-facing shores of Hawaii island remained in effect until 6 p.m. Sunday.

Surf was expected to be between 20 and 30 feet along north-facing shores of Oahu, Molokai and Maui, and north- and west-facing shores of Niihau and Kauai before gradually decreasing to between 15 and 20 feet late Sunday.

West-facing shores of Oahu and Molokai were expected to have surf between 15 and 20 feet, before decreasing to between 10 and 15 feet late Sunday.

Along west-facing shores of the Big Island, surf was expected to be 12 to 15 feet through Sunday.