Oahu could see heavy showers, possible thunderstorms and flooding tonight as the National Weather Service expanded a flash flood watch to include Oahu.

The flash flood watch is in effect through late tonight.

A flash flood watch means conditions may develop that could lead to flash flooding. A small craft advisory is also in effect until 6 a.m. Monday.

Forecasters say a weather system west of the islands is bringing moisture over Oahu and Kauai and making the air unstable. Thunderstorms associated with the storm could bring locally heavy rain overnight.

The system is forecast to weaken and move away from the islands over the next several days.