The National Weather Service has issued a flash flood watch for O'ahu, Kaua'i and Ni'ihau, effective tonight at midnight and continuing through Sunday.

A front approaching from the west combined with an upper-level trough is expected to make the atmosphere unstable near Kaua'i and O'ahu tonight, the weather service said. These conditions, combined with "abundant" low-level moisture will increase the likelihood of heavy rainfall and flash flooding over the western end of the Hawaiian chain.

A flash flood watch means that flash flooding is possible but not imminent in the watch area. Residents are advised to monitor weather forecasts and be prepared to act should a flash flood warning be issued.