The National Weather Service extended a flash flood watch for Oahu, Kauai and Niihau until Wednesday afternoon as a weather system northwest of the islands still poses a threat of thunderstorms despite moving away from the islands.

Forecasters say moist air from the south and unstable weather conditions over the islands may bring heavy rain and thunderstorms through tonight. Rain over the last several days has saturated the ground, creating a threat of flooding.

A flash flood watch for Maui County, which did not get as much rain as Oahu and Kauai, has been lifted.

The state Health Department is also warning people to stay out of the water in areas affected by storm runoff. A Brown Water Advisory is in effect on waters off Kauai and from Waikane to Waimea Bay on Oahu. The public is advised to stay out of flood waters and storm water runoff due to possible overflowing cesspools, sewer  manholes, pesticides, animal fecal matter, dead animals, pathogens, chemicals, and associated flood debris. If coastal waters are turbid and brown, the health department says stay out.

Forecasters say the weather should gradually improve after tonight and tradewinds could return Thursday.