The National Weather Service says potentially heavy rains, even thunderstorms, are heading for the islands. Forecasters issued a flash food watch for Oahu,  Kauai and Niihau starting at 6 p.m. Tuesday through 6 p.m. Wednesday.

Forecasters expect widespread heavy rains because of moisture in the atmosphere and a passing weather system that will create unstable weather conditions.

“The threat of flooding is great enough to warrant a flash flood watch,” the weather service said in an advisory. “However some uncertainty remains as to the timing and placement of the heavy rain.”

This afternoon’s forecast calls for the weather system to pass over Kauai and Oahu late Tuesday night.

“Intense rain fall rates are possible, especially over the slopes, and will likely last sufficiently long to produce flash flooding of streams” the National Weather service said.

The rains Tuesday and Wednesday over Oahu and Kauai could bring enhanced trade wind showers to Maui and Hawaii island Thursday, forecasters said.

There may be a short break in the rains Wednesday and Thursday on Oahu. But another weather system is approaching and could bring more rain this weekend.

The forecast for Honolulu calls for  the sunny skies this morning to turn partly cloudy tonight. Partly sunny skies are expectedTuesday with the chance of scattered or isolated showers.

The skies should become cloudy Tuesday evening through Wednesday night. There’s a 90 percent chance of rain Wednesday, with possible thunderstorms and locally heavy rain.

The skies should clear Thursday, Friday and Saturday. But rain is near 100 percent likely Saturday night, forecasters said.