The National Weather Service has issued a flood watch for all main isles of Hawaii, effective until 6 a.m. Saturday, ahead of a storm expected to arrive tonight.

Forecasters say a strong, upper-level disturbance will bring scattered, heavy showers and isolated thunderstorms to the isles tonight through Friday night, bringing the threat of flash flooding.

“The heaviest downpours will be hit or miss, but where they do occur, they will be capable of producing rainfall rates in excess of 3 inches per hour,” said NWS in the watch. “The threat for flash flooding is expected to be the greatest Friday through Friday evening. Conditions should steadily improve from west to east late Friday into Saturday.”

Forecasters say this strong, upper-level disturbance is unusual for this time of the year, and anticipate it will begin to move in tonight, bringing the threat of flash flooding and isolated, severe thunderstorms aross the state.

In a separate special weather statement, NWS said the latest guidance “indicates an increasing potential for strong to severe thunderstorms, which could produce damaging winds and large hail, as well as the possibility for localized intense rainfall and flash flooding.”

NWS said the main threat initially will be over the smaller isles of Kauai, Oahu and Maui County, which will then expand eastward to Hawaii island on Friday.

Windward and mauka areas are favored for rainfall, especially along Oahu’s Koolaus.

A flood watch means one should be prepared because conditions are favorable for flooding.

NWS advises monitoring later forecasts, and to be prepared to take action should flash flood warnings be issued, particularly for those in flood-prone areas.

NWS also reminds the public that flash flooding can also occur in locations where it is not raining, and can occur well downstream of heavy rains.

“If you notice signs of heavy rain or thunderstorms upslope, move quickly away from flood prone areas,” said NWS.

A high surf advisory is also in place for south-facing shore of all isles through 6 p.m. Friday due to a long-lived south swell.

Forecasters said large, breaking waves of 7 to 10 feet are expected along south-facing shores through Friday evening, and that strong currents will make swimming dangerous.

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The National Weather Service updated a “special weather statement” overnight, warning of the potential for severe thunderstorms and possible flash flooding affecting Hawaii tonight through this weekend.

Forecasters say “an unusually strong upper-level disturbance is expected to approach the islands later tonight into Saturday, bringing the possibility of very active weather to portions of the Aloha State.”

The forecasters say the latest guidance indicates a possibility for severe thunderstorms, with damaging winds, large hail, intense rainfall and flash flooding.

“Impacts could begin over the western end of the state as early as tonight, with a transition to more of a widespread potential Friday through Saturday,” said NWS in the statement. “Listen for later statements, as well as possible watches and warnings, as this event becomes more clear.”