Active weather that could bring strong winds and hail could arrive in the islands Friday due to an “unusually strong upper level disturbance moving over the state,” the National Weather Service said in a special weather statement late this afternoon.

According to the statement, the islands could see “strong to severe thunderstorms, which could produce damaging winds and large hail” this weekend, with the thunderstorms most likely to occur through Friday evening.

The statement accompanies a statewide flood watch, effective until 6 a.m. Saturday, caused by excessive rainfall.

Forecasters say the strong, upper-level disturbance will bring scattered, heavy showers and isolated thunderstorms to the isles 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.

“We are not looking at a widespread rain event. Rather, we are looking at a scattered heavy shower event with isolated strong to possibly severe thunderstorms,” NWS said in an area forecast discussion this afternoon. “Due to the scattered nature of the event, sudden changes of the weather should be expected. It could be sunny and fair weather one hour and you could be under a severe thunderstorm the next hour.”

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

In the special weather statement, NWS advised listening for later statements, watches and warnings “as this event becomes more clear.”

NWS also said 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 also can 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.

Conditions are expected to stabilize over the weekend, but “conditions will likely be more wet than normal especially during the afternoons.”

A high surf advisory is also in place for the south-facing shores 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.