HONOLULU  – A powerful Pacific storm predicted to grow to hurricane force churned toward Hawaii, bringing with it heavy rains and potentially destructive winds that could begin hitting the state later Friday.

Forecasters said that the storm likely will bring 40 to 50 mph winds to the Big Island’s Kau, Puna and South Kona districts.

“Right now, we expect the main impacts to be on the Big Island,” National Weather Service meteorologist Ray Tanabe said Thursday. The weather service has issued a tropical storm watch for the island.

Tropical Storm Ana likely will become a hurricane by Friday evening but return to tropical storm strength Saturday morning, Tanabe said.

That’s also when the storm is forecast to be closest to the island  – about 85 to 90 miles offshore to the southwest.

The storm will be farther from the coast than predicted earlier, and will be a hurricane for a shorter period than previously thought, they said.

The soil in the Kau district already is heavily saturated from recent thunderstorms, raising the risk of flooding there.

The weather service issued a flash flood watch for the entire state from Friday through Sunday, indicating flooding is possible anywhere in the archipelago, said Chris Brenchley, a weather service meteorologist.

Between 5 to 10 inches of rain may fall, though locally some areas could get 20 inches or more, he said.

Ana (AH – nah) is expected to lose some power as it moves northwest along the island chain.

It could bring 40 mph to 50 mph winds to Oahu  – which is home to Honolulu, the state’s biggest city  – and Kauai. Gusts could reach up to 75 mph near the storm’s centre.

“It will weaken, but it still will be packing some pretty strong winds,” Tanabe said.

The current forecast calls for Ana to approach Oahu and Kauai more closely than the Big Island, but it will likely be a tropical storm and not a hurricane then.

Kauai Mayor Bernard Carvalho, Jr. on Thursday proclaimed an emergency to help his county respond to storm damage on Kauai and Niihau islands.

State officials and Civil Air Patrol personnel have hiked and flown to Kalalau Valley, Polihale and other remote parts of Kauai to tell campers about the approaching storm, the county said.