HILO: Heavy rains hit parts of Hawaii and strong waves pummeled shorelines as a downgraded yet potent Pacific storm passed near the island state.

Though Tropical Storm Madeline was no longer a hurricane, the weather's uncertainty couldn't let Hawaii's Big Island relax.

There were periods of intense rainfall Wednesday as the National Weather Service downgraded Madeline, with winds falling below hurricane strength of 74 mph.

Wind speed diminished steadily throughout the day and by 11 pm (2 am PDT Thursday), they were swirling at 50 mph. Forecasters said continued weakening was expected.

Madeline's center was about 200 miles south of Hilo and moving west-southwest and away from the state at 14 mph. Though the storm was not predicted to make landfall on any Hawaiian island, the Big Island and Maui County remained under tropical storm warnings because of threats from wind and rain.

Strong winds were gusting on Oahu as a series of bright blue flashes lit up the night sky above Honolulu. The power went off in the hillside neighbourhood but the lights in most of downtown Honolulu remained on. There was no rain accompanying the blustery weather and only scattered clouds whizzed by overhead.

On the Big Island, waves crashed into a seawall that surrounds Liliuokalani Gardens Park at Hilo Bay. Water accumulated on the grass of the gardens, leaving stairs of a pavilion partially submerged.

"That heavy rainfall is interspersed with sunny patches," said Kanani Aton, spokeswoman for Hawaii County Civil Defense.

Officials said residents should continue to be prepared for more rain, strong winds and high surf overnight. The rainfall may lead to dangerous flash floods and mudslides, the weather service warned.

"It doesn't matter if it's a strong tropical storm or a category 1 hurricane," said Eric Lau, a meteorologist with the weather service. "If you have 70 mph winds versus 75 mph winds, it's still a strong storm, so residents still need to be prepared."

Meanwhile, Hurricane Lester was about 1,000 miles from Hawaii and expected to drop to a tropical storm by Sunday.

Earlier on Wednesday, merchants boarded up shop windows along Hilo Bay and shoppers snatched supplies of food and water from grocery store shelves after initially being told the island could be hit by its first hurricane in a quarter-century.

"We are not out of the woods," Aton said. "At this point it is still a powerful storm, and we are working to remind the public to be storm ready."

Elsewhere, a tropical storm warning was issued early Thursday for a section of the US East Coast as Tropical Storm Hermine approached Florida from the Gulf of Mexico.

The warning covered an area that extends from Marineland, Florida, northward to the South Santee River in South Carolina. A hurricane warning was already in effect for a section of Florida's Gulf coast from the Suwanne River to Mexico Beach.

The US National Hurricane Center said Hermine was expected to become a hurricane by the time it makes landfall on Florida's coast Thursday night or early Friday. As of 5 am EDT ( 2 am PDT) Thursday, Hermine was centered about 275 miles west-southwest of Tampa, Florida, and is moving north-northeast near 12 mph.

In Hawaii, Peggy Beckett, a retiree and beekeeper, stopped at a Hilo supermarket to pick up onion bagels, cheese, cold cuts and salad to add to her canned food at home. She also has a cooler with ice plus a portable burner and batteries to get her through the storm.

Noting the lines of people at the market, Beckett said people were getting prepared but weren't panicking.

"There's always a lot of disbelief on the island that the storms will really be as big and bad as forecast," she said, noting that she and her partner had taken precautions to protect their beehives.

Employees boarded up windows at Hulakai Store, a surf shop in Hilo. "We'll probably keep it up till Sunday, waiting for the second one to come through," said supervisor Renee Balanga.

Gov. David Ige has issued an emergency proclamation for both storms, allowing the state to quickly spend money.

The Hawaiian islands of Maui, Molokai, Lanai and Kahoolawe were under a tropical storm watch, but there were no alerts for Oahu or Kauai.