Hurricane Lane has weakened as it approaches Hawaii but was still expected to pack a wallop, forecasters said.
The National Weather Service said tropical-storm-force winds could begin as early as today on the Big Island.
The hurricane was about 490km south of Kailua-Kona and moving northwest toward other islands.
Meteorologist Chevy Chevalier in Honolulu says winds slowed overnight from 259km/h to 250km/h, prompting a downgrade of the hurricane from a Category 5 to a Category 4.
He says it may diminish to a Category 3 by tomorrow NZT but that would still be a major hurricane.
Chevalier says that by late tomorrow, the hurricane is forecast to be a Category 2 with winds up to 177 km/h and the centre located west of Hawaii Island and south of Honolulu.
"We expect it to gradually weaken as it gets closer to the islands," Chevalier said. "That being said, on our current forecast, we still have it as a major hurricane."
With winds to 209 km/h, the hurricane could cause catastrophic damage.
Hawaii residents have rushed to stores to stock up on bottled water, ramen, toilet paper and other supplies as they faced the threat of heavy rain, flash flooding and high surf.
Public schools on the Big Island and in Maui County closed until further notice.
Hawaii Governor David Ige is allowing non-essential state employees on the Big Island and Maui to go on administrative leave. Employees on Hawaii and Maui islands who work in disaster response as well as in hospitals and prisons are required to report to their jobs, the governor said.
The hurricane was moving at 15 km/h, said meteorologist Melissa Nye.
"We are starting to see some of the rain show up on the Big Island radar," she said. Offshore buoys also had detected wave action associated with the storm.
A hurricane watch remained in effect for Oahu and other smaller islands, meaning tropical storm-force winds, excessive rain and large swells could arrive starting today.
The central Pacific gets fewer hurricanes than other regions, with about only four or five named storms a year, and Hawaii rarely gets hit. The last major storm to hit was Iniki in 1992. Others have come close in recent years.
"Winds tend to steer storms away from there," said Princeton University climate scientist Gabe Vecchi. He also said upper level winds, called shear, tend to be strong enough to tear storms apart.
Kauai resident Mike Miranda was 12 when Hurricane Iniki devastated the island 26 years ago. "A lot of people are comparing the similarities between Iniki and Lane," he said.
Iniki's turn into the islands was sudden, he recalled.
"I remember how very little rain fell. But I remember the wind being the strongest force of nature I've ever witnessed and probably the scariest sounds I've ever heard in my life," he said.
Miranda said his family is used to preparing for hurricane season.
"A lot of people who moved here and never experienced a hurricane . they're the ones rushing to the store," he said.