Hurricane Iselle is 245 miles east of Hilo and is expected to make landfall on Thursday afternoon.
Hawaiian officials have urged residents to stock up on emergency supplies in the remote US state.
Another Pacific storm, Hurricane Julio, strengthened overnight but is too far out to determine whether it will hit the islands in the next few days.
Julio is now a category two storm, with maximum sustained winds of 105mph, as of Thursday morning.
The last cyclone to hit Hawaii, Hurricane Iniki in 1992, killed six and caused $2.4 billion in damage.
Hawaii County, the state’s largest island, is under a hurricane warning. The rest of the state remains on a tropical storm warning.
Iselle had maximum sustained winds of 80mph as of early Thursday morning.
Five to eight inches of rain are forecasted, along with heavy winds and potential flash flooding in some parts of the state.
Wave swells of up to 15ft to 25ft were expected on the Big Island, the Honolulu Star-Advertiser reported.
National Weather Service meteorologist Mike Cantin said the size and terrain of the Big Island would help weaken Iselle into a tropical storm as it passes Maui and Oahu.
The state’s department of emergency management has asked residents to prepare a seven-day emergency kit, including non-perishable food and water.
Hawaii’s remoteness from the mainland makes it hard to get emergency supplies to the state. Stores have had to quickly restock bottled water and other supplies.
Some tourists did not let the threat keep them from enjoying their vacation.
An inter-island airline has cancelled flights on Thursday between Maui and Lanai islands and all flights on Friday.
Schools on the Big Island, Maui, Molokai and Lanai will be closed on Thursday.
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