The named storm Arthur has strengthened to a hurricane in the Atlantic and is threatening to disturb Independence Day celebrations in parts of North Carolina.
The hurricane's maximum sustained winds are near 75 mph (120 kph) with some additional strengthening expected.
Hurricane warnings on the coast of North Carolina have been extended and now cover an area from Surf City to the Virginia border.
Hurricane Arthur is centred about 340 miles (545 kilometers) southwest of Cape Hatteras, North Carolina, and is moving north near 9 mph (15 kph).
The first named storm of the Atlantic hurricane season has prompted a mandatory evacuation for visitors to the Outer Banks' Hatteras Island. Residents have been advised to leave the island.
A voluntary evacuation was announced for the Outer Banks' Ocracoke Island, accessible only by ferry.
North Carolina Governor Pat McCrory warned residents and holidaymakers not to "put your stupid hat on".
"Although the current forecast doesn't indicate this will be a major impact, we are taking it very seriously," he said. "I don't want you to put at risk not only yourself but also people who may try to help you."
The islands are linked by North Carolina Route 12, which has been sliced apart twice in recent years as storms cut temporary channels from the ocean to the sound. Hatteras Island is particularly vulnerable to storm surge and flooding and the road is easily blocked by sand and water.
In addition to the hurricane warning, tropical storm warnings were in effect for coastal areas in South Carolina and Virginia.
The annual Boston Pops Fourth of July concert and fireworks show has been moved up a day because of potential heavy rain ahead of Hurricane Arthur. Organisers and public safety officials said the celebration was being rescheduled for Thursday, which appeared to be the best of two potential bad weather days.
The National Hurricane Centre predicts Arthur will swipe the coast early on Friday with winds of up to 85 mph. The storm would be off the coast of New England later on Friday and eventually make landfall in Canada's maritime provinces as a tropical storm, it predicted.