RODANTHE, N.C. — Arthur strengthened to a hurricane early this morning and threatened to give North Carolina a glancing blow on Independence Day, prompting the governor to warn vacationers along the coast not to risk their safety by trying to salvage their picnics and barbecues.

Forecasters expect Arthur to whip past the state’s Outer Banks on Friday without making landfall. One local remarked that he was more worried about his tomato plants than storm damage.

But North Carolina Gov. Pat McCrory warned: “Don’t put your stupid hat on.”

The first named storm of the Atlantic hurricane season prompted a hurricane warning for much of the North Carolina coast and a mandatory evacuation for visitors to the Outer Banks’ Hatteras Island as of 5 a.m. today. Residents also were advised to leave the island. A voluntary evacuation was announced for the Outer Banks’ Ocracoke Island, accessible only by ferry.

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.