RALEIGH, N.C. — Florence exploded into a potentially catastrophic Category 4 hurricane Monday as it closed in on North and South Carolina, carrying winds up to 130 mph and water that could wreak havoc over a wide stretch of the eastern United States later this week.

The South Carolina governor ordered the state’s entire coastline to be evacuated starting at noon Tuesday and predicted that 1 million people would flee.

The storm’s first effects were already being seen on barrier islands as dangerous rip currents hit beaches and seawater flowed over a state highway. Communities along a stretch of coastline that is vulnerable to rising sea levels due to climate change prepared to evacuate.

For many people, the challenge could be finding a safe refuge: If Florence slows to a crawl just off the coast, it could bring torrential rains to the Appalachian mountains and as far away as West Virginia, causing flash floods, mudslides and other dangerous conditions