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.

The storm’s potential path also includes half a dozen nuclear power plants, pits holding coal-ash and other industrial waste, and numerous eastern hog farms that store animal waste in massive open-air lagoons.