Florence was expected to build into potentially catastrophic Category 5 hurricane today as it closed in on North and South Carolina.

It exploded into a Category 4 storm yesterday, carrying winds up to 220km/h and water that could wreak havoc over a wide stretch of the eastern United States this week.

Communities along a stretch of coastline that's vulnerable to rising sea levels due to climate change were told to evacuate ahead of the storm. The South Carolina governor ordered the state's entire coastline to be evacuated by today and predicted that 1 million people would flee. North Carolina and Virginia yesterday declared states of emergency, while Virginia's governor ordered a mandatory evacuation for some residents of low-lying coastal areas.

South Carolina residents have been stocking up on supplies ahead of Florence's arrival. Photo / AP
South Carolina residents have been stocking up on supplies ahead of Florence's arrival. Photo / AP

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.

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

Airlines have allowed passengers to change travel plans that take them into the hurricane's possible path.

National Hurricane Center Director Ken Graham warned that Florence was forecast to linger over the Carolinas once it reaches shore. People living well inland should prepare to lose power and endure flooding and other hazards, he warned.

A warm ocean is the fuel that powers hurricanes, and Florence will be moving over waters where temperatures are peaking near 30C, hurricane specialist Eric Blake wrote. And with little wind shear to pull the storm apart, Florence's hurricane wind field was expected to expand over the coming days, increasing its storm surge and inland wind threats.

Two other storms were spinning in the Atlantic. Hurricane Isaac was expected to lose strength as it reaches the Caribbean, and Helene, much farther out to sea, may veer northward into the open ocean as the 2018 hurricane season reaches its peak.

In the Pacific, Hurricane Olivia triggered warnings for multiple Hawaiian islands.

- AP