Hurricane Florence churned yesterday toward the Eastern Seaboard as a storm of "staggering" size, forcing a million people to evacuate the coast. Many more were left to wonder where they might be safe if days of torrential rains unleash floods from the mountains to the sea.

"This one really scares me," National Hurricane Center Director Ken Graham warned.

Florence is so wide that a life-threatening storm surge was being pushed 480km ahead of its eye, swirling clouds that could deluge states from South Carolina to Ohio and Pennsylvania as it slows and then stalls over land.

"You're going to get heavy rain, catastrophic life-threatening storm surge, and also the winds," Graham said.

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There was little change in Florence's track during the 11am (EDT) forecast from the National Hurricane Center. Sustained winds were 215kmh yesterday morning, but it remains a Category 4 storm and is expected to intensify to near Category 5 status as it slows over very warm coastal waters.

Derek Mundy, left, and Kris Crouse boards up the Confederate House in preparation for Hurricane Florence in downtown Charleston, South Carolina. Photo / AP
Derek Mundy, left, and Kris Crouse boards up the Confederate House in preparation for Hurricane Florence in downtown Charleston, South Carolina. Photo / AP

The storm is currently around 640km south of Bermuda and is moving at 26kmh.

The eye of the massive storm is forecast to make landfall late Thursday or early Friday US time along a stretch of coastline already saturated by rising seas, and then meander for days. Seven-day rainfall totals are forecast to reach 25 to 50cm over much of North Carolina and Virginia, and even 75cm in some places.

Combined with high tides, the storm surge could swell as high as 12 feet.

"The water could overtake some of these barrier islands and keep on going. With time, the wind pushes the water into every nook and cranny you can think of," Graham said. "All you have to do is look up at your ceiling, and think about 4m. That, folks, is extremely life-threatening."

States of emergency were declared by Washington, D.C., Mayor Muriel Bowser for the nation's capital; requested by Virginia's governor; and approved by US President Donald Trump for North and South Carolina.

Trump says the federal government is "absolutely, totally prepared".

North Carolina Governor Roy Cooper said his state is "in the bull's-eye" and urged people to "get ready now".

The very centre of that bull's-eye may be Camp Lejeune, the sprawling Marine Corps training base, where authorities were opening emergency operation centres, staging equipment and urging families on the base to build survival kits with food and equipment needed to sustain themselves for 72 hours.

Mandatory coastal evacuations were in effect for civilians in South Carolina, North Carolina and Virginia, but the military base posted on Facebook that different chains-of-command would decide whether to release non-essential personnel, and some relatives vented fears that they wouldn't be able to evacuate in time.

Photo from October 15, 1954 shows high tides whipped in by Hurricane Hazel, shatter boats and buildings in Swansboro, North Carolina. Photo / via AP
Photo from October 15, 1954 shows high tides whipped in by Hurricane Hazel, shatter boats and buildings in Swansboro, North Carolina. Photo / via AP

Florence could hit the Carolinas harder than any hurricane since Hazel packed 209kmh winds in 1954. That Category 4 storm destroyed 15,000 buildings and killed 19 people in North Carolina. In the six decades since then, many thousands of people have moved to the coast.

Ahead of Florence's arrival, barrier islands were already seeing dangerous rip currents and seawater flowed over a state highway — the harbinger of a storm surge that could wipe out dunes and submerge entire communities.

Watches in effect yesterday forecast a storm surge of up to 4m at high tide from Cape Fear to Cape Lookout in North Carolina. A hurricane watch was in effect for Edisto Beach, South Carolina, to Virginia's southern border, with the first hurricane-force winds arriving late Thursday.

For many people, the challenge could be finding a safe refuge: If Florence slows to a crawl, it could bring torrential rains into the Appalachian mountains, causing flash floods and mudslides across a region getting lots of rain recently.

The last time the midsection of the US East Coast stared down a hurricane like Florence was in 1954 with Hurricane Hazel. Photo / via AP
The last time the midsection of the US East Coast stared down a hurricane like Florence was in 1954 with Hurricane Hazel. Photo / via AP

"This is going to produce heavy rainfall, and it may not move very fast. The threat will be inland, so I'm afraid, based on my experience at FEMA, that the public is probably not as prepared as everybody would like," said Craig Fugate, former director of the Federal Emergency Management Agency.

Florence's projected path 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. Duke Energy spokesman Ryan Mosier said operators would begin shutting down nuclear plants at least two hours before hurricane-force winds arrive.

All signs pointed to a stronger, slower, wider and wetter hurricane in the days ahead, forecasters said.

A warm ocean gives hurricanes their fuel, and Florence is moving over an area with water temperatures nearing 30C, hurricane specialist Eric Blake wrote. With little wind shear to pull the storm apart, hurricane-strength winds have been expanding to 64km from the eye of the storm, and tropical-storm-force winds 240km from the centre. Information gathered Tuesday by a hurricane-hunting aircraft suggests it will intensify again as it nears the coast, approaching the 253 kmh threshold for a worst-case Category 5 scenario.

This image provided by NOAA shows Hurricane Florence, left, yesterday, as it threatens the US East Coast. At right is Hurricane Helene, and second from right is Tropical Storm Isaac. Photo / via AP
This image provided by NOAA shows Hurricane Florence, left, yesterday, as it threatens the US East Coast. At right is Hurricane Helene, and second from right is Tropical Storm Isaac. Photo / via AP

Two other storms were spinning in the Atlantic as the 2018 hurricane season reaches its peak. Isaac became a tropical storm again approaching the Caribbean, while Hurricane Helene was veering northward, no threat to land.

In the Pacific, Olivia became a tropical storm again, on a path to hit the Hawaiian islands early Wednesday.

Airlines, including American, Southwest, Delta and JetBlue, have begun letting affected passengers change travel plans without the usual fees.

Liz Browning Fox was planning to ride out the storm on the Outer Banks, defying evacuation orders. She said it was built in 2009 in Buxton, North Carolina, up on a ridge, and made to withstand a hurricane. But even the most secure homes could be surrounded by water, or penetrated by wind-launched debris.

"You never know, there could be tree missiles coming from any direction," she said. "There is no way to be completely safe," but going inland might not be much safer. "I don't know where to go from here," she said.

- AP