The outer bands of wind and rain from a weakened but still lethal Hurricane Florence began lashing North Carolina on Thursday as the monster storm moved in for a prolonged and potentially catastrophic drenching along the Southeast coast.

Florence’s winds had dropped from a peak of 225km/h to 165km/h by midmorning, reducing the hurricane from a terrifying Category 4 to a 2. But forecasters warned that the widening storm — and its likelihood of lingering around the coast day after day — will bring seawater surging onto land and torrential downpours.

“It truly is really about the whole size of this storm,” National Hurricane Centre Director Ken Graham said. “The larger and the slower the storm is, the greater the threat and the impact — and we have that.”

An onlooker checks out the heavy surf at the Avalon Fishing Pier in Kill Devil Hills, North Carolina. Picture: AP

As of 11am local time (1am AEST), Florence was centred about 230km southeast of Wilmington, its forward movement slowed to 17km/h.

Hurricane-force winds extended 130km from its centre, and tropical-storm-force winds up to 315km.

Forecasters said Florence’s eye could come ashore early on Friday around the North Carolina-South Carolina line. Then it is likely to hover along the coast on Saturday, pushing up to nearly four metres of storm surge and unloading water on both states.

By midday, Spanish moss blew sideways in the trees as the winds increased in Wilmington. On North Carolina’s Outer Banks, water flowed through streets and between beachfront homes, and some of the few people still left in Nags Head took photos of angry waves topped with white froth.

Members of the Missouri Search and Rescue, part of FEMA, hold a team briefing at a staging area as Hurricane Florence started to make landfall. Picture: AFP

The forecast calls for as much as 102cm of rain over seven days along the coast, with the deluge continuing even as the centre of the storm pushes its way over the Appalachian Mountains.

The result could be what the Houston area saw during Hurricane Harveyin 2017: catastrophic inland flooding that could swamp homes, businesses, farms and industrial sites.

Sandbags surround homes on North Topsail Beach, North Carolina. Picture: AP

The police chief of a barrier island in Florence’s bullseye said he was asking for next-of-kin contact information from the few residents who refused to leave. “I’m not going to put our personnel in harm’s way, especially for people that we’ve already told to evacuate,” Wrightsville Beach Police Chief Dan House said.

North Carolina Gov. Roy Cooper urged residents to remain alert despite changing forecasts.

“Don’t relax, don’t get complacent. Stay on guard. This is a powerful storm that can kill. Today the threat becomes a reality,” he said.

A church is seen under storm clouds as Hurricane Florence nears. Picture: AFP

About 5.25 million people live in areas under hurricane warnings or watches, and 4.9 million in places covered by tropical storm warnings or watches, the National Weather Service said.

Weather Underground meteorology director Jeff Masters said Florence eventually could strike as a Category 1 with winds less than 160km/h, but that’s still enough to cause at least $US1 billion ($A1.4 billion) in damage. Water kills more people in hurricanes than wind does.

A South Carolina high school is used as a shelter. Picture: AFP

Scientists said it is too soon to say what role, if any, global warming played in the storm. But previous research has shown that the strongest hurricanes are getting wetter, more intense and intensifying faster because of human-caused climate change.

It’s unclear exactly how many people fled ahead of the storm, but more than 1.7 million people in the Carolinas and Virginia were warned to clear out. Airlines cancelled about 1200 flights and counting, and some airports in the Carolinas virtually shut down.

Locals gets some kiteboarding in before Florence hits. Picture: AP

Major US hardware stores Home Depot and Lowe’s activated emergency response centres and sent in around 1100 trucks to get generators, garbage bags and bottled water to stores before and after the storm.

Duke Energy, the nation’s No. 2 power company, said Florence could knock out electricity to three-quarters of its four million customers in the Carolinas, and outages could last for weeks. Workers are being brought in from the Midwest and Florida to help in the storm’s aftermath, it said.

Vickie Grate, left, waits in a shelter with her son Chris, centre, and his girlfriend Sarah. Picture: AP

Florence’s weakening as it neared the coast created tension between some who left home and authorities who worried that the storm could still be deadly. Frustrated after evacuating his beach home for a storm that has since been downgraded, retired nurse Frederick Fisher grumbled in the lobby of a hotel in Wilmington several kilometres inland.

“Against my better judgment, due to emotionalism, I evacuated,” he said. “I’ve got four cats inside the house. If I can’t get back in a week, after awhile they might turn on each other or trash the place.”