A WEATHER show in the US has aired a terrifying simulation of what it would look like if you were caught up in Hurricane Florence, as the storm prepares to hit the North Carolina coast

The Weather Channel in Atlanta, Georgia, showed viewers the possible impact the hurricane could have when it lashes the East Coast.

The graphic showed how high floods are expected to rise, with a clever green screen simulation placing reporter Erika Navarro in the middle of the storm.

Twitter users were blown away by the realistic simulation, referring to it as “amazing”, “effective” and “terrifying”.

Meanwhile, cities in the US are bracing for impact as the outer bands of wind and rain from a weakened but still lethal storm begins lashing North Carolina.

The monster hurricane moved in for a prolonged drenching along the Southeast coast overnight.

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 — 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.”

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 (Friday evening AEST) 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.

Yesterday, water flowed through streets and between beachfront homes on North Carolina’s Outer Banks, and some of the few people still left took photos of angry waves topped with white froth.

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 Harvey in 2017, which included catastrophic inland flooding that swamped homes, businesses, farms and industrial sites.

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.

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.

Scientists said it was 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.

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.

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.”