The outer bands of Hurricane Florence, a large and dangerous Category 2 storm and one of 9 deadly storms circling the globe, landed on the North Carolina coast Thursday morning (local time).

Sea water was already surging ashore along the Outer Banks, washing over roads. It marks the beginning of a prolonged assault from wind and water, which - by the time it's over - is likely to bring devastating damage and flooding to millions of people in the Southeast.

Conditions will deteriorate through Thursday (local time): Starting along the coast, winds will accelerate, the rain will intensify, and the ocean will surge ashore.

The storm's centre is expected to make landfall Friday (local time) in southeast North Carolina, which will coincide with the most severe effects. Storm surge, the rise in seawater above normally dry land at the coast, could rise a story high. On top of that, a disastrous amount of rain - 500mm, possibly even as much as 1m - is expected to fall.

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Flooding from both the storm surge and rainfall could be "catastrophic," the National Hurricane Center warned.

This same zone will be hammered by winds gusting up to hurricane force for nearly a day while tropical-storm conditions could linger twice that long. These unforgiving winds will damage homes and buildings, down trees and knock out power.

Even though the storm's category fell from a 4 to a 2 Wednesday (local time), forecasters stressed the category is only an evaluation of the storm's peak winds in a very narrow core near the center of the storm. The storm's size and area affected by hazardous winds have actually expanded, and the threat from storm and rain-induced flooding "have not changed" tweeted Rick Knabb, The Weather Channel's tropical weather expert and former Hurricane Center director.

"Put simply, Florence is a 'Category 5 #flood threat'," tweeted The Weather Channel."

Gradually, Friday through the weekend (local time), the massive storm - containing a zone of tropical-storm-force winds nearly 643km wide - will drift inland, engulfing much of South Carolina and southern North Carolina. Widespread rainfall amounts could reach 152mm to 300mm, spurring flooding. Some of the storm's wind and rain could even creep into eastern Georgia.

Enough rain could fall to break North Carolina's record for a tropical storm - 24 inches - set near Wilmington during Hurricane Floyd in 1999, said Greg Carbin, chief of forecast operations at the Weather Service's national prediction center.

Flooding from heavy rains is the second-leading cause of fatalities in tropical storms and hurricanes that make landfall.

The rain threat may not stop in the Carolinas. By early next week, a weakened but soggy Florence may drop rain on already saturated Virginia, West Virginia, Maryland, Washington and Pennsylvania. These areas are vulnerable to flooding and downed trees after heavy rains this summer.

Weather

The Hurricane Center reported heavy rain bands with tropical-storm-force winds had arrived in the North Carolina Outer Banks.

A weather station on Cape Lookout, N.C. recently clocked a sustained wind of 88km/h and gust to 112km/h.

A social media video showed the seas rising at Hatteras Inlet on the Outer Banks.

As of 1 p.m. Thursday, Florence's top winds were 105 mph, and it was marching northwest at 16km/h, about 185kms east-southeast of Wilmington, N.C., and 281kms east-southeast of Myrtle Beach, South Carolina.

This wind speed fell slightly from the previous update, but the Hurricane Center said the storm's presentation on radar and satellite had "improved" and that slight strengthening was possible as the storm moved over some warmer waters through this evening.

The storm's forward motion has slowed 11km/h since Wednesday (local time), which was predicted as the storm nears the coast. The slower motion will prolong effects from wind, rain and surge in the eastern Carolinas.

The Hurricane Center predicts the storm to maintain this strong Category 2 intensity until landfall, after which wind speeds will steadily decline.

Even as Florence's peak winds decreased Wednesday, the storm's wind field grew, the Hurricane Center said. Hurricane-force winds extend 128kms from the center, while tropical-storm-force winds extend 313kms outward. The storm's cloud field is about the size of four Ohios.

Hurricane warnings are in effect for the South Santee River in South Carolina to Duck, North Carolina, and Albemarle and Pamlico sounds. This includes Wilmington. A hurricane watch extends into the Charleston area. A tropical storm warning covers the area from north of Duck to the Virgina Tidewater area.

Because landfalling hurricanes commonly spawn twisters, a tornado watch was issued for eastern North Carolina through 9 p.m (local time).

More than 10 million people are under watches and warnings, the Associated Press reported.

Storm surge

Like a bulldozer, the storm's winds and forward motion will push a tremendous amount of water onshore when it makes landfall. The storm surge could reach up to more than 3.9m, if the maximum surge coincides with high tide.

The biggest surge should occur just to the north of where the eye of the storm comes ashore, which the Hurricane Center projects in southeastern North Carolina.

The surge will result in "large areas of deep inundation . . . enhanced by battering waves," the Weather Service said. It warned of likely "structural damage to buildings . . . with several potentially washing away," "flooded or washed-out coastal roads" and "major damage to marinas."

Storm surge warnings were issued from South Santee River in South Carolina to Duck, North Carolina. The Charleston area is under a storm surge watch.

The Hurricane Center projects the following surge heights above normally dry land, if the maximum surge coincides with high tide:

Rain

Models agree that excessive amounts of rain will fall in southeastern North Carolina.

"Floodwaters may enter numerous structures, and some may become uninhabitable or washed away," the Weather Service warned.

Where exactly the zone of heaviest rain sets up as the storm meanders inland is more uncertain, but models suggest that it may concentrate in southern North Carolina and northern South Carolina through the weekend.

Hurricane Florence churns over the Atlantic Ocean heading for the U.S. east coast as seen from the International Space Station. Photo / AP
Hurricane Florence churns over the Atlantic Ocean heading for the U.S. east coast as seen from the International Space Station. Photo / AP

It has become likely that the storm will reverse course early next week and turn back north toward West Virginia, Virginia, Maryland and Pennsylvania, albeit significantly weakened.

Wind

The strongest winds will occur where and when the storm makes landfall in a ring around the calm eye of the storm known as the eyewall. If the storm makes landfall as a Category 2, these winds will be damaging, sustained at up to 160km/h or so with higher gusts.

The zone where these intense winds occur will be narrow and they will last just a few hours, but the effects will probably be severe, similar to a tornado. The Hurricane Center describes the types of damage associated with Category 2 winds:

"Well-constructed frame homes could sustain major roof and siding damage. Many shallowly rooted trees will be snapped or uprooted and block numerous roads. Near-total power loss is expected with outages that could last from several days to weeks."

Outside this zone of destructive winds, damaging winds are still likely, even some distance inland from the coast, which would lead to minor structural damage, downed trees and widespread power outages.

A power outage model run at the University of Michigan projects that 3.2 million customers will be without electricity because of the storm, mostly in the eastern half of North Carolina.

Because the storm will slow it moves over the eastern Carolinas, these wind impacts will be magnified.

Path projections

While it is extremely likely that the eastern Carolinas will be hardest hit by the storm Thursday into Friday (local time), the storm's direction becomes far less certain over the weekend and next week.

Models agree the storm should strike land between the North Carolina-South Carolina border and the North Carolina Outer Banks, and then track across South Carolina. But then they gradually diverge. While all simulations show the storm turning back to the north Sunday or Monday (local time), exactly where that turns occurs is a big wild card.

The storm could track north through the Ohio Valley, the Appalachians or even closer to the Interstate 95 corridor. The specifics of the track early next week will have implications for where the heaviest rainfall occurs north of the Carolinas.