WASHINGTON, Sept. 12 - Hurricane Florence on Wednesday morning was forecast to slow down but remains "incredibly dangerous" and will hammer large swathes along the US East Coast for days.

As of Wednesday morning, Florence was centered more than 500 miles (about 805 km) southeast of Cape Fear, North Carolina, moving west-northwestward, according to the US National Weather Service (NWS).

Meteorologists now project the center of Florence to make landfall around the South and North Carolina border early morning Friday between midnight and 5 a.m. local time (0900 GMT) as a Category 3 or 4 hurricane.

However, strong winds are expected as early as Thursday morning along the coasts of the Carolinas, said the NWS, warning Wednesday is the last day for preparedness.

"#HurricaneFlorence is very large and incredibly dangerous," the service tweeted on Wednesday, "Follow local evacuation orders!"

"Prepare for life-threatening, catastrophic flooding over portions of the Carolinas and Mid-Atlantic states late this week into early next week," it warned.

"This will likely be the storm of a lifetime for portions of the Carolina coast," an NWS spokesperson said Tuesday night. "I can't emphasize enough the potential for unbelievable damage from wind, storm surge, and inland flooding with this storm."

Federal Emergency Management Agency administrator Brock Long warned on Tuesday that power in parts of the Carolinas could be knocked out for weeks.

Florence has the potential to become the most intense and damaging storm to strike the US East Coast in at least 25 years since Hurricane Hugo.