Hurricane Florence is about 50 miles off the coast of North Carolina with "life-threatening" winds of 100mph already battering the state.
Catastrophic flooding from lakes, rivers and freshwater reserves is expected over parts of North and South Carolina, the National Hurrican Center (NHC) warned.
At least 150,000 people are already without power, and forecasters have warned that conditions will only get more lethal as the storm pushes ashore in the early hours of Friday morning.
The hurricane’s surge could cover large swathes of the Carolina coast under as much as 11ft (3.3m) of seawater.
Although Florence has been downgraded to a category one storm, North Carolina governor Roy Cooper warned: Don’t relax, don’t get complacent. Stay on guard. This is a powerful storm that can kill. Today the threat becomes a reality.
There are fears that this storm could cause damage similar to what Houston suffered during Hurricane Harvey last year, when homes and businesses were inundated with floodwater.
Ken Graham, the director of the NHC, said: It truly is really about the whole size of this storm. The larger and the slower the storm is, the greater the threat and the impact – and we have that.
On Thursday evening, Florence’s forward movement was just 5mph. While hurricane force winds extended 80 miles from the centre of the storm, tropical storm force winds extended up to 195 miles. Overall, the storm is 400 miles wide.
Airlines have cancelled more than 1,500 flights, and coastal towns across the Carolinas are largely empty after 1.7 million people in three states were told to clear out.
One climate model is predicting that as much as 11 trillion gallons of rain will fall on North Carolina in the coming week – an amount that’s enough to fill the Empire State Building 40 times over.
Air Force General Terrence J O’Shaughnessy, head of US Command, said search and rescue is a top priority but that the magnitude of the storm may exceed the ability of rescuers.
He said there are about 7,000 US military forces currently in place and ready to respond to the storm – along with ships, helicopters and high-wheeled vehicles.
One electricity company fears that three-quarters of its four million customers will lose power as a result of the storm, and may not be reconnected for weeks.
Some Carolinians have expressed frustration after evacuating their homes for a storm that was later downgraded – but officials have pushed back at suggestions that Florence’s threat has been exaggerated.