As Hurricane Florence put a corridor of more than 10 million people in the crosshairs in the southeast of the United States, people in the northern Philippines were bracing for

Typhoon Mangkhut, the most powerful storm of the typhoon season.

As Florence closed in on the Carolinas, uncertainty over its projected path spread worry across a widening swathe of the US. Faced with new forecasts that showed a more southerly threat, Georgia's governor joined his counterparts in Virginia and North and South Carolina in declaring a state of emergency yesterday, and some residents who had thought they were safely out of range boarded up their homes.

The National Hurricane Centre's best guess was that Florence would blow ashore as early as tomorrow morning around the North Carolina-South Carolina line, then push its rainy way westward with a potential for catastrophic inland flooding.

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Florence's winds were down to 185km/h from a high of 225km/h, and the Category 4 storm fell to a Category 2, with a further slow weakening expected as the storm nears the coast. Tropical storm-force winds extended 315km from Florence's centre, and hurricane-force winds reached out 110km.

But authorities warned it would still be an extremely dangerous hurricane. "Do you want to get hit with a train or do you want to get hit with a cement truck?" said Jeff Byard, an administrator with the Federal Emergency Management Agency.

Hundreds of thousands have left coastal areas ahead of the arrival of Hurricane Florence. Photo / AP
Hundreds of thousands have left coastal areas ahead of the arrival of Hurricane Florence. Photo / AP

In the Philippines, meanwhile, officials ordered precautionary evacuations and closures of schools and offices and urged farmers to harvest their crops to reduce damage.

Forecasters said Typhoon Mangkhut, considered a super typhoon and the strongest this year, could hit northern Cagayan province tomorrow. Yesterday it was located about 800km away in the Pacific with sustained winds of 265km/h and gusts of up to 325km/h.

It could maintain the strength of a super typhoon when it hits land in the northeastern corner of Luzon Island.

With a massive rain band 900km wide, combined with seasonal monsoon rains, the storm could bring heavy to intense rains that could set off landslides and flash floods, state forecaster Meno Mendoza said.

Cagayan Governor Manuel Mamba said by telephone that northern coastal and island villages in the typhoon's projected path were to begin evacuating residents yesterday ahead of the expected onslaught. He said classes were suspended and offices, except those involved in rescue and relief work, advised to close from today.

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

At the White House, President Donald Trump both touted the Government's readiness and urged people to get out of the way of Florence.

"Don't play games with it. It's a big one," he said.

Hurricane Florence weakened, but is still expected to have a major impact. Photo / AP
Hurricane Florence weakened, but is still expected to have a major impact. Photo / AP

The hurricane centre said Florence would approach the coast tomorrow and linger for a while before rolling ashore.

Computer models of exactly what the storm might do varied, adding to the uncertainty.

In contrast to the hurricane centre's official projection, a highly regarded European model had the storm turning southward off the North Carolina coast and coming ashore near the Georgia-South Carolina line.

Reacting to the possibility of a more southerly track, Georgia Governor Nathan Deal declared an emergency but did not immediately order any evacuations.

"I ask all Georgians to join me in praying for the safety of our people and all those in the path of Hurricane Florence," Deal said.

The shift in the projected track spread concern to areas that once thought they were relatively safe. In South Carolina, close to the Georgia line, Beaufort County emergency chief Neil Baxley told residents they need to prepare again for the worst.

"We've had our lessons. Now it might be time for the exam," he said.

In Virginia, where about 245,000 residents were ordered to evacuate low-lying areas, officials urged people to remain away from home despite forecast changes showing Florence's path largely missing the state.

Mangkhut is expected to affect as many as 54.2 million people, according to the United Nation's Global Disaster Alert and Coordination System. After hitting the Philippines it is expected to hammer Taiwan before heading to Hong Kong and south China, bringing heavy rains and storm surges on its trail.

The Philippines, which is hit by around 20 typhoons a year, expects Mangkhut to be as powerful as Super Typhoon Haiyan, which left 7300 dead or missing and 5 million displaced in 2013.