Florence exploded into a potentially catastrophic hurricane Monday as it closed in on North and SC, carrying winds up to 140 miles per hour (220 kph) and water that could wreak havoc over a wide stretch of the eastern United States later this week.
Hurricane Florence's size is "staggering", National Hurricane Center Director Ken Graham warned.
Florence, it said, will bring "life-threatening storm surge and rainfall to portions of the Carolinas and mid-Atlantic".
Forecasters said parts of North Carolina could get 20 inches (50 centimetres) of rain, if not more, with as much as 10 inches (25 centimetres) elsewhere in the state and in Virginia, parts of Maryland and Washington, D.C.
"The water could overtake some of these barrier islands and keep on going. When you slow a system down that has tropical water with it, it is just going to dump rain".
Rain measured in feet is "looking likely", he said. "That, folks, is extremely life-threatening".
After the storm surge comes the deluge of rain.
Flooding and storm surge are a concern for officials in North and SC as they watch Hurricane Florence.
Category 4 hurricanes are unusual this far north. Hurricanes need warm water to thrive - and the sea surface temperature of ocean water cools as you get further away from the tropics. President Donald Trump tweeted Monday evening.
The president signed emergency declarations for the Carolinas and Virginia, a move that frees up federal money and resources.
More than a million people are expected to flee low-lying areas, as SC and Virginia ordered mandatory evacuations of coastal areas starting on Tuesday.
Larger animals like those at the Virginia Zoo in Norfolk, the property of which is partly under an evacuation order, are going to ride out the storm in the indoor and sheltered portions of their enclosures.
Speaking about the Wilmington area and its surrounding beaches in North Carolina, a Federal Emergency Management Agency (FEMA) official said the inland flooding threat was "extreme".
A storm surge warning was issued from South Santee River, South Carolina, to Duck, North Carolina, and the Albemarle and Pamlico Sounds, including the Neuse and Pamlico Rivers. "Plus the battering waves will cause massive damage and destruction". A Category 5 storm has 157 miles per hour winds.
"The South Carolina National Guard is on the frontline of support", said U.S. Army Maj.
Grace Beasley said: 'I'm now in the Philippines and the country is making huge preparations for typhoon Mangkhut, which has already caused devastation to Guam'. "This is the craziest one". The last storm to hit the United States at Category 5 strength was Hurricane Andrew, in 1992.
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