Miami — The eastern US braced on Monday for the impact of "major" hurricane Florence as it threatened catastrophic flooding in areas already soaked by rain.
The storm was 1,000km southeast of the British island territory of Bermuda early on Monday and was forecast "to become a major hurricane" just hours later, the National Hurricane Centre (NHC) said.
Florence "is expected to remain an extremely dangerous major hurricane through Thursday", the centre in Miami said.
Virginia governor Ralph Northam’s office described Florence as possibly the state’s "most significant hurricane event in decades", and warned of "catastrophic inland flooding, high winds and possible widespread power outages".
It added: "The largest threat to life from hurricanes is not the high winds. Flooding is the deadliest result of these storms."
The US Navy ordered ships at its major base in Hampton Roads, Virginia, to put to sea, saying "the forecast destructive winds and tidal surge are too great to keep the ships in port".
Heavy rain in the Washington area over the weekend had already led to flooding in historic Alexandria, Virginia, local media said, and the National Weather Service issued a flood watch for part of the Potomac River.
Florence is "rapidly strengthening", with maximum sustained winds increasing to 170km/h, making it a category 2 storm, the second-weakest on the five-level Saffir-Simpson hurricane scale.
"The centre of Florence will move over the southwestern Atlantic Ocean between Bermuda and the Bahamas Tuesday and Wednesday, and approach the southeastern coast of the US on Thursday," the NHC said.
The storm was moving west-northwest at around 15km/h, and was forecast to drench a large swath of the US East Coast running from northern Florida to New Jersey.
On its current track Florence is expected to slam the Carolinas and Virginia the hardest, and all three states have issued emergency declarations to speed preparations.
North Carolina governor Roy Cooper’s office said Florence is already being felt along the state’s coast, with large sea swells resulting in life-threatening rip currents and surf.
"Everyone in North Carolina needs to keep a close eye on Florence and take steps now to get ready for impacts later this week," Cooper said.
The storm "is too powerful and its path is too uncertain to take any chances", South Carolina governor Henry McMaster said, issuing his state’s emergency declaration.
Florence was producing large swells expected to reach from the northern Caribbean to the southern coasts of Canada’s maritime provinces.
At this height of the Atlantic hurricane season, Florence is being trailed on east-to-west paths by two other hurricanes, Helene and Isaac.
Helene — 490km west of the Cape Verde islands off the African coast — has winds up to 150km/h and is expected to continue moving west-northwest for another couple of days, the NHC said in its bulletin.
Hurricane Isaac — which late on Sunday became the fifth hurricane of the season — is heading west towards the Caribbean. Early on Monday Isaac, which the NHC called a small hurricane, was about 1,900km east of the Windward Islands — a region still recovering from 2017’s powerful Hurricane Maria — with maximum sustained winds near 120km/h.