More than 42 million people in the region were under a hurricane or tropical storm warning on Saturday, the NHC said.
The center warned that Henri could produce storm surges - when ocean waters are pushed above their normal level - of 3 to 5 feet (1 to 1.5 meters) in some areas, as well as bringing rainfall of 3-6 inches (7.5-15 cm).
Parts of Long Island and New Haven, Connecticut, were under hurricane and storm surge warnings. Other parts of New England, such as Martha's Vineyard and Nantucket, were under surge and tropical storm watches and warnings.
STAY HOME, NEW YORKERS
New York City, the largest city in the United States, was under a tropical storm warning. In a Twitter post on Saturday, Mayor Bill de Blasio urged residents to stay home on Sunday and to use public transport if they need to travel.
Emergency management officials said heavy rainfall and damaging winds could flood roadways and reduce visibility throughout the weekend.
"Secure outdoor objects and make sure you are in a safe location before the onset of wind and rain!," New York City Emergency Management Commissioner John Scrivani wrote in a tweet on Saturday.
On eastern Long Island, in the tiny Hampton hamlet of Amagansett, New York - where celebrities including Paul McCartney, Alec Baldwin and Gwyneth Paltrow have homes - residents packed supermarkets, hardware and liquor stores early Saturday morning.
At the IGA supermarket, shelves were stripped bare of toilet paper, paper towels and other supplies. Motorists waited in long lines at gas stations while stores ran out of flashlights.
Michael Cinque, the owner of the Amagansett Wine & Spirit, deliberated whether to board up the windows of his store as shoppers streamed in and left with bottles of tequila, vodka and expensive liquors.
"You always have to be prepared," said Cinque, who has owned the store for 42 years and also volunteers as an emergency responder. "You have to take it seriously."
Officials on Fire Island off the south coast of Long Island told residents and visitors to leave or face the potential of getting stuck on the island.
In Newport, Rhode Island, a coastal yachting community of 25,000 people, sump pumps, flashlights and generators were also in high demand.
Eversource, the largest electric utility company in Connecticut, warned residents to be prepared for power outages for up to five to 10 days.
Several airlines issued travel alerts and were offering vouchers for tickets on flights in and out of the region over the weekend. The Long Island Rail Road and Metro-North Railroad were suspending Sunday service while Boston's public transportation system said it was reducing service for Sunday.
New York Governor Andrew Cuomo declared a state of emergency on Saturday afternoon, warning people in areas prone to flooding to move to higher ground. Massachusetts Governor Charlie Baker and Connecticut Governor Ned Lamont activated the National Guard in their states to help in possible rescue, debris clearing and public safety efforts.
(Writing and additional reporting by Brendan O'Brien in Chicago and Frances Kerry in London Editing by Daniel Wallis, Diane Craft and Susan Fenton) ((firstname.lastname@example.org; (312) 408-8561;))