The United States is bracing itself for a direct hit from Hurricane Henri on the East Coast and "life-threatening" storms with winds of 75mph.
Fierce winds and heavy rain are set to pound Long Island, New York, or southern New England on Sunday, the US National Hurricane Center (NHC) said, putting 42 million people in the region under a hurricane or tropical storm warning.
"This is a life-threatening situation. Persons located within these areas should take all necessary actions to protect life and property from rising water and the potential for other dangerous conditions," the NHC said.
New York City, the largest city in the United States, is under a tropical storm warning.
New York Governor Andrew Cuomo declared a state of emergency on Saturday afternoon, telling people in areas prone to flooding to move to higher ground.
Thousands of people were ordered to leave Central Park and seek shelter as heavy thunderstorms hit.
New York City's homecoming concert was dramatically cancelled halfway through and 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 tweeted on Saturday.
Henri could produce storm surges of 1 to 1.5 metres along the coast in New England, 75mph or higher wind gusts and rains of 7.6 to 15.25cm with 25cm in isolated areas.
On eastern Long Island, 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.
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.
Eversource, the largest electric utility company in Connecticut, has warned residents to be prepared for power outages for up to five to 10 days.
Airlines have issued travel alerts and are offering vouchers for 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.
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.