Henri has weakened slightly to a tropical storm but is still expected to crash into a long stretch of northeastern US coastline, as millions on New York's Long Island and in southern New England brace for flooding, toppled trees and extended power outages.
The storm had top winds of 110km/h on Sunday morning, just shy of hurricane status.
With the centre of the storm projected to pass just off the eastern tip of Long Island by midday, warnings extended from coastal Connecticut and Rhode Island to near the old whaling port of New Bedford, Massachusetts, and across the luxurious oceanfront estates of New York's Hamptons.
The first thunderstorms bringing what could be up to 15cm of rain arrived late on Saturday, and flash flooding began in some areas overnight.
Bands of heavy rain overwhelmed storm drains and drivers ploughed through 30cm-deep water in a few spots in New York City, and Newark and Hoboken, New Jersey.
Tropical storm-intensity winds were beginning to strike the coast on Sunday morning. Rising tides threatened to produce a dangerous storm surge.
People in the projected path spent Saturday scrambling to stock up on groceries and petrol. Those close to the coast boarded up windows and, in some cases, evacuated.
Approaching severe weather on Saturday night cut short a superstar-laden concert in Central Park.
The show headlined by Bruce Springsteen, Paul Simon and Jennifer Hudson was meant to celebrate New York City's recovery from the coronavirus.
But officials asked concertgoers to leave the park during Barry Manilow's set amid the threat of lightning.
Major airports in the region remained open as the storm approached, though hundreds of Sunday's flights were cancelled.
The White House said President Joe Biden discussed preparations with northeastern governors. Biden later began approving emergency declarations with Rhode Island.
New York hasn't had a direct hit from a powerful cyclone since Superstorm Sandy wreaked havoc in 2012.
Some of the most important repairs from that storm have been completed, but many projects designed to protect against future storms remain unfinished.
With maximum sustained winds at 110km/h, Henri was moving north at 30km/h as of Sunday morning.
It was about 80km south-southeast of Montauk Point on the tip of Long Island.
Australian Associated Press