At one stage 3.15 inches of rain fell in the space of an hour across Central Park as the deluge enveloped the city and struck at essential services including the subway system.
New York state governor Kathy Hochul declared an emergency as the remnants of the storm caused massive flooding in the country's financial and cultural capital, leaving the boroughs of Brooklyn and Queens badly hit, AFP reports.
Police gave no details on how the seven deaths in the city occurred.
State governor Phil Murphy has also declared a state of emergency in neighboring New Jersey, where CNN reported at least one person was killed in the city of Passaic, bringing the overall death toll from Ida to 15.
“We're enduring an historic weather event tonight with record-breaking rain across the city, brutal flooding and dangerous conditions on our roads,” Mayor Bill de Blasio said in a tweet as he declared a state of emergency in the city.
I’m declaring a state of emergency in New York City tonight.
We’re enduring an historic weather event tonight with record breaking rain across the city, brutal flooding and dangerous conditions on our roads.
— Mayor Bill de Blasio (@NYCMayor) September 2, 2021
Hundreds of flights have been cancelled at nearby LaGuardia and JFK airports, as well as at Newark, where video showed a terminal inundated by rainwater.
Authorities urged residents not drive on flooded roads.
“You do not know how deep the water is and it is too dangerous,” the New York branch of the National Weather Service (NWS) said in a tweet.
The city earlier issued a rare flash flood emergency warning urging residents to move to higher ground, AFP reports.
“Significant and life-threatening flash flooding is likely from the Mid-Atlantic into southern New England,” the NWS said in a bulletin, adding that three to eight inches of rain could drench the region through Thursday.
Flash Flood Emergency including New York NY, Brooklyn NY, Queens NY until 11:30 PM EDT pic.twitter.com/44k7Am5AyI
— @NWSFlashFlood (@NWSFlashFlood) September 2, 2021
The NWS said steep terrain and even city streets were particularly vulnerable to a band of severe weather that extended to Massachusetts, where tornado warnings were issued early Thursday.
In Annapolis, 30 miles (50 kilometers) from the U.S. capital, a tornado ripped up trees and toppled electricity poles.
AFP contributed to this story