New York Governor Kathy Hochul declared a state of emergency on Thursday as the remnants of storm Ida caused massive flooding in New York City and across the northeastern United States.

"I am declaring a state of emergency to help New Yorkers affected by tonight's storm," Hochul tweeted, after Ida, which slammed into the southern United States as a Category 4 hurricane on Sunday, caused tornadoes and flooding as it steamed north.

"Significant and life-threatening flash flooding is likely from the Mid-Atlantic into southern New England," the National Weather Service said in a bulletin, adding three to eight inches of rain could drench the region through Thursday.

New York City issued a rare flash flood emergency warning on Wednesday, urging residents to move to higher ground. 

Flooding in Maryland killed a 19-year-old man and left another person missing after a building was inundated on Wednesday, bringing the death toll from Ida to seven.

Ida slammed into the southern United States on Sunday.

It has since weakened but has caused severe flooding and tornadoes in Pennsylvania, New Jersey and Maryland, while in Annapolis, 30 miles (50 kilometres) from the US capital, a tornado ripped up trees and toppled electricity poles.

The NWS warned the threat of tornadoes would linger through Wednesday night, with tornado watches in effect in parts of southern Connecticut, northern New Jersey, and southern New York.

"Take shelter NOW. Flying debris will be dangerous to those caught without shelter. Move to a lower floor and stay away from windows," tweeted New York City's emergency notification body, later announcing multiple road closures across boroughs of the major metropolis, including Manhattan, The Bronx and Queens, due to flooding.

"This is extremely dangerous and potentially deadly flash flooding ongoing in Somerset County, as well as others in the area," the NWS account for Philadelphia metropolitan area tweeted late Wednesday, along with a video of a car trapped in churning water.

Ida is expected to continue steaming north and bring heavy rainfall on Thursday to New England, which was hit by a rare tropical storm in late August. 

US President Joe Biden is due to travel Friday to Louisiana, where Ida destroyed buildings and left more than a million homes without power. 

Hurricanes are common in the southern United States, but scientists have warned of a rise in cyclone activity as the ocean surface warms due to climate change, posing an increasing threat to the world's coastal communities.