Tropical Storm Henri was on course to make landfall on the US east coast Sunday, with millions in New England and New York's Long Island preparing for flash flooding, violent winds and power outages.

The US National Hurricane Center said in its 7:00 am (1200 GMT) advisory that Henri 50 miles (80 kilometers) southeast of Montauk Point in New York state.

Forecasters downgraded Henri from a hurricane but warned heavy rainfall and the risk of surging seas as the storm churned in the Atlantic, packing maximum sustained winds of 70 miles per hour.

As the surface layer of oceans warms due to climate change, cyclones are becoming more powerful and carry more water, posing an increasing threat to the world's coastal communities, scientists say. Storm surges amplified by rising seas can be especially devastating.

A swath of the northeastern coastline, including New York City, was under alert as the storm approached. If Henri is upgraded again then it would be the first hurricane to hit New England in 30 years.

Nasty weather that preceded Henri late Saturday forced New York City to halt a star-studded Central Park concert billed as a "homecoming" for a metropolis hard hit by the pandemic.

The approaching then-hurricane had prompted New York Governor Andrew Cuomo to announce a state of emergency and the deployment of 500 National Guard soldiers in anticipation of response efforts.

"It's as serious as a heart attack," he warned.

He said the storm was expected to make landfall on Long Island, home to the plush Hamptons villages where wealthy New Yorkers retreat in summer, around noon (1600 GMT) on Sunday.

"It will be about a 26-hour event," Cuomo added, telling New Yorkers to expect "significant power outages" and "significant flooding" in some suburbs of the Big Apple.

Henri was anticipated to miss New York City by several miles, but still caused tropical storm conditions that began Saturday night.

The National Weather Service said 1.94 inches of rain fell in the park between 10 and 11 pm Saturday, the wettest hour on record in New York City.

In the park an announcer cut off pop legend Barry Manilow mid-song to urge revelers to proceed swiftly but calmly to the nearest exit.

"I guess for safety it makes sense. I mean I can hear the thunder," said attendee Maria Fuentes.

- 'Dangerous' -

The NHC warned of "a dangerous storm surge, hurricane conditions and flooding" in areas of southern New England and Long Island.

Henri is expected to produce three to six inches of rain (7.5 to 15 centimeters) across the region, with isolated maximum totals near 10 inches, the NHC warned.

The heavy rainfall "may result in considerable flash, urban, and small stream flooding" as well as river flooding, it added, saying storm surges of 5 feet were possible in coastal areas.

High winds are expected to knock out electricity for hundreds of thousands of people across the region and delay countless flights.

Officials in New England -- which includes Connecticut, Maine, Massachusetts, New Hampshire, Rhode Island and Vermont -- have warned people to get ready.

"The last hurricane to make landfall onto New England was Hurricane Bob in 1991," Dennis Feltgen, an NHC spokesman, told AFP. That storm killed at least 17 people.

It has been almost a decade since such severe weather threatened the region.

"The last time we had hurricane watches issued for the area was for Hurricane Irene, back in late August of 2011," tweeted the National Weather Service in New York City.

The last hurricane to make landfall in Long Island was Gloria in 1985.

The warnings have reignited memories of Hurricane Sandy, a more powerful storm that knocked out power for much of Manhattan and flooded subways in 2012.

The US PGA Tour postponed the final round of the Northern Trust tournament in suburban New York to Monday because of Henri.