(CNN) - Isaias slammed into the East Coast overnight, causing rapid flooding, water rescues and the threat of New York's strongest winds since Superstorm Sandy in 2012.

Isaias hurled sustained winds of 85 mph and became a Category 1 hurricane before reaching land around 11:10 p.m. ET near Ocean Isle Beach, North Carolina, the National Hurricane Center said.

It was downgraded to a tropical storm Tuesday morning, with maximum sustained winds down to 70 mph. But that doesn't mean the danger is over.

Tropical storm warnings extend from North Carolina all the way to the Maine/Canada border, CNN meteorologist Michael Guy said. Tropical storm conditions will last through Tuesday and into the overnight hours.

In addition to torrential rain, major storm surges and the threat of tornadoes, "this is going to be a power problem," CNN meteorologist Chad Myers said.

By mid-Tuesday morning, Isaias had knocked out power to more than 500,000 electricity customers, Myers said. That means well over a million people are left in the dark.

And with the coronavirus pandemic still raging across the country, recovery from this storm could be made much more difficult.

Flooding, fires and 65-mph winds

Howling wind and water washing across in "one to two foot swells" closed a bridge Monday night in Sunset Beach, North Carolina, the Sunset Beach Police said on Facebook. Streets in Holden Beach became rivers as water quickly rose, Jessi Viox told CNN.

"Getting ready for Round 2," Viox said. "The eye has moved around us, and now here comes the back end."

Brunswick County, North Carolina, reported "numerous calls" for water rescues, structural fires, structural collapses and people trapped in flooding houses," Oak Island Water Rescue said on Facebook.

Before Isaias even made landfall, the top of the Apache Pier Pavilion was seen lifting off in the wind.

And multiple structures in Ocean Isle Beach were reported to be ablaze, according to the Horry County Fire Rescue in South Carolina.

After pummeling the Carolinas, Isaias is forecast to gradually weaken as it brings strong winds Tuesday all along the East Coast, including in Washington DC, Philadelphia and New York.

The storm is expected to lash Philadelphia with winds of up to 65 mph, while New York will get pounded with winds of 65 to 70 mph.

The system could bring the strongest winds to New York City since Superstorm Sandy almost eight years ago, said Ross Dickman, the meteorologist-in-charge at the National Weather Service office in New York.

"The wind and flooding impacts from Isaias will be similar to what the city has seen from some of the strongest coastal storms," such as nor'easters, he said.

"But we haven't seen one this strong in many years."

New York City is implementing flood measures in lower Manhattan, including installing temporary barriers to prevent flooding.

And in Maryland, Gov. Larry Hogan suspended Covid-19 testing operations at community-based sites for Tuesday.

Curfews and evacuations in parts of North Carolina

Ferocious winds of up to 70 mph are expected and could bring down power lines and trees. Tornadoes are also possible in North and South Carolina, each state's emergency management department said.

Parts of the North Carolina coast, like Cape Fear, were given curfews Monday as Isaias drew closer,CNN affiliate WWAY-TV reported. Most of the curfews started around 5 and 6 p.m. Monday and were due to end between 6 and 9 a.m. Tuesday.

In other parts of the coast, residents and tourists were evacuated. The North Carolina Department of Transportation (NDOT) evacuated more than 3,000 people off Ocracoke Island Monday, CNN affiliate WAVY-TV reported.

"The most important thing is to get out of harm's way if you are told to evacuate," Gov. Roy Cooper said Monday. "Try to have a plan to stay with friends or family outside the danger zone."

North Carolinians rushed to stock up on supplies.

"You never know. We've been hit with worse surprises, so there really is no amount of over-preparations that you can do," Eli Thompson of Avon told WAVY-TV.

Storm surge and tropical storm warnings

Tropical storm warnings were in effect for much of the mid-Atlantic and East Coast, all the way up to Stonington, Maine.

A warning south of the Savannah River on the Georgia-South Carolina border has been discontinued.

But storm surge warnings were in effect for Edisto Beach, South Carolina, to Cape Fear, North Carolina; Pamlico and Albemarle sounds in North Carolina, and Oregon Inlet, North Carolina, to the Virginia border.

Delaware Bay, Tidal Potomac River, Chesapeake Bay and Long Island Sound are among the areas along the East Coast expected to get battered Tuesday.

And by Wednesday morning, heavy rain will fall on New Hampshire and Maine.

CNN's Joe Sutton contributed to this report.

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