Buxton, N.C. ? A monstrous Hurricane Irene tightened its aim on the Eastern Seaboard on Thursday, threatening 65 million people along a shore-hugging path from North Carolina to New England. One of the nation’s top experts called it his “nightmare” scenario.

The Category 3 storm with winds of 115 mph — the threshold for a major hurricane — would be the strongest to strike the East Coast in seven years, and people were already getting out of the way.

Tens of thousands fled North Carolina beach towns, farmers pulled up their crops, and the Navy ordered ships to sea so they could endure the punishing wind and waves in open water.

All eyes were on Irene’s projected path, which showed it bringing misery to every city along the I-95 corridor, including Washington, New York and Boston. The former chief of the National Hurricane Center called it one of his three worst possible situations.

“One of my greatest nightmares was having a major hurricane go up the whole Northeast Coast,” Max Mayfield, the center’s retired director, told The Associated Press.

He said the damage will probably climb into billions of dollars: “This is going to have an impact on the United States economy.”

The head of the Federal Emergency Management Agency said damages could exceed most previous storms because so many people live along the East Coast and property values are high.

“We’ve got a lot more people that are potentially in the path of this storm,” FEMA Director Craig Fugate said in an interview with The Associated Press. “This is one of the largest populations that will be impacted by one storm at one time.”

The storm would “have a lot of impact well away from the coastline,” he added. “A little bit of damage over big areas with large populations can add up fast.”

Irene was massive, with tropical-force winds extending almost twice as far as normal, about the same size as Katrina, which devastated New Orleans in 2005.

“It’s not going to be a Katrina, but it’s serious,” said MIT meteorology professor Kerry Emanuel. “People have to take it seriously.”

The governors of North Carolina, Virginia, Maryland, Delaware, New York and New Jersey declared emergencies to free up resources, and authorities all the way to New England urged residents in low-lying areas to gather supplies and learn the way to a safe location.

Irene was expected to come ashore Saturday in North Carolina with 115 mph winds and a storm surge of 5 to 10 feet. Warnings were out for the entire coast. It could dump a foot of rain, with as much as 15 inches falling in some places along the coast and around Chesapeake Bay.

With heavy rain and storm surge predicted for the nation’s capital, organizers postponed Sunday’s dedication of the Martin Luther King Jr. Memorial on the National Mall.