"When they tell you to evacuate, you need to evacuate," he said. "Don't delay, don't pause, don't question the instructions that are being given, because this is a powerful storm."
Sandy, which killed 69 people in the Caribbean before making its way up the Atlantic, began to hook left at midday toward the New Jersey coast.
New Jersey Gov. Chris Christie said people were stranded in Atlantic City, which sits on a barrier island. He accused the mayor of allowing them to stay there. With the hurricane roaring through, Christie warned it was no longer safe for rescuers, and advised people who didn't evacuate the coast to "hunker down" until morning.
While the hurricane's 90 mph winds registered as only a Category 1 on a scale of five, it packed "astoundingly low" barometric pressure, giving it terrific energy to push water inland, said Kerry Emanuel, a professor of meteorology at MIT.
Image: This photo provided by Dylan Patrick shows flooding along the Westside Highway as Sandy moves through the area Monday, Oct. 29, 2012 in New York. Much of New York was plunged into darkness Monday by a superstorm that overflowed the city's historic waterfront, flooded the financial district and subway tunnels and cut power to nearly a million people.