Forecasters said that because of giant waves and high tides made worse by a full moon, the metropolitan area of about 20 million people could get hit with an 11-foot wall of water.
"This is the worst-case scenario," Uccellini said.
New York Mayor Michael Bloomberg warned: "If you don't evacuate, you are not only endangering your life, you are also endangering the lives of the first responders who are going in to rescue you. This is a serious and dangerous storm."
New Jersey's famously blunt Gov. Chris Christie was less polite: "Don't be stupid. Get out."
New York called off school Monday for the city's 1.1 million students and announced it would suspend all train, bus and subway service Sunday night. More than 5 million riders a day depend on the transit system. The New York Stock Exchange announced it will shut down its trading floor Monday but continue to trade electronically.
Officials also postponed Monday's reopening of the Statue of Liberty, which had been closed for a year for $30 million in renovations.
In Washington, President Barack Obama promised the government would "respond big and respond fast" after the storm hits.
"My message to the governors as well as to the mayors is anything they need, we will be there, and we will cut through red tape. We are not going to get bogged down with a lot of rules," he said.
He also pleaded for neighborliness: "In times like this, one of the things that Americans do is we pull together and we help out one another And so, there may be elderly populations in your area. Check on your neighbor, check on your friend. Make sure that they are prepared. If we do, then we're going to get through this storm just fine."
The storm forced the president and Mitt Romney to rearrange their campaign schedules in the crucial closing days of the presidential race. And early voting on Monday in Maryland was canceled.