New York City storm surge: How high will the flood waters go?

In an unprecedented move, New York City is evacuating nearly 400,000 residents of low-lying sections of Manhattan, Brooklyn, and Queens. Just as unprecedented, the city's airports and subways are being shut down, and Con Ed is warning that if it has to cut electricity to Wall Street, it could take three days to restore it.

The National Hurricane Center warned that hurricane Irene was still expected to arrive in New York Sunday night at or near hurricane strength.

The big concern: Storm surge.

The Miami-based National Hurricane Center has forecast a storm surge of up to 8 feet for Long Island and metropolitan New York when Irene passes, Reuters reports.

Storm surge is caused primarily by strong winds pushing on the surface of the water, creating a higher than normal tide. Other factors contributing to storm surge can be large amounts of rain, and in this case, a new moon on Sunday.

"Higher than normal astronomical tides are occurring this weekend," said Daniel Brown, a meteorologist for NOAA's National Hurricane Center. "Coastal and river flooding will be highest in areas where the peak surge occurs around the time of high tide."

A storm surge model developed by MIT, shows that areas around Manhattan, including lower Manhattan, could show a surge of four feet on top of the high tide.

Another model by The Stony Brook Storm Surge Research Group at the State University of New York, offers an interactive map that's updated every 12 hours. It indicates that the peak surge for The Battery is at 5:15 am Sunday. If you click on the buoys on the map around New York, you can see the surge charted in specific areas over the next 12 hours.

The Stony Brook site advises that this is a research model still under development and shouldn't be use for evacuation decisions.

But clearly NYC officials are taking the storm surge threat seriously.

Consolidated Edison said it wouldn't shut down the city's electricity pre-emptively, but could cut power early Sunday in low-lying parts of Manhattan, including to 6,000 homes or businesses south of the Brooklyn Bridge. The area includes New York's Financial District, according to Reuters.

Con Ed officials say it could take three days to restore power in lower Manhattan, if power is shut off, reports The New York Daily News.