What will Manhattan look like if Irene brings a severe storm surge to the city this Sunday? In 2005, the Canadian Climate Center used Google Earth to simulate a category 2 hurricane on top of a high tide in order to visualize how vulnerable New York City is to the kind of surge that a powerful hurricane can bring. The results would be a five meter storm surge.
The visualizaiton shows that the areas that are inundated under these conditions roughly correlate with the portions of the city already under a mandatory evacuation order from Mayor Bloomberg.
Update: Simulations of the storm surge from Irene are now projecting only 1 meter of surge, not including the high tide from the new moon.
Here's a visualization from WNYC's excellent evacuation zone map:
While it's unlikely that Sunday's surge will be anywhere close to this one, the result could still be billions of dollars in damage due to flooding and unusual amounts of rainfall.
As dire as that sounds, it could be even worse. Here's a video showing what would happen if the city were hit by a full-on category 4 hurricane. (Hurricane Katrina was only a category 3.)