My BIG worry in NYC (besides being ground zero for 'man mad disasters') is what is called "The New York Bight Scenaro"

If you look at a topographic map of NYC, and particularly one with a fathom curve, you'll notice that we have a large area of fairly shallow water offshore, and we are right at a sharp bend in the coastline

If a major (or even not so major) hurricane came up the shore as struck in NY Harbor (or actually worse, the eye goes ashore with a glancing blow into NJ so the east wall of the eye is in NY Harbor), there would be a MASSIVE storm surge. NYC actually has some shoreline flooding during the monthly high tides if there is onshore wind. Large sections of lower Manhattan, Downtown Brooklyn, and the parts of Brooklyn and Queens on the south shore of Long Island are in serious risk of flooding in even a Cat 1 hurricane

The problem is, there are SO many people that have to be moved, that they will have to start early, and given human nature, and the facts that 1)It hasn't happened in a long time and 2)NYC probably has the least cars/capita of just about anywhere in the USA, it's going to be "fun". In addition, being that the evacuation would have to start 72 hours or so before expected landfall, the cone of uncertainty is large enough that Nassau and Suffolk counties to our east, and parts of NJ will be evacuating themselves (want some fun? Look at a cross section or good topo map of Long Island - basically, it's a line of hills on the North Shore (the Terminal morrain) and a runout to an aluvial plain of sand to the south shore - the south shore is FLAT)

I know that NYC figures that if they are going to get hit with a cat III, they have to evacuate slightly more than 2.3 MILLION people, and I thing Nassau and Suffolk will be evacuating something like 1.5 million people at the same time. Given that there are exactly 8 bridges/tunnels off this island, and only 4 of them DON'T go to Manhattan...

(That's why when I bought my house, I actually looked at evacuation maps, and Topo maps, and bought a house on one of the highest hills in the city - I figure if the storm surge hits 110ft, we've got other problems)