New York City prepares to face Irene

>> we want to move to the north and the city that has seen it all, except for this, new york city is under a hurricane warning tonight, extremely rare. so are the fact that mandatory evacuations are underway. lester holt is inside the evacuation zone, in lower manhattan . lester, we have had minor flooding over the years where they have taken some folks out of nursing homes, perhaps a hospital or two with, but this kind of thing has never happened in new york city ?

>> reporter: you know, brian, it's interesting you said seen at all. there are 8 million of us living in new york city , we do feel at times we have seen it all. heck we had an earthquake this week. but with mandatory evacuations in a lot of this city, it got a lot of folks' attention. tonight urgent reality is gripping the big apple . the naked city has never felt so exposed. residents in low lying neighborhoods are now being urged to get out.

>> i don't know what's going on, i don't know what's going to happen. that's always a scary feeling.

>> reporter: the city has issued a map showing the relative flood risk, now zone a is under an evacuation order. today seniors at this brooklyn hospital are among the first to be relocated.

>> i can't stress enough, please, nature is a force more powerful than any of us and it's better to be safe than sorry.

>> reporter: we're less than a half mile from the financial district . this would be the first area that would be overwhelmed by a storm surge . experts say the entire area could be buried in water, even flooding the holland tunnel that carries car traffic between lower manhattan and new jersey.

>> new york city is perhaps the most vulnerable megacity in the world. ive a major hurricane comes through new york city , we're very vulnerable.

>> reporter: wind is the other worry and construction crews are making plans to secure towering cranes. here at the world trade center rebuilding site, larry davis has 11 rigs in use.

>> if they're not secured, they could collapse in the high, high winds . they're designed for 100, 110-mile-per-hour winds.

>> reporter: the subway is the life blood of new york city , carrying half a million passengers a day. but the tracks are susceptible to flooding. like 2007 when rain falling at two inches an hour from a tornado related storm did this. irene is expected to drop rain at more than twice that rate. tonight with train service shutting down soon, those new yorkers not under evacuation orders are gaming out when and how they'll get out if and when the time comes.

>> you have to start your preparations to leave right now. keep in mind, after noon tomorrow, you're not going to have the advantage of mass transit to help you do that.

>> now imagine being a city official with the to do list when you're facing a hurricane in new york city . and you have to think about tourists, there are tens of thousands of them in the city right now. peak tourism season. i found i had to deliver the news to them and they're trying to figure out what to do. the airline also start paring down flights soon and at some point the airlines will stop flying when that hurricane comes through here.

>> i talked to some first responders last night who were worried about water actually breaching the pit, ground zero , the site of the new museum because it's not far from the river, of course there, and second flooding the planes, and the run ways at both jfk and laguardia which are both adjacent to bodies of water . .

>> reporter: they both sit on water, the run ways are about this level over the water. so yes, the airports could be shut down for quite some time. i was at the ground zero site a few days ago, and we all remember from the disaster time, it's a bathtub there. there's a wall there to separate it from the river. so right now they're susceptible to anything. they're just a stone's throw from where i acm right now.

>> lester holt in lower manhattan , we'll be talk a lot