STEVE INSKEEP, HOST:
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Half a million people have been ordered to evacuate from low-lying areas in the path of Hurricane Sandy. Many of them are in New York City, where a storm surge may squeeze into the harbor and beyond. More than 7,000 flights have already been canceled. Amtrak has shut service in the northeast corridor. And we are hearing from different coastal spots throughout the program as people prepare. NPR's science correspondent Jon Hamilton is following the science of this storm.
Jon, good morning.
JON HAMILTON, BYLINE: Good morning.
INSKEEP: And one thing that calls for an explanation is why this is such a big deal. Category one hurricane, 85 mile per hour winds, yes, a big storm, but you've seen worse. So why is this a big deal?
HAMILTON: Yeah, exactly. There are two major reasons. The first one is that even though this isn't the most powerful storm we've seen, it is one of the largest storms ever to hit the East Coast. All right? So it's enormous. You know, we talked earlier this morning about how...
INSKEEP: In sheer size, you're talking about.
HAMILTON: The sheer size. You know, we talked about how it's going to cut this swath across the country maybe 1,000 miles wide. That makes it a really huge storm.
There's a second reason, though. And that is that this storm is expected to slow down a lot as it approaches land. So any given place in its path is going to have a much longer period of high winds and heavy rains. So that means more wind damage, more flooding. You know, in some places people could have two full days of really, really rough weather and see up to a foot of rain.
INSKEEP: Is this because you've got these winter storms coming the opposite direction and the storms basically collide and that slows down the hurricane?
HAMILTON: It's a combination of factors. I mean, what's happening is that this storm is going north. And usually, as we know, hurricanes go north and they turn right out to sea. OK? This storm is going to encounter something called a blocking system. It's a high pressure ridge up in Canada. And when that happens it's going to turn it the opposite direction that you usually have the winds going. So it's going to turn left and it's going to head west. Then, of course, it's going to run into - eventually run into the prevailing winds and sort of stall and stop and eventually turn north and dissipate.
INSKEEP: So you have this possibility of a slow moving storm, it dumps tons of rain. You've got the possibility of a large storm surge in some areas. What about the moon and the tides? Do they also have an affect here?
HAMILTON: Yeah. You talk about things coming together. So tonight I believe is the full moon. And when you get full moon you also get extra high tides. So what you're going to get is the storm surge produced by Sandy will be in addition to a tide that is already high. And so that can turn - if say it's a four or five foot storm surge, it could look like, you know, six or eight foot because of that.
Also, because the storm's moving so slowly, the surge is probably going to coincide with two or three different high tide cycles. And that could be really a bummer if you're in somewhere like New York City.
INSKEEP: Which is one of the reasons I suppose that people have used the phrase perfect storm. Is this going to be a perfect storm?
HAMILTON: People have compared this storm to the perfect storm - the so-called perfect storm of 1991. What happened there is that a hurricane - a much smaller hurricane called Grace - was headed north. And, like this storm, ran into a huge winter storm system and became this kind of hybrid, where it's not really a tropical storm anymore. It has the characteristics of a big rotating winter storm. And that, of course, produced a huge amount of damage in New England, which is one of the reasons people are so worried now.
INSKEEP: OK. And very briefly, if people are in the northeastern United States - you said it's a slow moving storm - if people are not yet prepared, how much time is left?
HAMILTON: Very, very little. And that's because this storm is so big that landfall is when the center of the storm meets the land. And you could have 10 or 12 hours before then the highest winds and heavy rains already onshore.
INSKEEP: Hours beforehand, you could have...
HAMILTON: Many hours beforehand.
INSKEEP: ...serious, serious conditions?
INSKEEP: OK. Jon, thanks very much.
HAMILTON: You're welcome.
INSKEEP: That's NPR's Jon Hamilton. We'll continue hearing from him throughout the program, as well as from reporters up and down the East Coast of the United States. Transcript provided by NPR, Copyright National Public Radio.