Monday, August 29, 2011

On hurricanes and predictions

Updated 30 August
 
It's become fashionable in the last 24 hours, now that Irene has come and gone from New York City, to say that officials overreacted and that NYC's mayoral administration, in particular, had to live down its dismal performance during last year's snowstorm. Here's how a local street looked for several days after that storm:


But I don't think Mayor Bloomberg overreacted to the threat Hurricane Irene posed (for one thing, it's too easy to say that after the fact; for another, there were big trees down in my neighborhood and flooding not so far away). Once again, interpreting the information we had gives a lesson in how important it is to understand the basis for the news.

In Irene's case, the issue was the likelihood of a storm following a particular track, and its intensity when it arrived. After the fact, I think that Irene changed course slightly, hitting land to the west of NYC, not the east, and weakened in intensity. It clearly caused a lot of damage - several people have been killed, there has been extensive flooding, and lots of property and tree damage. The damage was lighter here. In advance that was not so certain - and the storm did hit at high tide, with high winds and lots of rain. Sites like weather.com provide useful quick predictions, and lots of scary graphics. But they're interpreting readily available sources of information, and you can look at them yourself and  make your own judgments.

In addition to the map I posted Friday from the NY Times, a site I found very useful was the National Weather Service National Hurricane Center site. Click through it to the Irene Graphics Archive and you can see that the NWS' prediction of Irene's path was fairly accurate. The cone moves, but here's a screen shot of the five day cone/warning from one of the Wednesday briefings:


(The New York Times carried a story yesterday about how much better meteorologists are getting at predicting storm tracks, and how hard it can be to predict the intensity of a storm at any particular time. And BTW, I'm impressed with the Times for taking down its paywall for the duration.)
UPDATE: Here's a link to NPR's story aired Monday, 29 August, on the same subject. (Water temperature, land topography, air temperature to name a few elements.)

There was a lot of talk about Nate Silver's tweet stating that a Category 4 hurricane making landfall in New York City would cause damage roughly comparable to the damage caused in Japan by the earthquake/tsunami combination. If you read the full post, (always worth doing with Silver) you'll see all of Silver's qualifiers - starting with the crucial fact that a Category 4 storm has never hit the Northeast US. And in any case, there was a great deal of concern that Irene might come ashore here as a Category 3 storm (it wound up a Category 1 here).

After scooting around on a lot of sites, I decided that I should be a little, but not a lot, worried. So how did I prepare? I found Melissa Clouthier's list to be the most helpful. I knew I had bleach and paper goods, and I figured guns and ammo would not be an issue here in NYC. Mostly what I did was buy fresh fruit and fill water bottles (wondering how we had accumulated so many) and check the candle supply.

As usual: forecasters are basing their predictions on statistical models. Weather reporters are not always trained as meteorologists. Excitement means big news. So - take what the reporters are telling you with a grain of salt, and draw your own conclusions. And then turn on the Weather Channel to watch the waves crashing.

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