Friday, September 02, 2011

Hurrivcanes: when too much information is not enough


An hurricane put people of science - meteorologists - on television and all around us. Of Irene, much was said before, during, and after. There was a high probability, but not a certainty, that it could be a catastrophic storm. The danger was not the strength of the storm; there have been plenty of stronger storms. The biggest danger was the storm's target, namely New York City.
For us, in central Massachusetts, the predictions were consistent and accurate - a strong storm with a lot of rain. At one point a couple of days before landfall, Irene was predicted to travel along the west end of our town. The final track was perhaps 30 miles west of here. I call that an excellent forecast.
It isn't a decision when we have a complete set of immutable facts. That's a conclusion. A decision is something you make when you're trying to assess probability and risk based on as much information as you have available.
We had plenty of information about Irene's forecasted track and too much about what every news outlet thought about it. I tried to avoid the news coverage of the anticipated storm. We'd have the tv on, but muted, turning on the sound only when the meteorologists were talking. (I like Kevin Lemanowiczon Fox 25 and the team at WCVB.)
Starting more than a week before, we had computer models giving us forecasts that also included confidence levels. As the hours and days progressed, the forecasts were tweaked and the confidence levels adjusted upward.
One of the big problems that we have with tropical storm forecasts is our over-reliance on the The Saffir-Simpson Scale. This metric categorizes storm strength by sustained wind speed and correlates a storm surge. Irene's greatest destructive power was not, however, its wind speed or storm surge, as feared. According the scale, it was barely a category 1 storm, with sustained winds in the upper 70s, as it hit the New England coast. The 11" of rain that destroyed much of central and southern Vermont isn't included in the model. As a result, it's easy to underestimate the potential danger of a storm if we focus primarily on the wind speed.
There have been occasional efforts (see Wilma's Rage Suggests New Hurricane Categories Needed )  to replace the S-S scale with something that assesses rainfall potential.
To date, however, we've not seen the meteorologists agree on a new metric. Herbert Saffir says that we should keep it simple. Simple may be accurate, but it's not sufficient.

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