The bogusness of category six

June 1, 2006

As one would expect, the newspapers in Florida are full of hurricane related stories today. Storm surge, the Colorado State forecast, and FEMA are just some of the reasonable topics that went to print today. The Ledger, on the other hand went a different way: 

A new category for super hurricanes? 

Before the monstrous Hurricane Katrina slammed Louisiana and Mississippi last year, it strengthened into a jaw-dropping Category 5 — the benchmark for the most destructive hurricanes on the Saffir-Simpson rating scale.

Katrina grew well beyond the 156 mph that put it into the highest possible category. Before its run in the Gulf was over, its winds intensified to 175 mph.

While advisories at the time stated that "maximum sustained winds (were) near 175 mph", the official post-storm report put Katrina's maximum strength at 155 knots (171 mph).

Further along the article states:

If weather gurus were to expand the Saffir-Simpson scale, Category 6 would probably start at 175 mph. And storms like Katrina would make the cut.

The first sentence is probably wrong. None of the categories start on a multiple of five; instead the multiple of five is the upper limit (i.e. 155 mph is the max for category four). So, a category six would start at 176 mph. By that mark, only Wilma and Rita would have made the cut.

Of course, neither of those storms made landfall at that strength. And ultimately, that's what matters when you are talking about the Saffir-Simpson scale. After all, it was designed to describe the damage caused by storms when they hit land. The only two storms in the records as having gone ashore with winds greater than 175 mph are 1969's Camille and 1988's Gilbert.  It is hard to argue that either storm had damage that was above and beyond the damage described by the category five description. Also, I don't see anything overturning the dynamical constraints that restrict hurricanes from holding category five strength for an extended period of time (thereby reducing the probability of category landfall).

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