As Cyclone Favio makes landfall in an already flooded Mozambique–striking the provinces of Inhambane and Sofala as a Category 3–I am prompted to reflect a bit on what the South Indian cyclone season of 2006-2007 has shown us so far.
There have now been three storms that we can classify as Category 4s according to traditional Saffir-Simpson categorization: Bondo, Dora, and Favio. Being a Category 4 in this particular basin means that at some point, based upon estimates from satellite images, these storms were determined to have maximum sustained winds of 115 knots or higher for at least one advisory. (The forecasting center that officially does the advisories for this region of the world, or at any rate the one I’ve been listening to, is the Joint Typhoon Warning Center.)
A glance back over records that I’ve been keeping, however, shows that Dora and Favio have this in common: Each storm was only a Category 4 for one official advisory. Dora briefly reached 115 knots on February 3; Favio jumped up to 125 knots on February 20 but then dropped down again to 100 knots (or Category 3) the next day.
By contrast, Bondo of last year apparently reached Category 4 strength on three separate occasions. Other Category 4 and of course Category 5 storms have lasted at these intensities considerably longer than any of these three 2006-2007 storms, the record-setter being last year’s Hurricane and Supertyphoon Ioke, which was a Category 4 or higher for 198 consecutive hours.
The question then becomes: Even though we call Bondo, Dora, and Favio Category 4s as if they’re interchangeable, aren’t we missing something important by doing so? I would argue that we probably are; more importantly, so would MIT’s Kerry Emanuel, who has been a strong critic of classifying hurricanes according to traditional attributes such as intensity and frequency.
In terms of the amount of power dissipated, a storm that reaches weak Category 4 status for one advisory doesn’t really compare with a long-lived Category 4 or 5 storm. Metrics like Accumulated Cyclone Energy (ACE) or Emanuel’s Power Dissipation Index (PDI), which try harder to take into account how long storms are intense rather than merely sampling the peak intensity that they achieve, are much better at capturing that. And get this: In terms of its ACE rating, Ioke was apparently more powerful than all 2006 storms in the Atlantic combined.
So in short, while catchphrases like “Category 3” and “Category 5” may capture our attention and are easily understood by the public, they’re really fairly limited in terms of what they tell us about storms. Not only do these classifications ignore how long a storm lasts at a strong intensity; they also don’t say anything about the incredibly important factor of storm size.
Speaking of which, the next South Indian storm to worry about after Favio is Gamede, which looks considerably larger (as you can see from the image below), and which may be on a track to intensify and hit Madagascar….
UPDATE 4 pm ET: Gamede has already intensified faster than the Joint Typhoon Warning Center expected, and is now a strong Category 1 storm and expected to grow considerably stronger.