Concerns arise over defining storms by category
NEW ORLEANS — Why were people in Plaquemines Parish and other coastal areas of Louisiana and Mississippi so blase about Hurricane Isaac? The answer could have something to do with the yardstick most commonly used to measure storms.
The measure that many in the meteorological field use is known as the Saffir-Simpson scale. That is the system that ranks storms in “categories” — Isaac, for example, was a Category 1 Hurricane, the weakest level of hurricane by this scale.
Experts have long argued that the scale is flawed because it focuses on the power of wind and not the surge of water that a hurricane pushes before it. Climatologists like Kerry Emanuel at the Massachusetts Institute of Technology have said that any classification should include both wind speed and surge. Otherwise, he argues, coastal residents can be easily misled. Hurricane Ike in 2008 was a Category 2 storm, on the low end of the Saffir-Simpson scale when it made landfall in Galveston, Texas, but its surge wreaked grievous damage.
Isaac, too, packed a surge surprise. At a briefing Wednesday evening for officials of the Army Corps of Engineers, a staff hydrologist, Maxwell E. Agnew, noted that the surge from Isaac was not much smaller than that of Hurricane Katrina, a Category 3 storm when it slammed into Louisiana on Aug. 29, 2005, whose surge had a catastrophic effect on the metro area’s flawed floodwalls and levees. Katrina’s surge at the corner of Lake Borgne, east of the city, was 15.5 feet; the wall of water pushed by Isaac, measured at roughly the same place, came to about 14 feet.
That surge overwhelmed the locally built levees of Plaquemines Parish.
While considering the current version of the Saffir-Simpson scale in 2011, the National Oceanic and Atmospheric Administration said that surge varies depending on coastal conditions, so there is no simple solution to incorporate surge into models of hurricane strength, and the proposed storm surge scales “do not consider these local factors that play a crucial role in determining actual surge impacts.”
NOAA has instead championed its use of a case-by-case “probabilistic surge product” and explicit storm surge warnings that could be issued by the National Weather Service “to provide specific and qualitative information to support decision-making at the local level.”
In East Asia, where storms similar to hurricanes are known as typhoons, meteorological agencies resisted using the Saffir-Simpson scale for many years. Fearing that people might not take Category 1 typhoons seriously, the common position of the agencies until the Past few years was that all typhoons were dangerous. Even a Category 1 typhoon could easily sink some of the region’s many fishing vessels, they contended.