Hurricane Sally was a Category 2 at landfall. Hurricane Michael was adjusted upward after landfall, to Category 5, when later data showed that to be realistic. Hurricane Dorian was not a Cat 6; the scale stops at Cat 5.
The Saffir-Simpson Hurricane Wind Scale has been used for over 40 years to give the category of a hurricane, based on the highest-sustained wind speed anywhere within the storm. It does not account for wind gusts. It was named after Herb Saffir, an engineer, and Robert Simpson, a former director of the National Hurricane Center. The scale originally included storm surge, but storms like Katrina made it clear the landfall wind and storm surge don’t always match up.
If you use the scale for what it is — just a wind scale — it’s fine. If you want more information, it’s not. There are ongoing research projects and proposals to improve or replace the scale. Hurricane threats include wind, storm surge, rain and tornadoes. How does one put all of those into one scale, as the threats can all be independent? How would you rate the impact of these scenarios?
- A weakening Cat 1 makes landfall with high storm surge and produces dozens of tornadoes, hundreds of miles inland.
- A Cat 5 in the Gulf makes landfall as a tropical storm but lingers to leave 40 inches of rain.
- A fast-moving, small Cat 3 hits in late November, when trees have fewer leaves, and steadily moves inland, with no tornadoes and little rain.
It would be a tremendous undertaking to replace the Saffir-Simpson scale with an impact scale because you have to account for construction type and age; population density; land cover; tree health, age and type; soil moisture; time of year; bathymetry; and topography. Those all then have to be integrated with precise projections of flood, surge, tornado, wind, storm radius and storm motion. Impact also means accounting for human action in preparation for things, which if not completed might result in projectiles and debris, clogged drainage, overturned trucks, blocked evacuation routes and runaway boats and barges.
Predicting the impact of a hurricane landfall is like predicting the outcome of a two-car collision. The speed, size and weight of each vehicle is a big deal, but so is the angle of collision, the age and design of the cars and whether they have airbags or not. Critical to the outcome is whether the passengers are wearing seat belts.
A hurricane impact forecast leads to the issue that a storm that is meteorologically identical to another will have widely different impact in different regions and at different times of the year.
A single number, category or designation to describe an incoming storm and all of its threats and impacts would still mean you must digest the entire forecast from a trusted source. You’ve also got to follow the recommendations and instructions of your local emergency management for the threats particular to your area. I state this, categorically.
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