In Defense Of The Saffir-Simpson Scale

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As of this writing, flood waters continue to rise in many areas of North Carolina and points north and east due to the tremendous rainfall brought by (former) Hurricane Florence. Both North and South Carolina set records for the greatest precipitation associated with tropical systems.

As Hurricane Florence approached the Carolina coast, forecasts called for the storm to make landfall as a “major” hurricane. This is a technical definition based on wind speed. A storm with sustained winds over 110 miles per hour is considered a major hurricane. This designation is based on the Saffir-Simpson scale.

Category 1: 74-95 mph

Category 2: 96-110 mph

Category 3: 111-129 mph

Category 4: 130-156 mph

Category 5: 157+ mph

As the hurricane approached the U.S. mainland, the winds began to slowly decrease. In the 36 hours prior to landfall, the storm went from a Category 4 down to a Category 1 based on the reduced wind speed noted by the National Hurricane Center.

As the storm’s winds were decreasing, many news stories had headlines with the words “weakening” or “downgraded” in them. At the same time, professional meteorologists were very vocal in (correctly) reminding people that this was still a very dangerous storm. Let’s not forget that a Category 1 hurricane IS STILL A HURRICANE and the threat to life and property is exceptionally high.

In the case of a weakening storm, the inertia of the storm surge build-up, takes a long time to decrease. In these instances, a storm’s surge is more reflective of a storm at least a full Category stronger.

A lot of discussion the last few days has focused on inadequacies of the fabled Saffir-Simpson Scale. Did the Category accurately or effectively convey the expected impacts? For several days before landfall, forecasters were warning of catastrophic flooding due to the unusual track and slow forward motion.

Foretasted versus actual rainfall from Hurricane Florence. Source: https://twitter.com/NWSWPC/status/1042412919424729090National Weather Service

Without question, the meteorologists “nailed” both the track and the rainfall forecasts for Florence. As with Hurricane Harvey in 2017, the forecast of an epic rainfall event was highlighted days in advance and came to fruition with amazing accuracy.

The numbers are still coming in but the damage from Florence will certainly be over $10 billion and the death toll, tragically, may exceed 50 lives. Since Florence was “only” a Category 1 storm at landfall, does it indicate inadequacies in the Category system of ranking hurricanes? The answer is yes, and no.

Remember that the Saffir-Simpson Category is only a reflection of peak sustained winds at any given moment. It does not make any predictions about rainfall. That said, two of the three biggest storm hazards, wind damage and storm surge, are highly correlated with wind speed – which the Saffir-Simpson Scale accurately characterizes. (Note: the storm surge is a primarily a function of wind speed and coastal bathymetry [forward speed and wind radius are also factors].)

Wind speed at landfall versus total U.S. damages for landfalling hurricanes: 2002-2017. Source: Wikipedia.Brian Brettschneider

The other primary hazard with tropical systems is rainfall. Slow moving systems, with access to deep tropical moisture, can produce prodigious rainfall totals (see: TS Allison, Hurricane Harvey, Hurricane Lane, and Hurricane Florence). This characteristic of storms is unrelated to the wind speed (Category) at landfall.

Most members of the general public living in coastal regions affected by hurricanes are familiar with the Saffir-Simpson Scale. They understand that a Category 5 storm making landfall will cause more wind and surge damage than a Category 1 storm. This is a good thing. With the current state of scientific literacy being what it is, science communicators and emergency planners should feel good that people have a known, reliable metric at their disposal to gauge the intensity of a storm. It also provides a common reference marker for people across geographical boundaries to make shared decisions.

Beyond the Saffir-Simpson Scale, it is the job of the scientific community, the media, local, state, and federal agencies to effectively characterize the hazards posed by any given storm. In the case of Hurricane Florence, the National Hurricane Center did an exemplary job identifying where the storm would make landfall. The Weather Prediction Center did an amazing job identifying potential rainfall totals. Local governments issued evacuation orders, and most people were able to heed those warning. The system worked.

Perhaps we need new ideas to communicate tropical hazards. Maybe a 1-10 scale? Perhaps a color-coded scale? Whatever it is, it will likely be tied to or tiered off of the Saffir-Simpson scale. It has been around a while, it works, and the public knows/accepts it. Let’s not throw the baby out with the bath water.

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