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Using the Saffir-Simpson Hurricane Intensity Scale Wisely

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The Saffir-Simpson Hurricane Intensity scale is a convenient reference for quickly describing the intensity of a hurricane, and this system has been in use for decades.  The Saffir-Simpson scale (see illustration) delineates hurricane intensities based on categories founded primarily in sustained wind speed.  Also, the Scale correlates these intensities with anticipated storm surge.  The problem with casually relating wind speed to storm surge is that is does not always work.  There are multiple physical characteristics and phenomena associated with hurricanes that just don't necessarily associate with Saffir-Simpson but are instrumental to understanding the potential impact a storm may have.

Size.matters.  Although tiny storms, such as Camille (1969) and Andrew (1992) can reach Category 5 intensity wind speeds and produce massive local storm surge, their absolute potential for destruction is the result of several other important factors, including the storm's size.  The size of the wind field can vary greatly from storm to storm, and this feature is especially important to how much coastline will be impacted by a storm.  One measure of storm impact size is the radius of maximum wind (RMW). In Camille the RMW was only about 20 miles across.  While devastating locally, that impact has a significantly smaller footprint than, say, Beulah (1968) or Carla (1961).  Compared to Supertyphoon Haiyan (2015), the impact of Andrew was less than 1/10 of the coastal swath.  

Storm surge is the biggest killer from tropical systems.  While the more intense systems generate bigger storm surges, the relationship is complex.  The total water volume lifted and transported by a tropical cyclone (i.e. the storm surge) depends on the storm's pressure differential between its eye and its environment, the size of the storm, and the length of time the storm has maintained a certain intensity.  Therefore, the storm with the lowest pressure relative to its surroundings that has maintained its intensity for a long time, and is relatively large, will produce a larger storm surge over a larger landfall area.  

Another major killer from tropical cyclones is flooding from extreme rainfall.  Two landmark examples of this phenomenon are Hurricane Harvey (2017) and Hurricane Florence (2018).  A storm that is slow moving with a deep tap to abundant oceanic heat content can result in monumental rainfall totals, despite not having an intense wind field.  Emergency managers and forecasters pay close attention to the speed that a storm is moving.  The slower the storm, the more of a rainfall threat it can become.

There are other unique elements to tropical cyclones that are not directly factored into the Saffir-Simpson scale, including production of tornadoes, extreme wind gusts, and the vulnerability of the impact location.  These are reasons why each tropical system must be examined as a unique mosaic of the many varied features that create risk.  It is unwise to assume that you know what is coming if you experienced a certain hurricane in the past and try to compare its impact to what may occur in the future.

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