Is it time to say goodbye to the Saffir Simpson Wind Scale?

Among Meteorologist, the limitations of the Saffir Simpson Wind Scale is well known; but that doesn't really apply to the general public. The talk about the need to upgrade or just do away with the Saffir Simpson Wind Scale has been around for quite some time. But here we are in 2018 still talking about it.

What is the Saffir Simpson Wind Scale:

The Saffir Simpson Wind Scale was devised by civil-engineer Herbert Saffir and meteorologist Bob Simpson in 1971.  The Scale has always been about assigning a category from 1-5 based on maximum sustained wind speed.  The measurement is a one minute average at 10 meters above the surface.  The scale also associated a minimum central pressure and most likely storm surge values.  The surface pressure was included to help assign a category to the hurricane. Because at the time it was very difficult to accurately measure surface winds from the recommence aircraft.  But to help measure those surface winds, dropsondes were developed in the 1990's. A dropsonide is a device dropped from the reconnaissance aircraft to measure storm conditions within a tropical cyclone. As it collects the data it transmits it back to the aircraft.  Then in 2005 the improved stepped frequency microwave radiometer (SFMR) was deployed.  The SFMR  Is a small sensor under a hurricane hunter aircrafts wind. that measures surface wind speed, that uses 6 microwave frequencies to accurately measure hurricane force winds. Because storm surge for a peculiar Category is sometimes higher or lower than the scale would call for.  Because of this the National Hurricane Center now only uses the wind scale without including surge height.       

 

   

A recent history lesson:

I started looking at a particular  tropical wave over Africa on Aug 26, 2018. Even then I could see the potential. On the 28th, the National Hurricane Center (NHC) started to officially track the wave; at that time they gave it a 20% chance for development over the next 5 days. The NHC dropped the chances to 0% the next day.  On the 30th, the wave that would become Major Hurricane Florence move off of Africa and into the Atlantic Basin. On Sept 1st the tropical wave, was officially given the name Tropical Storm Florence. On the 4th of September she became a Category 1 hurricane. The next day she became a Category 4 storm. But by the next day she was downgraded to a tropical storm. On September 9th she once again became a hurricane. The next day she was once again a Category 4 with max sustained winds of 140 mph.  Over the next nearly 4 days she remained a Major Hurricane ; as she dramatically slowed down. Then Florence rapidly weakened but continued to increase her wind field..  She made landfall near Wrightsville Beach, North Carolina as a Category 1 hurricane. Her main impact as she moved inland aside from a 10 to 12 foot storm surge, was days of heavy rain that caused catastrophic flooding. Some locations received 4 feet of rain. 

Here are some Maps that show Florence's track and some of the dates and times that I refer to above.

 

 Meteorology and Florence:

Most weather outlets called Florence unpredictable, Odd, and weird. Many thought she took a bizarre track. While it is true, by September 4th she was in a location where tropical cyclones typically recurve north and out to sea.  Florence made history on the  7th of September. On that date, no tropical cyclone in recorded history had  passed within 100 miles of that location and still make an US Landfall. She was also the farthest north Atlantic Basin Category 4 hurricane  in recorded history.  She played lots of tricks. But I wouldn't call her unpredictable.  Over a week out I forecasted Florence would most likely make a landfall between Wilmington and the Outer banks (Cape Lookout). The setup in the Atlantic was perfect for a storm like Florence to take the path she did. Hurricane models couldn't see it. But a person doing her homework and looking at the overall setup could see it. This is why models are only one of the tools available to a meteorologist. Meteorology is a complicated science far too complex for our current weather models to handle. Maybe in 20 to 30 years the models might be very accurate. But not today. The only thing that can unwrap the complexity is the human mind.  Meteorologist all have different interpretations, understanding, and methods. This is why forecast can be quite different from each other. Sometimes these different approaches can cause confusion.

There has to be a better way:      

The fact that she fooled many people (including the National Hurricane Center) and her rapid weakening two days before landfall, along with her slow forward speed. Caused a lot of confusion and most likely contributed to the loss of life that occurred.  This is where the SSWS comes into play.

The private weather forecasting outlets like Weatherbell Analytics  have come up and use different impact scales.  I've talked and wrote on my belief that we need to change the way we convey the dangers of severe weather to the general public. Both the Storm Prediction Center (SPC) and the National Hurricane Center (NHC) need to update how they go about things.  The good news is the SPC has been moving in the right direction in this regard.  However, the NHC has been much slower to make needed changes. The NHC needs to upgrade not only the Saffir Simpson Wind Scale but also their "Cone Of Uncertainly"  I've never been a fan of tracking just the center of the storm. It needs to be upgraded to show the breadth of the dangerous tropical weather. The public has a very poor understanding of severe weather. When it comes to the NHC cone they figure the most danger must be in that center that is being tracked. But this kind of thinking is very precarious and can lead to deadly outcomes. Wind and water are a very deadly combination. The roots of the public misconceptions lie at the heart of the Saffir Simpson Wind Scale.

The public misconception involving hurricane categories:

Hurricanes like Florence and Sandy lead to dangerous assumptions from the public.   The news media is fixated on the category number. Part of this goes back to my post on weather hype. But when the public learns that a hurricanes max sustained winds are diminishing the danger they are in also goes down. When Florence rapidly loss intensity going from a major hurricane to a category 1; many people stopped paying attention and decided to not evacuate. The Same thing happen with hurricane Sandy when she came ashore in 2012. Prior to Sandy's landfall the NHC issued hurricane warnings; But two days before landfall she was downgraded to a post tropical storm. The reason the NHC did this had to do with procedural reasons and nothing to do with impact. When Sandy made landfall near Brigantine, New Jersey she came in with the force of a major hurricane. I call her a major not because of her wind category but because of her central pressure, size, and impact. The storm surge that came in with Sandy was catastrophic for the New Jersey and New York State coastlines.  In Sandy's wake, 72 people died in the Mid-Atlantic and Northeast. Would people have evacuated Breezy Point, the hardest hit area during Sandy, if the emphasis was less on names and categories and more on impact? The stories people had who rode it out say yes they would have.

Florence weakened before landfall, why?:

  Why did Florence weaken approaching the North Carolina Coast?  One reason was that she continued to grow in diameter. As she went through eye wall replacement cycles, instead of dropping her central pressure, she instead became a larger storm.   The 2nd reason is the bigger the hurricane the faster it will weaken approaching the coast. Small hurricanes like Andrew like to strengthen approaching the Coast. But large storms like Florence or giant storms like Sandy normally won't.  The reason for this is the bigger the storm the more effect the land has on the hurricanes inflow, cutting off  part of the moist warm air that is feeding the storm; also the land interaction causes a distortion in the pressure pattern in and around the hurricane.  When the focus is on a number the main dangers from a land falling hurricane are glossed over or just forgotten.  Believe it or not, wind speed is not the main reason people die in hurricanes. Most  of those who die in land falling hurricanes drowned.

But Florence was trending South?:

As the NHC Cone Of Uncertainly shifted south, people north of the center of circulation thought they were safe. But nothing could have been farther from the truth. The area to the right of the storms direction of movement is called the right front quadrant. The right front quadrant of a tropical cyclone is the most dangerous area of the storm. This area due to the direction of the winds has the most destructive  winds and the highest storm surge. It is also the area most likely to see tornadoes; due to the fact that surface winds are stronger in this sector; this caused a change in  the wind speed and direction of winds with altitude. This is called wind shear. The more veering wind shear you have the greater the likelihood for tornadoes.        

Why the Saffir Simpson Wind Scale needs to change or be replaced:

The problem with the modern Saffir Simpson Wind Scale is the fact it doesn't account for size, central pressure, rainfall potential, storm surge, and tornadoes. Other important factors not considered are , forward speed, angle of approach, and the shape of the coastline.  

The overall size of a hurricane is a huge deal. The larger the wind field the greater the area impacted.  The larger the hurricane the more people effected and longer they will be impacted by hurricane force winds. The cumulative effect from a long duration hurricane's wind field is staggering. Florence was the perfect example of this.

Storm surge is the component that has the greatest potential to kill people and cause destruction. Storm surge is a wall of sea water approaching and moving ashore. Surge is very much like a tsunami. Wind driven waves sit on top of the surge.  Storm surge is a factor of the  size of the hurricane, forward speed, shape and characteristics of the coast line, central pressure of the hurricane, and the angle of impact. All of this has to be considered when dealing with storm surge. If a hurricane's right side is approaching a part of the coast that has a concave shape. The height of the surge will be higher, because the coast shape is cupping the flow of the water.  The slope of the continental shelf the surge is moving over is also important. A shallow slope will mean higher heights of sea water coming ashore.

 Rainfall from a slow moving storm like Florence will cause massive inland flooding. When a tropical cyclone makes landfall people drown in both salt and fresh water.  Bill, Irene, Lee, Harvey, and Florence brought catastrophic flooding rains to regions far removed from the point of landfall.  Tropical cyclones often have a one two punch aspect.  Wind and surge impact the coastal region, then a larger area deals with the rainfall.  If you drop 20, 30 or over 40 inches of rain on flat ground you will get fresh water flash flooding and urban flooding.  If you drop a foot or two of rain in hilly or mountain terrain the flooding problem is much worse.  Florence took advantage of a very wet summer in the Mid Atlantic; the ground was already saturated from near record to record rainfall. Florence crawling along dumped copious amounts of rain quite dangerous because of the rain type that falls.  There are two types of rain, "cold rain" and "warm rain" . The type of rain we in the Northeast are acquainted with is cold rain.  Cold rain is non tropical. It starts high in the cloud as snowflakes; the snow melts into raindrops on the way down. Warm rain is tropical in nature. It starts as small droplets. As they descend they collide with other droplets that are moving slower. As a result of the collision the droplets can split or merge. The ones that split fall and hit other droplets. The ones that merge grow into very small drops and continue downward. a little faster. The small drops crash into other drops splitting or merging and falling even faster, This process continues all the way to the ground. This is why tropical rainfall has large rain rates and so much collects is a very short time.  Warm rain, is a very dangerous affair, and has lead to many deaths.    

How much force does wind have anyway?:

There are groups that have been working on all these aspects. A team from Purdue University: Dan Chavas, Kevin Reed, and John Knaff came up with an approach that uses Integrated Kinetic Energy. There is a lot of high level math involved. But the kinetic energy of an object depends on the square of its speed, and is directly proportional to the mass of the object.   When we're outside we can feel the kinetic energy force of the wind.  On a gusty day we can sometimes hear the house respond to that force. Let's say a 50 mph wind gust hits our house inside we can surely hear the air collide with the house.  How much force is being slammed into the house? The answer to that is easier to show using math. But I will forgo that and try to give you and understanding just how much power wind has. 

The mass of an object is its density multiplied by the volume. After doing the math (trust me I did and this is right) Moisture laden air has a density of close to 1 kilogram per cubic meter, and 50 mph is about 22 meters per second.  After doing more math, we find that a standard 2-liter coke (soda for you Yankees :) )  bottle filled with 50 mph storm wind, has about half a joule of kinetic energy (A joule is amount of force required to move an object by one Newton force that moves the object one meter). That much force expended would feel like someone poked you in the arm.  If we filled a small car with that 50 mph storm wind we would end up with about 700 joules of wind energy.  Now imagine all of that kinetic energy slamming into your house over and over for a hour then four hours.....Now take into account all the houses in your town or city they are also getting hit with that much wind energy.  One thing to remember, is we're only talking 50mph wind force. That is far from hurricane force. As winds increase the force of the wind grows exponentially. That should give you a sense of the power a hurricane has.  This is why the larger the hurricane is the more dangerous it is. This is also why a larger category 1 storm or a category 1 storm with a lower pressure pushes a greater amount water.   A large category 1 hurricane can have a lot more destructive potential than a small category 4 hurricane.   It is all of this that the Saffir Simpson Wind Scale doesn't convey.

 

A multi faceted rating system:

When issuing tropical advisories and warnings to the general public, the conversation needs to be about impact and not intensity.  I would like to see the Saffir Simpson Wind Scale completely replaced. The reason for this, is that I think air pressure is a better indication of a hurricanes potential than peak wind speed. The difference between the central pressure and the pressure outside of that is known as "Central pressure deficit."  But because of the public's familiarity with the Saffir Simpson category system replacing it is impractical.  When the public hears a category number they instantly know what that means. So any updated rating system has to start with the Saffir Simpson Wind Scale.  Any rating system must take intensity, duration, and size of a storm into account.  It is the only way to know the true damage potential.    

No two tropical cyclones are the same. The aspects across a tropical cyclone are different and some are worse than others.  Each of these aspects can cause different outcomes at landfall. We need a scale that accounts for the size, intensity, and pressure of a tropical cyclone.  Besides a wind rating from 1-5, we need a system that has a 1-5 rating for Surge, Pressure, size and rainfall potential.  These impact scales would be flexible as the storms move inland over different types of terrain.   The surge scale should take into account the characteristics of that specific coastline. The surge scale should also take into account how far the surge is expected to travel inland.  Rainfall could also be identified. A hurricane could have a rainfall category of 2 at landfall. But has the system weakened and merged with frontal systems  or stalled over an area the category could be raised to 4. Something like that would have been useful when Irene and Lee drowned parts of Pennsylvania, New York State, and New England  in 2011.   A rating system 1-5 for central pressure would be a far better indication of a storms destructive potential.  The size of a storm would also have to be integrated into the scale either with a 1-5 category type deal or another way to figure it into the scale.

 

Well that's about it. I do hope I was able to explain why the Saffir Simpson Wind Scale must be upgraded to something that shows the true damage potential of a Tropical Cyclone.