In the weeks since Ike devastated the upper Texas coast, I’ve had a number of people approach me in disbelief, asking for my assurance that Ike was more than a Category 2 hurricane.   But the answer is no, Ike was not a “major” hurricane as defined by the Saffir-Simpson Hurricane Scale.  It was only a Category 2 hurricane.  And Ike’s Category 2 winds were confined to a very small area well east of the center.  Winds around most of Ike’s center were actually in the Category 1 range of 74-95 mph (or lower).   How is it that Ike caused so much damage?  Why was Ike’s storm surge so much higher than expected for a typical Category 2?    Better yet, could there be a better way to classify hurricanes that takes into account now only wind speed but a hurricane’s size?

I can answer that last question first – yes, there is a better way.  After the 2005 hurricane season, I began working with coworker and meteorologist Bob Weinzapfel to develop the Hurricane Severity Index (HSI), a 50-point scale that considers both a hurricane’s intensity and the size of its wind field.  Ike scored 30 out of a possible 50 points on the Hurricane Severity Index, making its destructive potential on par with some of the great major hurricanes of the past.   More on HSI later.

While most people are familiar with the Saffir-Simpson scale, very few understand that it’s only a wind scale.  A hurricane’s Saffir-Simpson rating is given by the peak sustained (1-minute) wind found anywhere in the hurricane.  That’s it – it’s a wind scale.  But over the years, some have attempted to associate an average storm surge to each category, similar to the table below:

The problem with this is that storm surge is independent of a hurricane’s peak wind speed (its Saffir-Simpson rating).  A hurricane’s Saffir-Simpson rating is not even considered when calculating storm surge.  It’s the size of the wind field that’s more of a factor in determining potential storm surge.  That, and the general topography and bathymetry of the coast near the point of landfall, the forward speed of the hurricane and the angle at which the center crosses the coast.

A hurricane with a larger area of hurricane force or greater winds will transport a larger volume of water into the coast at landfall, producing a larger storm surge than a smaller hurricane.  We learned this lesson in 2005 when Category 3 Hurricane Katrina produced a storm surge that was larger than the surge produced by Category 5 Hurricane Camille in 1969.  Residents of the upper Texas coast from Galveston to Sabine Pass were taught this lesson again by Ike.   Even though downtown Galveston resides behind a 17-foot seawall constructed after the Great 1900 Hurricane (a Category 4), most of the island was submerged by Ike’s storm surge.

It’s clear that not all storms are alike, as the image below demonstrates.  On the left is 2004’s Hurricane Ivan.  On the right is 2005’s Hurricane Dennis.  According to the Saffir-Simpson scale, there’s no difference between them – both had 120-mph winds at landfall.  But the red shaded area represents the coverage of hurricane-force winds for each hurricane.  Ike’s hurricane-force wind field was extremely large compared to that of Dennis.  A larger wind field means not only a greater storm surge at landfall, but also a greater chance that a location in or near the path of the hurricane will be impacted by hurricane-force winds.  The larger wind field also produces much larger waves covering a much greater area.  Ivan produced significant waves (the highest 1/3 of all waves) in the 50-55 foot range across the northern Gulf of Mexico.  Dennis produced waves closer to 25-30 feet.

Back to Hurricane Ike.   Though Ike was a Category 2 hurricane, its wind field was very large – larger than Rita’s, as large as Ivan’s and almost as large as that of Katrina.  This large wind field resulted in a storm surge of between 12 and 20 feet that inundated much of the upper Texas coast on the morning of September 13th.   So much for a Category 2 producing a 6-8 foot storm surge, right?  There has to be a better way (than Saffir-Simpson) of estimating a hurricane’s true destructive potential. There is.   It’s called the Hurricane Severity Index (HSI).

After the 2005 season, Bob and I came up with a new hurricane scale to supplement (or replace) Saffir-Simpson.  What we came up with is an index that not only considers a tropical cyclone’s maximum sustained winds, but also the size of its wind field.  It’s a 50-point scale – 25 points are allotted for the maximum sustained winds, the other 25 points are assigned according to the size of the wind field.  I won’t go into all of the details here, you can read about the HSI in great detail at the two links below:

http://impactweather.com/pdf/hsi_explained.pdf  - a full explanation of HSI

http://impactweather.com/pdf/hsi.pdf  - a full description of how HSI is calculated

We can graphically depict some well-known past hurricanes to see where they rank on the HSI:

On the graphic above, a tropical cyclone’s intensity is represented by the horizontal scale.  A tropical cyclone’s wind field size is represented by the vertical scale.  The top right of the graphic represents very large and powerful Category 5 hurricanes.  The lower left of the graphic would represent relatively small and weak tropical storms.  Yes, even tropical storms are classified with the HSI, as opposed to the Saffir-Simpson scale which only classifies hurricanes.  On the HSI, Ike scored 30 points at landfall  – 10 for its Category 2 winds but 20 out of a possible 25 points for the size of its wind field.  It was those 20 size points which was the reason for Ike’s extremely high and extensive storm surge.

So Ike wasn’t “only a Category 2” hurricane, it was a massive Category 2 hurricane that produced huge waves across the northwest Gulf and a storm surge that was much greater than that of a typical Category 2 hurricane.   But it’s incorrect to say that Ike packed a “Category 4 surge”.  That simply isn’t true.   Storm surge is not a function of a hurricane’s Saffir-Simpson rating.  Don’t mix the two.  Category 2 hurricanes can produce a very large storm surge.  In fact, even if Ike would have lacked that small area of Category 2 winds, it could have produced the same storm surge as a Category 1 hurricane.    Think about that – a Category 1 hurricane could put Galveston Island and much of the southeast Texas coast under water.

Many people will have to rethink the wisdom of declaring “I’ll ride out a Category 1 or 2, but not a Category 3 or higher.”  If you’re in a surge zone then you need to get out for any hurricane, no matter what category.

Over the coming months, we’ll be continuing our work on the Hurricane Severity Index.  The plan is to create scales/guidelines that equate a hurricane’s wind field size with potential storm surge threat or possible rainfall.  Combining the wind field size and intensity will always provide a better estimate of the potential threat from an approaching hurricane.

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