Do We Need a Category 6 for Hurricanes?

[Randy Zipser]

Don’t the NHC surge maps encompass these factors, via the color scale of surge inundation?

They might, in broad, general terms. But the idea behind creating a new WRAS is to assign a specific scale number to the anticipated landfall location and surrounding coastal region, similar to the SSHWS for wind risk (Category 1-5). Coastal residents might respond with more evacuation urgency to numbers than just to "broad-brush" color-scale schemes on maps.

Admittedly, the downside to having both wind- and water-risk scales is that both wind and water start with the letter "W," so the messaging might cause confusion with the public (at least at first). One way to resolve that messaging problem, however, would be to have a "W/S" [Wind/Surge] scale designation. For example, instead of Category 4, public hurricane warnings would say "W4/S3" [or "Wind 4/Surge 3"] where both scales have 5 levels of severity (because the public is already used to a 5-level scale). So, there may be no need for a Category 6 after all...

 

[Randy Zipser]

Lastly, the direction (onshore vs. offshore) of winds relative to the coastline due to the direction of approach of the tropical cyclone would have a huge impact on the storm-surge potential.

...to have a "W/S" [Wind/Surge] scale...

It would be possible to have a W5/S1 event, which makes a huge difference as to how the potentially-affected residents would prepare/respond in terms of evacuation.

 

[WallsC]

They might, in broad, general terms. But the idea behind creating a new WRAS is to assign a specific scale number to the anticipated landfall location and surrounding coastal region, similar to the SSHWS for wind risk (Category 1-5). Coastal residents might respond with more evacuation urgency to numbers than just to "broad-brush" color-scale schemes on maps.

Admittedly, the downside to having both wind- and water-risk scales is that both wind and water start with the letter "W," so the messaging might cause confusion with the public (at least at first). One way to resolve that messaging problem, however, would be to have a "W/S" [Wind/Surge] scale designation. For example, instead of Category 4, public hurricane warnings would say "W4/S3" [or "Wind 4/Surge 3"] where both scales have 5 levels of severity (because the public is already used to a 5-level scale). So, there may be no need for a Category 6 after all...

The question probably becomes one of public utility, both with relation to your point specifically, and with regard to the question of this thread about the need for a "category 6."

The Saffir Simpson scale is basically the other side of the coin of the Enhanced Fujita scale we use for tornadoes. We have chosen for practical purposes to define a tornado's wind speed retrospectively by assessing the damage it did, and the SS scale attempts to predict damage based on wind speed. As you note, it is imperfect as a warning metric because it fails to account for the biggest killer in hurricanes - surge. Fortunately, raw wind speed does tend to correlate well with surge, although diameter of the wind field is probably more important from a scientific perspective.

Although more precise, the general public may not digest a dual-rating system as well as we might hope. It is easy to picture someone who doesn't even know what the W and S stand for saying, "Who cares if it's an S4, it's only a W2, I'm fine," and then try to ride it out in their beach house.

A cat 6 rating isn't needed because practically speaking, the damage would be similar. We could probably argue that an EF6 tornado category could be created, but why bother when an EF5 will already strip your house to its foundation just as effectively? It would also be unwise to dilute the scale. Will people take a cat 3 or 4 less seriously if they know it's on a scale of 6?

Scientifically, the min pressure and storm diameter are much more important to assessing ACE and true power of a storm than max wind speed ever will be. The SS scale is primarily a tool for disseminating information to the public in a way that is both concise and actionable.

 

[Randy Zipser]

Scientifically, the min pressure and storm diameter are much more important to assessing ACE and true power of a storm than max wind speed ever will be.

Very true and point well taken. That level of technical "scientific" detail will only likely confuse the public, however. How many people in the non-meteorological community even know what normal sea-level pressure is in millibars, let alone in the inner core of a small-diameter Category 5 hurricane or typhoon (in either millibars or inches of mercury)? And what about the relationship between minimum "eye" diameter and maximum eye-wall wind speed? Such concepts will go right "over the head" of the vast majority of the lay public. As you point out, the simpler SS scale is probably just the right amount of detail that the public needs to know to act upon for safety.

 

[Boris Konon]

This proposal I find most interesting, and has merit.

Surface Pressure a More Skillful Predictor of Normalized Hurricane Damage than Maximum Sustained Wind

https://journals.ametsoc.org/view/journals/bams/101/6/bamsD190062.xml

Continuing the discussion...
The Saffir-Simpson scale is outdated IMHO. This was already shown after 2005 when pressure and storm surge were removed from the table (Katrina was the catalyst - 920 mb pressure at landfall, but "only" 110 kt winds, yet the surge was the highest ever recorded in the U.S. (18 ft or more was Cat 5, and Katrina's surge was 28 ft)!

This obsession w/ wind is often misplaced. Of the 3 main risks in a hurricane, wind is the *least* of the risk overall. And wind becomes moot once well inland as the TC decays, but the freshwater flooding risk often *increases*.

And using category 1-5 misleads the public and officials as to overall risk. Sandy was only a category 1, but resulted in a record storm surge in NJ and NYC, equivalent to what you;d typically see in a category 3.

Keeping it simple in this case I think does more harm than good now.

We should change the scale to an *impact* scale, as that is what really matters in the end to the public. The WSSI (Winter Storm Severity Index) does this well.

Winter Storm Severity Index (WSSI)

Winter Storm Severity Index Web Display

WSSI plots get very specific, accounting for things like elevation.

Not all hurricanes are the same for impact. For instance, does the hurricane hit at low or high tide? It is moving fast or slow? How large is the RMW and radii of gale-force winds? Is there baroclinic interaction or not? Using just wind for designated categories oversimplifies the situation.

And all this would line up w/ what the NWS has become more over time - Impact-Based Decision Support Services (IDSS).

In the idea that we keep the current category scale...
As for any additional catagories, I think I had mentioned before, would one prepare *any* different from a Cat 5 or 6? The answer is NO! Once you get to 160 mph, the damage is complete and the risk extreme, so adding any more is like pouring water into an already full bucket.

I suspect this category notion is pushed to further the climate change agenda ("look, we had to *add* category to hurricanes b/c of climate change!"). Also, it is merely a tool for the media to hype more. In addition, we as a society are *obsessed* w/ labeling things into nice, neat packages/categories, but that can only be taken so far, and more is not always better. Wx phenomena typically exists on spectrum, not into solid categories. It's like the three types of supercells, LP, CL, and HP., but how often do odd hybrids and combination exist?

Having just 1-5 is a psychologically satisfying range. Ending in brackets of 0 or 5 is a good thing in communications and messaging. Sounds more official. There is reason why we see top 10, 25, 50, or 100 for lists everywhere!

 

[WallsC]

This obsession w/ wind is often misplaced. Of the 3 main risks in a hurricane, wind is the *least* of the risk overall.

I definitely agree with this. All it takes is a quick look at the list of most costly US hurricanes of all time. The top two are so far ahead of #3 it's scary - and neither one of them was a cat 5 at landfall. It was the surge/flood/water impact that led to their absolute armageddon-level catastrophic impacts.

In some morbid way, wind speed is way sexier than other metrics. It is also far easier to quantify and predict in advance. That is probably why it continues to be the fall back as the core metric emphasized to the public.

Unfortunately, it is unrealistic to expect the general public to be able to confidently interpret anything beyond a simple 1-5 scale. The best way forward might be to modify or replace SS with something that takes expected water impact into account "behind the scenes," and continue to distill this out as a 1-5 scale for the public.

Alternately, change the warning system entirely. Take the emphasis off of the storm itself, and target the warnings as expected impacts to different areas. There has already been a move this direction with surge warnings and extreme wind warnings, but these products often get released late in the game. Additional products like "severe landslide warning" for mountainous areas could be useful if released far enough in advance. Unfortunately, such warnings would pose significant forecasting challenges to get them in front of the public in a time that would allow for a meaningful response without excessive false alarms.