An F6 Tornado?

[Scott Blair]

Some food for thought regarding wind speeds at varying levels close to the surface...

During several hurricane research trips I was associated with, we used two meteorological towers... one at 2m and 10m. It was quite impressive in the difference of wind speeds throughout the duration of the storm. Take Hurricane Rita for example. The towers were deployed just west of Johnson’s Bayou, LA (the landfall site of Rita) in a maritime environment (no significant obstacles to obscure winds). During the climax of the storm, the sustained and wind gusts varied by nearly 35mph stronger at 10m than 2m. This is significant when considering a 35mph difference in only a 26ft depth.

To relate towards the subject at hand... the difference of wind speeds from 2m (6.5 feet) versus winds at 30m (98.4 feet) would be unbelievably significant. This would imply the May 13th Knox tornado might have had winds near the surface (damage region) of weak F2 caliber, instead of the official moderate F3 rating (2m~120mph versus 30m~177mph). It would appear this tornado was probably overrated in regards to winds found at the surface where damage would have existed. (Now vertical velocities may induce more damage than what’s represented here by just considering horizontal winds... but overall the vertical velocities should be relatively negligible when compared to the horizontal.)


Rob wrote:
As kind of an aside for Scott...nice picture in the June Storm Data...and it kind of ties in with the tornado database...SD has it listed as an F0 with a 30 mile path, but the narrative says "four"different" tornado touchdowns over a 30 minute period northwest of Ekalaka Mt....the tornadoes came from different parts of a large mesocyclone" "at times the tornadoes were multiple vortex" Note they say "tornadoes"...so this brings me to another problem with database accuracy...and maybe this should be a different thread...can you give a better account of what happened there? Was it 1 "long tracked tornado" Or 4 tornadoes? The vagueness of SD submissions is most frustrating.

Yeah, I was somewhat surprised to see Storm Data list all four tornadoes into one tornado in the official database. And you’re right... this is a perfect example of inconsistency when related to the entire national tornado database. This is especially tough when considering the lack of tornado reports in such a sparsely populated region of southeastern Montana.

There were four separate tornadoes in the course of a 45 minutes. The first tornado was a rope and touched down east of the wall cloud. The second tornado was the longest duration and developed in association with the wall cloud. This tornado actually marry-go-rounded across the entire cusp of the wall cloud for a total duration of ~10 minutes. The last two tornadoes lasted ~3 minutes each and developed in-time well separated from each other.

My only guess why it was imputed into Storm Data like this was not enough detailed information was available at the time to the WCM on when each tornado began and ended. If there were only quick gaps between each “tornado/condensed vortexâ€￾, then it would make plenty sense to call it one tornado with intermittent gaps. However, this was not the case on 6/6/05.

Scott Blair
http://www.targetarea.net/

 

[Glen Romine]

To relate towards the subject at hand... the difference of wind speeds from 2m (6.5 feet) versus winds at 30m (98.4 feet) would be unbelievably significant.

The difference is not this bad - you should be relating the 30 m obs to 10 m height, not 2 m, as 10 m is the meteorological standard used for the F-scale. Nevertheless, the winds are probably stronger at 30 m than at 10 m, though that said, nobody really knows that for sure and if so how different are they. There is no reliable analytical model for relating tornado wind speeds at any one height with those at another (we struggle even doing this in an ordinary boundary layer with any consistent accuracy), and nor is there a reliable tool for accounting for centrifuging effects of debris sampled by the radar that dominates the doppler velocity signal. Also, anyone that's looked at closeup tornado video knows there can be significant vertical velocities even very near the surface, so how important is this (unmeasured) wind speed in damage production? I think it was a mistake to 'upgrade' the Knox county tornado, not that I wouldn't want a more accurate database of wind intensity within tornadoes, but because you have to have a consistent means of classification within a database to be able to derive meaningful statistics from it. If I were using storm data for tornado intensity distribution estimates, I'd have to pull this ob out.

Glen

 

[Scott Blair]

Yes, I’m aware 10m is the standard meteorological height. However, in this example it was important to show the differences between winds at varying levels from 2m, 10m, and 30m where damage to my first floor of the house, the top of my roof, and the top of my tall pine tree respectively. There will likely be stronger winds at 30m versus 10m and an even greater difference of wind from 30m to 2m. And agreed, the Knox County tornado was a poor decision based on the aforementioned wind differences at varying levels and the inconsistency with classification.

Scott Blair
http://www.targetarea.net/

 

[Glen Romine]

The biggest problem facing us by using DOW data to justify a specific rating without damage is the simple climatological inconsistency that will result.

Also, for those who may not be up on the latest literature, there has been an effort to try and 'calibrate' winds measured with the DOW's with damage occuring at the ground with the Spencer, SD tornado - (See Wurman and Alexander, MWR 2005). Additional studies such as this one could some day be used to estimate the relationship between radar measured winds and near surface wind damage - but many more studies (~30 for statistical significance) need to be completed. Regardless, I agree the DOW observations shouldn't be used to justify a higher tornado rating - as it diminishes the value of the database. Instead, it might eventually offer a more realistic climatology of typical wind speeds in tornadoes with all of the cases they keep accumulating.

Also Scott, I'm sure you were aware of the 10 m height, but most readers here are not, and your post I thought could easily be misinterpretted.

Glen

 

[Saul Trabal]

Exactly... An F6 will NEVER occur - even if they record winds well in excess of 318MPH, it's still an F5. Why? Because the Fujita scale is purely a damage scale... You can only destroy something so much until there is nothing left to destroy (an F5 would do that).

If this is the case, then the idea of "F6 to F12" is ridiculous. Why bother if what you say is true? That just seems silly. :roll:

 

[Mike Smith]

Go to the middle of page 4 of this thread. Fujita defined it as both wind and damage. There can be an F-6 tornado.

 

[Scott Blair]

I keep hearing the same arguments of why there can be a F6 with no logic. It seems some people would be devastated themselves if they accept what is so obvious. Perhaps this will be another thread that goes in circles, not because there is real evidence a F6 could exist, but because some just cannot admit science and the basis of the scale does not support such a fictional rating.

In attempt to summarize to those that ‘still believe’ in the F6 to come...

1) you have read the last few posts regarding near-surface wind speeds and frictional effects.

2) you understand that the modern day F-scale ranges from F0-F5.

3) The F-scale is based off damage to estimate wind speeds, and total destruction cannot exceed total destruction. F5 is the highest rating possible.

4) Recent research has shown the estimated wind speeds in the current scale are too high.

5) No surface wind speed measured has ever come that close to a high end F5. Again, the fastest wind speed “recordedâ€￾ in a tornado was May 3, 1999 found between 300-318mph. However... this wind was sampled at 50-100m above the surface!!! This is 160-300 feet above the ground!!!

If you still believe in the F6, fine... but just know science and the basis of the scale itself doesn’t support it.

Scott Blair
http://www.targetarea.net

 

[Mike Smith]

Scott,

Re: Point 2. Are you saying Fujita didn't have the right to define his own scale? Are you saying that 1992 isn't "modern day"? I guess I don't understand your point. Fujita (see my post on pg. 4 for this thread) CLEARLY defined F-6.

Point 3. The Fujita scale is defined as both wind and damage. Read the reference I cited from Fujita himself.

Fujita's damage examples for f-5 were based on homes and similar structures. As indicated earlier, an f-5 struck a skyscraper and left it standing. If a skyscraper (for example) were toppled, it might be damage-based evidence of f-6 conditions.

We don't know what the winds were in the Tri-State Tornado, the Woodward Tornado, Xenia and others. All we know is the damage. I don't know whether winds in a tornado have ever exceeded 319 mph and neither does anyone else.

Fujita clearly defined an F-6. Lets see what science measures the next decade or so.

 

[Scott Blair]

Let me make this real simple for you Mike...

1) F-scale rating is based off damage. Wind speeds are derived from this.

2) F5 is THE highest damage possible. It does not matter if every downtown building in some arbitrary city and their puppies are swept clean. It’s still F5 damage.

If you can understand these two simple points, then you can accept there will not be a F6. Period.

I can’t make it much more simpler than this... time to go play in the near-blizzard conditions outside

Scott Blair
http://www.targetarea.net

 

[Mike Smith]

Scott,

What is your source for your contentions?

I have referenced Fujita's own work.

Mike

 

[David Brookshier]

What makes an F5 the final rating in determening damage? thier have been many stuctures such as skyscrapers that have although heavily damaged remained intact after an assualt from an F5. Fujita indicated in his scale that the maximum alloted damage in an F5 is 318 mph. I have heard numerous points about the Fujita scale being strictly a damage scale, if this is truly the case why then did he include an F6 rating. Perhaps it is because he knew that an F5 cannot cause absolute destruction. Many structures have survived without total annhiolation and that's why we need to accuratly measure windspeeds so far as technology advances the process beyond all error then all will have to see an F6.

 

[rdale]

"if this is truly the case why then did he include an F6 rating."

I'd suggest reading this thread... It's been made quite clear.

"Perhaps it is because he knew that an F5 cannot cause absolute destruction."

Then why did he say F5 is absolute?

 

[Mike Smith]

I would like to try this again, so I am re-posting from page 4 of this thread:

I went to Dr. Fujita's own discussion on page 31 of his book, "Mystery of Severe Storms" published in 1992 by the University of Chicago Press. The F-Scale chart clearly shows BOTH wind speeds and damage as F-Scale determinators.

If the F-Scale rating is made using damage then a lower case "f" is to be used before the numerical value.

If the F-Scale rating is made using wind speeds then an upper case "F" is to be used.

So, if one documented a tornado with 325 mph wind speeds, it would clearly be F-6. Fujita DID define the wind speeds associated with F-6 and even used an adjective, "inconceivable", to describe it. He just (at least at that time) didn't speculate how one would tell the difference.

He is absolutely clear on the point: An F-6 is possible and he does not rule out an f-6 (although he did not define what the damage would look like).

A number of people posting on this thread have stated unequivocally that a f- or F- 6 is "impossible." Please reference where Fujita explictly states that it cannot occur. Opinions don't count.

 

[Scott Taylor]

3) The F-scale is based off damage to estimate wind speeds, and total destruction cannot exceed total destruction. F5 is the highest rating possible.

I agree with just about everything you've covered, but just FWIW, I cannot find the words "total destruction" in any of the Fujita scale references I've studied. All of them simply state "Incredible Damage" and go on to describe strong frame houses lifted off of foundations and carried considerable distances to disintegrate, automobile sized missles fly through the air in excess of 100 meters, etc, etc...

The references that do document how Fujita described F6 damage state "Inconceivable damage. These winds are very unlikely. The small area of damage they might produce would probably not be recognizable along with the mess produced by F4 and F5 wind that would surround the F6 winds. Missiles, such as cars and refrigerators would do serious secondary damage that could not be directly identified as F6 damage. If this level is ever achieved, evidence for it might only be found in some manner of ground swirl pattern, for it may never be identifiable through engineering studies."

So while I agree that we won't likely ever see a tornado rated F6, the door appears to have been left slightly open for that to happen. Just because we can't conceive something doesn't mean it isn't possible. =)

Scott

 

[Robert Dewey]

I would like to try this again, so I am re-posting from page 4 of this thread:

I went to Dr. Fujita's own discussion on page 31 of his book, "Mystery of Severe Storms" published in 1992 by the University of Chicago Press. The F-Scale chart clearly shows BOTH wind speeds and damage as F-Scale determinators.

If the F-Scale rating is made using damage then a lower case "f" is to be used before the numerical value.

If the F-Scale rating is made using wind speeds then an upper case "F" is to be used.

So, if one documented a tornado with 325 mph wind speeds, it would clearly be F-6. Fujita DID define the wind speeds associated with F-6 and even used an adjective, "inconceivable", to describe it. He just (at least at that time) didn't speculate how one would tell the difference.

He is absolutely clear on the point: An F-6 is possible and he does not rule out an f-6 (although he did not define what the damage would look like).

A number of people posting on this thread have stated unequivocally that a f- or F- 6 is "impossible." Please reference where Fujita explictly states that it cannot occur. Opinions don't count.

Good points... I've been reading this discussion since my original post, and I am starting to change positions. After reading your post, it makes complete sense - the lowercase "f" is purely for damage, and the upper case "F" is purely for wind. Why has the scale been twisted so much? Now EVERY rating is an uppercase "F", but not for wind. And, Fujita DID say that the only way to identify (yep, identify) and f6 (damage) would be some sort of ground swirl pattern. So, if a DOW clocks a wind speed of 400MPH and that ground swirl is present - then what?