Decreasing Tornado Intensity?

[Alex Goldstein]

So we all know that this past week there was an EF5 in Iowa. When I learned this, I looked back at the old F scale and realize that this tornado would have been a high end F3 maybe an F4. With that in mind I ask the question: are tornadoes in general weaker now than they were years ago? It seems recently, there have not been nearly as many storms that would get high ratings on the old F scale.

Take for instance the fact that from 1999 to 2006 there wasn't a single tornado rated F5 (I didn't read the Greensburg damage survey, but I am assuming it would've been F5 on the old scale). Obviously this takes into account the fact that the ratings are made by damage estimates, and an F5 tornado in an open field that does no damage will get a rating of F0/EF0, but it just seems to me that while the sheer number of tornadoes may or may not be increasing, their intensities are decreasing.

What do you all think? Is it possible that overall, tornado strength has decreased, or is this merely a product of stronger structures?

 

[Chris Foltz]

So we all know that this past week there was an EF5 in Iowa. When I learned this, I looked back at the old F scale and realize that this tornado would have been a high end F3 maybe an F4.

The Parkersburg, IA would have been an F5 under the old scale just as it was rated an EF5 on the Enhanced Fujita scale. What changed was the estimated wind speeds to cause various amounts of damage. Through research, it was shown the wind speed/damage correlation from the old F-scale was overdone on the upper portion of the scale. In other words, it takes a lower wind than previously estimated to cause the same amount of damage. I would highly recommend reading this link for a thorough explanation of this.

With regard to whether tornado strength has decreased, there is no scientific evidence to support this. As with everything regarding weather, it's very cyclical. Not to mention that survey teams were beginning to take into consideration the structural integrity of structures when rating a tornado. The 2002 LaPlata, MD tornado is a perfect example. It was initally rated F5 but was downgraded to an F4 when shoddy construction practices were taken into consideration.

 

[Nick Dawson]

I would agree that there is a lack of evidence really to show that tornado intensity is decreasing. If you think this is true then by all means try to back up your theory. Maybe do a little research if you are really wanting to know and show people. I would be interested to see some results.

 

[Alex Goldstein]

I would agree that there is a lack of evidence really to show that tornado intensity is decreasing. If you think this is true then by all means try to back up your theory. Maybe do a little research if you are really wanting to know and show people. I would be interested to see some results.

I have no idea if its true...
I was just wondering what people thought. It just seems to me that lately we have not seen nearly as many violent tornadoes as we saw in the 90's and even in the early 2000's. Perhaps I am incorrect. I have no idea.

 

[Chris Nuttall]

This pretty much also goes along with the large increase in the number of reported tornadoes. In years past (before 2000), typically, the only tornadoes that were reported were those that were impacting populated areas.

Now, with the advent of cell phones, mobile internet, spotter network, increased media coverage, and an overall higher number of chasers/spotters, more tornadoes are being reported. Since a lot of these tornadoes are over open country, they are going to get lower F/EF ratings.

Also, as was previously mentioned, quality of building construction is also being closely looked at during damage surveys. Houses may appear to have "good quality construction" on the outside, but the frames may not really be ideally built when closely examined, such as proper attachment to the foundation.

 

[Alex Goldstein]

I kind of see what your saying, however, even in the surveys done by the NWS offices they estimate the wind speeds and it seems that the tornadoes are not nearly as strong.

I understand the whole tornado in a field=EF0 idea, but are you saying that because the structures are stronger, the wind estimates are kust wrong and are generally going to be on the weak side? If so I suppose I understand, but doesn't that also mean that there is a problem with the rating system, and that there needs to be a better way to estimate the winds?

Obviously I have no idea what this improvement may be...

 

[Chris Foltz]

Alex,

That's exactly the intent of the Enhanced Fujita Scale. As quoted from the SPC page on the EF scale...

Over the years, the F-Scale has revealed the following weaknesses:

• It is subjective based solely on the damage caused by a tornado
• No recognition in difference in construction
• Difficult to apply with no damage indicators
  • if the 3/4-mile wide tornado does not hit any structures, what F-scale should be assigned?
• Subject to bias
• Based on the worst damage (even if it is one building or house)
• Overestimates wind speeds greater than F3

And the F-Scale has had its misuses over the years:

• Too much reliance on the estimated wind speeds
• Oversimplification of the damage description
• Judge the F-scale by the appearance of the tornado cloud
• Unrecognizing weak structures
  • mobile homes
  • modified homes

And from the discussion on the Enhanced Fujita scale...

When using the EF-Scale to determine the tornado's EF-rating, begin with the 28 Damage Indicators. Each one of these indicators have a description of the typical construction for that category of indicator. Then, the next step is to find the Degree of Damage (DOD). Each DOD in each category is given and expected estimate of wind speed, a lower bound of wind speed and an upper bound of wind speed.

It has been known for some time now that the original Fujita scale overestimated the wind speed necessary to cause significant structural damage to a well-built structure. Not sure if you've read over the link I provided previously, but I feel it would answer a lot of the questions you have. It's not that tornadoes are weaker now, it's that research has shown wind speeds have been overestimated for years and the Enhanced Fujita scale was created to help address some of the "shortfalls" of the original F-scale.

 

[Alex Goldstein]

I did read that but still am a bit skeptical. I understand that the new EF scale was intended to overcome the shortcomings of the F scale.

My question now becomes, how many tornadoes get their winds clocked by radar? I suppose that would give me a better idea of whether there really is a decrease in intensity. Obviously the problem there is that there is no way of being able figure out where the especially violent tornadoes will be beforehand, so tracking their wind speeds would be tough...

Also, what about the tornadoes that were rated F5 before the new scale was put in place. How many would be rerated as less than F5 (EF5) now? In other words, how many of these storms were no brainer F5's?

 

[Chris Foltz]

My question now becomes, how many tornadoes get their winds clocked by radar?

Not sure I understand your skepticism of the EF scale but with regard to the above question, I have but two words... Very few. Obviously, the DOW team along with the Howie Bluestein's group from OU and a few others are working hard trying to better document the ground-relative wind speed of tornadic storms but they can't be on every single tornado obviously. Wind engineering studies and experts such as Tim Marshall (among many, many others...) have provided much needed input into the wind speed classification of tornado damage. If you have a lot of free time, and by free time I mean a few hours, you would probably want to read the full document explaining the make up of the Enhanced Fujita scale. It can be found here...

http://www.spc.noaa.gov/efscale/ef-ttu.pdf

Hope this helps.

 

[Chris Nuttall]

My question now becomes, how many tornadoes get their winds clocked by radar? I suppose that would give me a better idea of whether there really is a decrease in intensity. Obviously the problem there is that there is no way of being able figure out where the especially violent tornadoes will be beforehand, so tracking their wind speeds would be tough...

Unless the radar gets extremely close or we can get an instrument to survive a direct hit, we probably will never know what wind speeds are near the ground. Sure the radar can clock velocities, but they will always be way above ground level. It's simple radar physics. Most of the research done to find the wind speeds for the EF scale was done at Texas Tech using a cannon and wind tunnels.

On the whole, I seriously doubt we are seeing any increase or decrease in tornado intensity.

Also, what about the tornadoes that were rated F5 before the new scale was put in place. How many would be rerated as less than F5 (EF5) now? In other words, how many of these storms were no brainer F5's?

As time as gone on, most damage surveys and tornado events that have happened since the 1980s have been increasingly better documented. We'll probably never know for sure because construction practices weren't taken into account until recently. But, I can tell you for sure the 4/26/1991 Andover, KS, and 5/3/1999 Bridge Creek/Moore, OK, tornadoes are "no brainer" F/EF-5s.

 

[Shane Adams]

Ignore the windspeed estimates and instead look at the damage caused. The only thing that changed from the F to the EF scale is the estimated windspeed required to do a particular type of damage, which is why people think that tornadoes aren't as powerful as they once were (they see lower numbers, they think less intensity). It's hard to make casual interests understand that what's changed isn't the intensity of tornadoes, but rather the 'estimated' windspeeds required to do the damage. If they'd left the name alone, there would be zero confusion.

I'm all for making it as accurate as possible with improvements, but I still don't understand why the name was changed. Anyone in the scientific community knows the scale has been 'enhanced', and Joe Public on the outside doesn't care. If the name had been kept the same, there wouldn't have been a single thread anywhere questioning the change. The windspeed chart would've been adjusted to make science happy, and the public would be none the wiser.

Instead, the name changed, which of course sparked curiosity from the public, who were largely ignorant of the original windspeed estimates, thus causing even more confusion over the new scale. I still get questions from non-chasing people about the EF scale, and all I tell them is "It's the exact same, they just changed the windspeed estimates and the name." But this of course just leads to more questions, because most people don't understand it's a damage scale, not a wind scale.

Which is why I've always wondered why windspeed estimates are even put there in the first place. The public doesn't need to know an F4 'might' have windspeeds around 160mph....they only need to know it can flatten their home.

 

[Greg Stumpf]

The Enhanced scale includes many more Damage Indicators than the old Fujita Scale, and these DIs are separated out by specific Degrees of Damage. This wasn't done for the old Fujita Scale.

Yes, if one is using the FR12 DI (single- or double-family residences) to rate damage, the resulting scale value will be identical (EF3= F3), but the wind estimates will be different. However, if some of the new damage indicators are used, the rating could be higher than using FR12 alone, since these damage indicators couldn't be used for a rating in the old scale.

But then why are ratings going down? Not because of the EF scale, but because of serious efforts to scrutinize construction quality, and include surrounding nearby damage (e.g., trees) to make sure the damage observed isn't due to poor construction. We can thank Tim Marshall for that. He's been a proponent of careful damage analysis from a structural engineering perspective for many years (several decades) prior to the advent of the EF scale but his ideas have only recently gained a wider acceptance (since the La Plata MD event in 2002).