8/18 Wisconsin tornadoes

[mikegeukes]

More information on the F-Scale, also there are many more links.

A Guide to F-Scale Damage Assessment
http://meted.ucar.edu/resource/wcm/ftp/Fin...ssmentGuide.pdf

Mike

 

[Brett Adair]

I am currently talking with a NWS-MKE meteorologist. He is trying to get the tornado upgraded to F-4 as we speak. I will give you more reasoning as I recieve it.

Edit: Bill thinks that the key is the tornado was nearly stationary at some points in time. He has taken photo's and there are areas of where F-4 damage was reached he said. Also, the entire office found it interesting that debris was strown 60-70 miles away from the tornado at the NWS office. He thinks he has the evidence to get the WCM to get it upgraded to F-4 sometime this week.

 

[Jeff Snyder]

I am currently talking with a NWS-MKE meteorologist. He is trying to get the tornado upgraded to F-4 as we speak. I will give you more reasoning as I recieve it.

Edit: Bill thinks that the key is the tornado was nearly stationary at some points in time. He has taken photo's and there are areas of where F-4 damage was reached he said. Also, the entire office found it interesting that debris was strown 60-70 miles away from the tornado at the NWS office. He thinks he has the evidence to get the WCM to get it upgraded to F-4 sometime this week.

I still don't know how damage being thrown long distances is any good indicator of tornado strength. If I'm not mistaken, mid and upper-level flow was strong that day, so any debris that made it to the midlevels of the storm would quickly be whisked away downstream. I wish I still had the presentation, but Tim Marschall made a talk a couple of years ago regarding the La Plata tornado. Areas that were preliminarily rated F4-F5 because the "foundations were wiped clean" were later found to be as low as F1-F2 IIRC. Some houses are built very poorly and not attached to the foundation very well (if at all). Again, IIRC, Tim found that some houses were "wiped clean" by winds of only 130-140mph -- he deemed this "sliders". It does NOT always take a lot of wind to demolish a house! I'm not saying that this isn't the case this time, especially if you're getting it from a (seasoned?) NWS met. I just think that some folks see a clean foundation and automatically think F4-F5.

 

[Glen Romine]

I'd certainly like to see the detailed survey report - describing what damage the team found, and the basis for the assessment. I've seen some images that superficially look F3, nothing that clearly would imply F4, but really there isn't that much available on the net that I've yet seen to make an even somewhat educated guess. As for the possibility of an upgrade to F4, I thought that power was removed from local NWS offices and only a QRT can assign F4 or greater intensity. maybe someone more in the know can comment further.

Glen

 

[Andy Wehrle]

From my observations, many of the houses in the Williams Drive neighborhood where the worst damage was observed are well-built frame houses. So are the ones in my neighborhood, where the fatality occurred. This tornado was quite capricious, even by tornado standards. On the north end of my neighborhood (north of Skaalen Road) most of the houses are still standing, some with all or part of the roof missing, some with very little damage at all. Two are completely razed to the foundation, and one is missing the entire second story with the first story intact (the Orlofske's house, where the fatality occured).

I think topography may have played a role in the damage here. Many of the houses in my neighborhood are built into the side of a hill (so they appear as 2-story from the bottom of the hill and 1-story from the top). Much of the hill is densely covered by trees. I believe several houses (including mine) were sheltered from the worst winds because of their position in relation to the hillside and trees. The houses that were completely destroyed were located such that they were fully exposed to the wind.

The pattern of damage (flattened house next to relatively intact house) is also consistent with a multiple-vortex tornado. I believe the "ambient" tornado circulation was relatively weak, perhaps F1-low F2, but it contained imbedded intense suction vorticies that produced the F3/possibly F4 damage.

My house was either in the edge of the tornadic circulation or in an inflow jet. Either way it was very close- <50 yards.

 

[Robert Dewey]

I thought the F-scale was primarily a damage scale, and then an estimated strength is assigned based upon the damage. Over time, houses will continue to be built stronger and better - so what exactly is the "norm" when comparing damage? It would seem that over time, tornado ratings would be skewed given the change in housing materials, construction methods, etc..

I guess there should be some "norm" as to what a strong framed house is, and that "norm" should remain the same for years to come. This means that tornadoes which were rated in the 1970's may have actually been weaker when compared to today's standards, or then again - maybe tornadoes today are rated too weak compared to the "norm" that was used years and years ago... Just some food for thought.

Honestly, I think that the WI tornado, based on Andy's description, would probably be very high-end F3, or more likely an F4. A tornado which occured in MI during the July 2, 1997 outbreak generated an F3 rating, and didn't do nearly as much damage - perhaps that one was rated too strong?

 

[Jeff Snyder]

Just a reminder that most tornadoes only produce "max damage" over a very small part of their lifetime/track. For example, the "average" F3 tornado only produces F3 damage over a very small part of their track. I think the Moore/Bridgecreek OK 5-3-99 F5 only produced F5 damage over well less than 1% of the total area affected by the tornado. All you need is a single structure that was deemed to be "F5-level" damage for the entire tornado to receive an F5 rating, regardless of it's "strength" over the rest of its track.

It's been noted that the Fujita scale was mathematically developed so that F12 = mach 1. Now, I'd REALLY like to know if Fujita developed the F-scale based on a derivation of probable damage from particular wind speeds, or a derivation of the probably wind speed for given damage. It's generally understood that the F-scale is a DAMAGE scale, as Rob noted. However, if that were the case, then "structural engineering" wouldn't matter, since a flattened house is a flattened house. However, the fact that we've begun to realize that not all houses are built equally implies that probable wind speeds are being incorporated in damage assessment, and thus the F-scale isn't an entirely damage-based scale. Well, maybe it is, but the refinement of the scale strongly relies on an examination of the type and degree of damage relative to wind speeds.

 

[Alex Lamers]

I am currently talking with a NWS-MKE meteorologist. He is trying to get the tornado upgraded to F-4 as we speak. I will give you more reasoning as I recieve it.

Edit: Bill thinks that the key is the tornado was nearly stationary at some points in time. He has taken photo's and there are areas of where F-4 damage was reached he said. Also, the entire office found it interesting that debris was strown 60-70 miles away from the tornado at the NWS office. He thinks he has the evidence to get the WCM to get it upgraded to F-4 sometime this week.

The Fujita Scale has its limitations and there is the over/under 1 rule that from what the tornado is ruled it could THEORETICALLY be one rating higher or lower. So under that rule of thumb, this tornado could be anywhere from F2 to F4. Second, you need to consider building construction and other factors that you cannot merely determine from a photograph. Although the damage looked bad (and it was for sure) "armchair surveying" typically isn't too accurate although it can be an interesting challenge if you are a little bored or intrigued.

In this particular case, and any case for that matter, you cannot judge the intensity of a tornado based on debris being lofted miles away. Surely it takes a strong updraft, but there are no conlcusive studies (at least that I am aware of) that suggest that you can measure intensity by that factor, nor is it common practice to use such information in NWS Storm Surveys despite how interesting it is.

Now on to the actual point at hand...F4 or not? Bill has said...while there is evidence of...and I am placing this in quotes due to the subjectiveness of the Fujita scale..."F4-type damage", it is not sufficient enough or conclusive enough to classify the Stoughton tornado as an F4 at this time, hence the F3 rating that you see on the Public Information Statement which is accurate based on current information. Reports from eyewitnesses said that the tornado was moving relatively slow over Stoughton. The exact figures are not available for you yet but lets just say that several minutes of exposure to 200 mph winds would cause "F4-type damage" but this is not necessarily indicative of an F4 tornado. The quoted information is not necessarily accurate in the sense that the high-end F3 rating seems plausible.

As you all may imagine, the process of evaluating double-digit tornadoes spread across a CWA is not a quick and easy process. I know you are all eager for information, and the office is working to get the information out as quickly as possible for all interested parties. As far as detailed damage assessments and county maps, those will be out within the week...no promises on exact timing.

Hope this helps clear up some issues...

Alex Lamers/Bill Borghoff
WFO MKX

 

[nickgrillo]

I thought the F-scale was primarily a damage scale, and then an estimated strength is assigned based upon the damage. Over time, houses will continue to be built stronger and better - so what exactly is the "norm" when comparing damage? It would seem that over time, tornado ratings would be skewed given the change in housing materials, construction methods, etc..

I guess there should be some "norm" as to what a strong framed house is, and that "norm" should remain the same for years to come. This means that tornadoes which were rated in the 1970's may have actually been weaker when compared to today's standards, or then again - maybe tornadoes today are rated too weak compared to the "norm" that was used years and years ago... Just some food for thought.

Honestly, I think that the WI tornado, based on Andy's description, would probably be very high-end F3, or more likely an F4. A tornado which occured in MI during the July 2, 1997 outbreak generated an F3 rating, and didn't do nearly as much damage - perhaps that one was rated too strong?

Yeah, and that thought comes to mind when reviewing alot of climotology. Taking a look at MI's climo data, there hasn't been a violent (F4/F5) for nearly 30 years... What explains that? Prior to 1977, there were violent tornadoes pretty often -- with two F5's occuring in the same decade. Then, as the years progressed, you'd see weaker and weaker ratings. There hasn't been a single tornado rated higher then F2 for nearly 10 years...

Is that the case? Obviously, houses are built much stronger then they were 30 years ago, so if a tornado was rated F4 based on demolished houses in 1975, then what would it be rated today?

 

[mikegeukes]

Getting off-topic somewhat.
The Fujita scale came about in 1971, 1980 the NWS started to verify warnings, in fact looking at Storm Data, the F-Scale started to appear
in the early 80's in Storm Data. The Super Outbreak in Storm Data, gives no F-Scale Rating.

From: A Guide to F-Scale Damage Assessment

The Fujita scale became the standard for estimating the intensity of tornadoes in the mid-1970s.

Along with this windspeed scale, Fujita included a description of the damage associated with each category – those in Table 1 are taken from the original reference (Fujita 1971).

In addition, Fujita used a set of damage photographs (Fig. 2) to illustrate the intensity categories. With the introduction of these materials, it became possible for someone surveying the damage from a tornado to estimate the F-scale (implying an estimate of the range of windspeeds).

Further, using newspaper accounts and photographs, it became possible to assign an F-scale to historical events, a project which was undertaken by the National Severe Storms Forecast Center (NSSFC), with the support of the Nuclear Regulatory Agency in 1976. Students were hired to do the necessary research into old newspapers and other accounts of storms.They assigned an F-scale rating to as many historical events as possible based mostly on newspaper accounts and photographs.

In 1973, the official authority for doing Storm Data passed from the state
climatologists to the NWS offices. From the late 1970s to the present, it has been expected that all tornadoes that become part of the Storm Data record will have an F-scale number assigned to them, as well as estimates of the path length and width.

Moreover, the actual application of the Fujita scale has been complicated by the frequent absence of standard structures by which the intensity could be estimated.

When a tornado passes through open country (or through vegetation for which no reliable standards exist) during some or all of its path, there is no obvious way to apply the standards developed by Fujita’s work for those segments of the path.

Therefore, it becomes challenging to know what the windspeeds were. Further, the F scale estimate applies to the entire event, whereas the rating is based on what might only be a single point of the worst damage. Of course, Fujita recognized that a tornado damage path can be complex in terms of the variation in damage along it.

For Michigan, since 1990, for F3 and higher tornadoes occurred on the following dates: 09/14/90, 03/27/91, 05/28/91, 06/21/96, and 07/02/97

Mike

 

[Scott Currens]

Nick Grillo wrote:

Obviously, houses are built much stronger then they were 30 years ago, so if a tornado was rated F4 based on demolished houses in 1975, then what would it be rated today?

Are homes really built stronger today? I don't have any facts to back this up, but I think builders cut more corners now than ever.

Shane have you seen a change in the quality of new home construction?

 

[Gabe Garfield]

Are homes really built stronger today? I don't have any facts to back this up, but I think builders cut more corners now than ever.

Good point, Scott. One of my good friends was a house framer about a year ago, and he told me that house constuction is terrible these days. In fact, he told me a story about some home builders who built the house an entire foot off of specifications. They had to make some major adjustments to even make the house look somewhat presentable. A house is as only good as those who built it.

I also wanted to say that the existence of a strong updraft is at least qualitatively an indicator of tornado intensity. If you think about it, a strong updraft would lead to strong evacuation of air mass. This evacuation of airmass would cause the development of a strong low pressure center, which would in turn create a strong horizontal pressure gradient. Finally, a strong horizontal pressure gradient would lead to strong horizontal winds (i.e. winds that cause the damage at the surface).

Just the mere fact that the Stoughton tornado was able to loft debris to such incredible heights (reaching 500 mb? :shock: ) suggests that it was at least a "strong" class tornado. IMO, I think the Stoughton tornado was at some point capable of producing violent class tornado damage. The damage survey apparently hasn't produced any compelling evidence that this was the case, however. But, as Jeff pointed out, tornadoes generally produce extreme damage in relatively localized areas, so it is possible that this tornado was violent, but wasn't hitting anything at the moment.

Gabe

 

[John Farley]

Even after some downward revision in the preliminary number of tornadoes for August 18 (from 28 to 24), the total number of tornadoes in Wisconsin for 2005, now standing at 54, easily breaks the old record for number of tornadoes in a year. A public information statement providing details on this can be found at:

http://kamala.cod.edu/wi/latest.nous43.KMKX.html

As many tornadoes as there have been this month, there were (as of today) even more in June of this year.

 

[Aaron Kennedy]

Gotta disagree with you Gabe....

I also wanted to say that the existence of a strong updraft is at least qualitatively an indicator of tornado intensity.

Studies have shown that updraft intensity actually decreases around the time of tornadogenesis. This makes sense considering the mesocyclone is typically going through occlusion. The occlusion cuts off the updraft, weakening it... often (for cyclic supercells), a new UD develops ahead of the surging RFD to the southeast of the primary circulation.

Aaron

 

[Shane Adams]

Also, there are photos from Stoughton of a home where it was completely demosished down to parts of the basement. That leans me more towards and F-4 rating in some locations.

Don't get your hopes up. The 6-12-04 Mulvane, KS tornado completely leveled a two-story home and removed two walls from the basement, and still wasn't upgraded to F4 from F3 (which several chasers believe is a mistake). Apparently, you'll need debarked trees to confirm an upgrade from F3 to F4; leveled houses and cars thrown hundreds of yards aren't enough. I can understand the house construction making a difference...but a car is a car.