NOW RATED A F4 TORNADO: ROANOKE ILLINOIS

This is another one of those events in which the tornado could have easily reached F5 damage intensity, but because it was over open land much of the time we'll never know.
 
I wonder what it was about the obliterated houses that differentiated them from the standard F5 obliterated house - I would like to see a bit more detailed report that describes the structural issues surrounding the final decision.
 
I wonder what it was about the obliterated houses that differentiated them from the standard F5 obliterated house - I would like to see a bit more detailed report that describes the structural issues surrounding the final decision.

Are there any photos of the houses? Usually, F5 sweeps everything off the foundation, and scatters the debris so far apart that you can barely tell a structure was there (unless there was a foundation). F4 will also level the structure, but the debris won't be scattered as much, leaving a pile of "mess" by the foundation.
 
Looks like a lot of bulldozing has already happened before the pics. This makes it difficult to determine a rating using these pictures
 
Looks like a lot of bulldozing has already happened before the pics. This makes it difficult to determine a rating using these pictures

David is correct, some bulldozing had ocurred. I saw the damage the day before I did the more detailed survey and it was certainly characteristic of F4, however.

Scott
 
Just a reminder here that the Fujita scale is a damage intensity scale, not a tornado intensity scale. So, there is no such thing as an F5 that didn't cause significant damage so was rated lower. Does the damage rating scale provide an accurate measure of the peak magnitude of the tornadic winds? Only in a gross sense. It is extremely difficult to assess a posteriori how sound a particular structure or vegetation was. Even with this knowledge, you would still be far short of being able to accurately determine the peak wind speed that object was exposed to - much less the peak within the tornado. So how intense was the Roanoke tornado? We'll never really know, but based on the currently adopted Fujita damage scale winds likely exceeded 200 mph.

Glen
 
I wonder what it was about the obliterated houses that differentiated them from the standard F5 obliterated house - I would like to see a bit more detailed report that describes the structural issues surrounding the final decision.

Happened several times this year -- the Harper F4 comes to mind right away. The house was completely leveled, with no structure remaining that was above-ground. However, I'm assuming two things here that kept the rating at F4:

1. The debris remained in the general area.
2. The structural integrity of the house(s) remained questionable.

Now, the first one may valid the most, but I haven't seen many pre-cleanup pics yet. The problem with using the first reason, however, is that this may be a strong function of tornado residence time over a location. For example, take a tornado with winds near 200mph, and keep it over one house for 30 1 minute, and I will bet that the debris is MUCH more scattered than a 250mph tornado that moves over a location for only 10 seconds. Part of the reason why the Jerrel pics showed parts of the damage path being completely devoid of debris likely has to do with the fact that the tornado moved quite slowly.

The second reason also is can be sketchy, since it seems legit if there are stronger structures that remain standing, but seems sketchy if all other structures (trees, etc) are gone. For the first, take La Plata. Some houses were initially rated F4 or higher because the foundation was wiped clean. However, some detailed work (see Tim Marshall's talk at http://www.nssl.noaa.gov/nsww2003/talks/Ti...im_Marshall.htm ) showed that the houses literally "slid" off their foundations when winds could have been <150mph! However, for the second, one of the main drawbacks of the Fujita scale resurfaces. Say one tornado hits a weak house and the house is completely obliterated, while a second tornado hits a strong house and the house is completely obliterated. Can we say anything about the strength of the two tornadoes relative to each other? Was the second tornado necessarily stronger than the first? No. Both completely obliterated the houses they hit. We can know with more certainty that the second tornado (which hit the 'strong' house) was likely violent, but we can't necessarily conclude the first tornado (that hit the weak house) was weaker than the second, nor can we conclude that it wasn't weaker than the second. This deals with the same problem of an obviously strong or violent tornado staying in a field in western KS and not hitting anything. Everyone may know it was a strong or violent tornado, but *technically*, it may only be able to be classified as an F1.

I can imagine that there are some signficant difficulties when "weak" houses get completely destroyed, since we can usually only say then that the tornado was "at least a strong F3", or something to that tone. It is at this time that the damage assessment team must examine surrounding damage to see if they can rate it higher. How did trees fare? What about the roads? Was asphalt completely scoured of does a road reman? Indeed, in cases such as this, it's much easier to put a minimum rating on a tornado (e.g. - this tornado was at least a weak F4) than to determine it's max rating, since, as has been discussed a plethora of times, there may not be any recognizable damage to say with certainty that a tornado was an F5...

I don't want to rehash the oft-discussed drawbacks of the Fujita scale, since it has been discussed indepth a multitude of times before, and is likely of little utility in the absense of the power to do something about it, but thought I'd atleast give my thoughts...
 
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