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Just a super chaotic and unstable environment inside a tornado vortex, especially as it traverses varying terrain/surface features, ingests and expels huge amounts of debris, etc. Multiple vortices are a function of a tornado's swirl ratio (basically the balance between the air flowing into the tornado and the updraft), so anything that affects that balance will affect the overall structure. To give you an idea of the typical timeframes, most papers classify long-lived subvortices as any that last longer than 15 seconds. Obviously most are even more brief, like a few seconds or so.

I have seen papers that mention especially persistent vortices lasting maybe a minute or two, but I don't recall much beyond that. Which isn't necessarily to say that it's impossible, of course. My minimally-informed guess is that anything longer than that is probably super rare, if it happens at all. And re: fluctuations in size, I'd assume so? I don't really know for sure but I don't see why that wouldn't be the case, at least with those vortices that last long enough. Pretty much nothing about a tornado is stable usually.
Perhaps tornadoes that travel over extremely flat land are able to maintain individual subvortices for much longer due to the more stable and consistent terrain? Dixie tornadoes definitely have more erratic damage paths and distribution of intense winds, I'm sure the hilly/craggy terrain of the Deep South is the culprit here.
 

buckeye05

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Violent damage from the 4/27/2011 Shoal Creek AL EF4.
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I mentioned many pages back that I’d been looking for photos of fairly significant scouring that occurred near a ranch in the Shoal Creek/Ohatchee area. While these are from a different angle from the ones I originally saw, looks like you found photos of the area I remember.
 
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2 videos of Parkersburg that I've never seen anywhere and only now are being recommended to me on YouTube (unless they've already been posted here and I need to pay attention):

1. It's formation, amazing that it formed as a massive wedge instead of transitioning into one:



2. Apparently this is when it's directly over Parkersburg, you can see that the thing was as wide as its wall cloud:

 
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I haven't seen/read too much on that April 1999 tornado; what little I've found on it was from old-school storm chaser accounts that have mostly dropped off the web by now (except for the one that is linked, and perhaps one or two others). Purely visually based on those photos it looks like one of those tornadoes that probably had F5 potential at some point. That was a high risk day and there was another F4 in Iowa with an even longer track than this one, but the outbreak is best known (to the extent that it even is remembered at all, mostly among weather-nerd circles like us) for the deadlier Cincinnati-area F4 which struck overnight/early morning hours of the 9th.
 

MNTornadoGuy

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I haven't seen/read too much on that April 1999 tornado; what little I've found on it was from old-school storm chaser accounts that have mostly dropped off the web by now (except for the one that is linked, and perhaps one or two others). Purely visually based on those photos it looks like one of those tornadoes that probably had F5 potential at some point. That was a high risk day and there was another F4 in Iowa with an even longer track than this one, but the outbreak is best known (to the extent that it even is remembered at all, mostly among weather-nerd circles like us) for the deadlier Cincinnati-area F4 which struck overnight/early morning hours of the 9th.
The tornado swept clean entire farmsteads according to the NWS though I can’t find any damage photos.
 

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This looks to be the first video I've ever seen of the Stockton MO tornado from 5/4/2003, which was the second in a series of intense tornadoes from the same storm that produced the Girard/Franklin KS F4. The user has disabled embedding, so just click "watch on Youtube".
 

andyhb

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I have questions about several of the surveys that day...

Mostly the tornadoes associated with that particular supercell, but I also think the Gladstone MO tornado in the KC area was overrated.
 

MNTornadoGuy

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Stockton was also a possible violent from that day. It "completely destroyed a frame house and two out buildings sweeping debris across the road. This house did appear to have the walls anchored to the foundation" which once again sounds more like F4 damage.
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buckeye05

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NWS Springfield is still just awful when it comes to surveys to this very day. Them and NWS Memphis are about neck and neck when it comes to bad surveys. They will frequently mention a structure is well-built or anchored with bolts, and proceed to rate it well below the expected rating for the DI, with zero explanation. Another example is how they rated the 5/22/2011 Southwest City, MO tornado an EF3, while acknowledging that it leveled a well-built brick home, tossed a vehicle hundreds of yards, and mangled it beyond recognition. I think their rating process is more along the lines of “because I said so”, rather than carefully balancing the construction quality with contextual evidence.
 
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Robinson lee

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Does anyone here have a photo of the damage caused by the F4 tornado in Edmonton, Canada on July 31, 1987 or some views on the damage caused by the tornado? I've heard that tornadoes may cause F5 damage, but I don't see any damage supporting this statement at present
 

locomusic01

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Does anyone here have a photo of the damage caused by the F4 tornado in Edmonton, Canada on July 31, 1987 or some views on the damage caused by the tornado? I've heard that tornadoes may cause F5 damage, but I don't see any damage supporting this statement at present
There are a bunch here: https://sites.google.com/a/ualberta.ca/tornado/

There's also an excellent damage survey paper (I believe from Environment Canada?) but apparently I didn't put it in my Edmonton folder; I'll find it and post it + some more photos later. Some of its damage was awfully impressive.
 

MNTornadoGuy

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Does anyone here have a photo of the damage caused by the F4 tornado in Edmonton, Canada on July 31, 1987 or some views on the damage caused by the tornado? I've heard that tornadoes may cause F5 damage, but I don't see any damage supporting this statement at present
The F5 damage rumors are false. The worst damage was some leveled metal industrial buildings which is F4 damage.
 
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i wanna know more about the purdin missuori ef2 tornado that occured on october 24. even though it was over shadowed by the two ef3's further south i feel like this one was likely one of the strongest of this year.

i heard people say that velocities came back to up to 230 mph winds. and the tornado looked like several violent tornadoes. can anyone confirm this?
 

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i wanna know more about the purdin missuori ef2 tornado that occured on october 24. even though it was over shadowed by the two ef3's further south i feel like this one was likely one of the strongest of this year.

i heard people say that velocities came back to up to 230 mph winds. and the tornado looked like several violent tornadoes. can anyone confirm this?'
Pretty sure the velocity of Purdin never reached 230mph, far from It actually.
 

MNTornadoGuy

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hmm. well i dont know where those claims come from but it definitely look hella violent. was definitely not max ef2 in strength.
There was no DOW or other mobile radar on it so I have no idea where you got those wind speed claims from. There is really no damage evidence of it ever reaching EF5 intensity or even EF4 intensity.
 
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