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I don't think "slow forward speed" is a valid reason to downgrade a tornado's rating at all. In Jarrell, that NIST study concluded the homes in Double Creek were impacted by tornadic winds for upwards of three minutes, but keep in mind that tornadoes are measured by the width of their EF0 damage contour. That means the homes were impacted by winds of at least 65 mph (40 mph on the old scale), for three minutes. It doesn't mean the homes were impacted by F5 winds for that amount of time.Something that keeps nagging at me is the idea that the forward motion of a tornado matters, at least in whats relevant to a rating.
We have countless videos showing that tornadoes basically do all of their damage within the first second of even touching a structure. At best being slow moving just mulches the dirt more and maybe granulates the debris a bit.
Edit: I never got an answer to this question when I asked it back in April, but I have been feverishly hunting down the source of this Minneola KS EF3 photo (2019) since basically the same day. I only ever saw it once, during the event, and since then I have never been able to find the original source. Which sucks because its one of my favorite tornado photos (the colors and mood are exquisite); of one of my favorite tornadoes.
Any ideas?
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In all cases I'm aware of, the most extreme damage a tornado causes is by its 'inner core' and/or subvortices. In those cases, the most extreme winds responsible for EF4/5 damage only last a couple seconds, as can be observed in the video of Joplin taken at the Fastrip gas station.
As for your photo, unfortunately neither Google or Yandex image search could find anything. I suspect it came from a social media post that has since been deleted.