Ethan Moriarty posts about Rochelle: He doubles down on EF4, and states that 200 mph was an overestimation and the true winds were about 170-180 mph. Looks like Rochelle is rapidly exiting the "universally recognized EF5 candidate" category - which is a shame because it absolutely was an EF5. Oh well.
I'm late to this.
Very strange. Perhaps he thinks he's redefining his methodology in general, but this seems less about new evidence and more about a shift in analytical philosophy. I suppose that's fine, but unless you're disregarding the EF scale's own definitions (of course, he would be FAR from the first engineer to do that), 200 MPH - or, to be more straightforward, a rating that basically said "borderline EF5" - remains well within reason for that tornado.
If memory serves me right, the house in question had anchor bolts embedded in a poured concrete basement foundation. There were no significant terrain effects or unusual topo shielding (that could explain "exaggerated damage" or whatever). IF memory serves me right, nearby objects and vehicles also sustained intense damage (again, I'm open to correction). An elevated subfloor oculd introduce some vulnerability, but the anchoring method and lack of widespread failure elsewhere in the structure don't suggest much.
The EF scale guidelines specify that devastation of a well-constructed home with anchor bolts and a basement suggest winds around 200 MPH (fitting with the EF4-5 crossover zone). That is why experts leaned toward the high end, especially because the structure was just totally eradicated. His revised estimate - 170-180?? - assumes a very significant downward adjustment *without clear new evidence.* It's a totally different interpretation. I think that's extremely conservative, to put it mildly, especially without a compelling structural reanalysis. I guess some fluid dynamics concepts have him convinced of something. But I don't think the forensic evidence, so to speak, supports that. The real-world destruction from Rochelle was extremely impressive. I don't see it as "let's reduce the estimate from high-end EF4 to almost low-end."
And, of course, that was far from the only EF4 level damage point on the tornado's path.
Moriarty does a good job overall and produces interesting content. If he's being this, IMO, unduly conservative - and, frankly, almost speculatively conservative - it really makes me wonder about the rest of the engineering type folks in the tornado rating business.
I'm not overawed by theoretical revisionism.
As for the sidewalk, I assume what he's getting at is: approximations of wind speed based solely on surface effects, like the sidewalk, might overstimate actual tornado intensity. That's probably not entirely wrong from a modeling perspective, but using that logic to downplay observed catastrophic damage is .... something. For one thing, the EF scale was supposed to be designed with boundary layer effects in mind. It doesn't just assume a straight 200 MPH wind at ground level, it interprets the effects of those winds. Second, there were multiple indicators - besides the sidewalk - of extreme winds. Third, again, this is essentially overriding the scale in a sense. I also find that sidewalk situation simply hard to explain without violent winds.
A slab of concrete flush with the soil has no structural entry points for wind to get under unless the soil itself is disturbed or the pressure gradient is high enough to, I guess, essentially excavate it. Any lift has to come from pure wind force overcoming the mass and the downward friction of earth. That's gotta be brute strength of a pressure differential or something. There was also no obvious erosion or water damage. The entire ground-level wind field seems to have been strong enough to move a large, low-profile thing against friction and gravity.
Really, I would think that since wind (in theory) decays closer to the surface, the wind must have been even stronger aloft to cause that much force at ground level.
And finally, there are some damages that fit what Fujita said for F5: extraordinary phenomena will occur. Concrete slabs being cleanly moved ... folks, that's not strong, that's EXCEPTIONAL. Downgrading that to 170 MPH using boundary-layer caveats seems like intellectual overcorrection or something.
I'd probably have fewer words if he just left it at "it was a high-end EF4" or something instead of suggesting 170-180.
Edit: please note also: I am not an engineer. I know engineers, and my Dad is one. I was trained in history and political science, and work in agriculture. But I try to think critically and learn.
