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Severe Weather Threat 5/19-5/22/2024

Sawmaster

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"The legacy F-scale wind speed ranges may ultimately provide a better estimate of peak tornado wind speeds at 10–15 m AGL for strong–violent tornadoes and a better damage-based intensity rating for all tornadoes."

So in their quest to improve the accuracy of damage-based wind speed estimates with the EF scale, they caused them to be more inaccurate? That's food for thought. What we have here is a comparison of two methods, both of which aren't measuring wind directly where the damage is being done, and both having some admitted inaccuracy involved.

I still clamor for actual wind measurements at ground-to-rooftop level. Though it could take many years to achieve the goal, by placing a grid of anemometers in areas known for strong tornadoes like Moore OK, Tanner AL, etal we could obtain those direct measurements eventually. Given today's low cost of manufacture and cheap compact data-logging it would probably be affordable enough to deploy hundreds of individual self-contained devices which could be analyzed post-event even if they were otherwise destroyed. They could be powered indefinitely by (what else?) wind, and this would have the advantage of measurement of any passing tornado- not just the strong ones. Devices blown away could be found and analyzed later, and the placement of them located to give best accuracy. Even with a 80% level of destroyed unreadable devices you'd still have maybe a dozen or more giving that accurate on-site data we've all been looking for. Even if 100% were destroyed, you'd still get an accurate measurement up to that point which would be of value anyway. Maybe Texas Tech could develop these devices given their expertise and available resources.

When what you are doing isn't giving the results you want, it's always good to consider other approaches to achieve your goals.
 

Maxis_s

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there are grounds to believe that the actual near-surface winds were (considerably?) lower than indicated by radar
Yes. Look at the wind turbines that were collapsed in the Greenfield tornado. They were completely demolished, but every tree on the ground right next to them were untouched. This leads me to believe that the winds may extent outwards more the further up you go, and also become substantially more intense just a few meters above the ground.
 

Sawmaster

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FWIW, I believe the EF scale windspeeds are defined at the reference height of 10 metres.
That's a standard for anemometer height for normal wind measurement as that is where 'clear air' unaffected by it's surroundings begins to be found. That probably influenced the decision. Yet most of the damage occurs below that for the simple reason that few buildings and structures exceed two stories in height with much more being toward the one-story range. What happens at 10m will affect little compared to below that so it's not very logical to measure at 10m in order to ascertain what happens below that.
 

SouthFLwx

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It's not all of them, either. They all don't have photos so either them being EF4 was a bug, or them being EF3 is a bug, or something else. The northeast corner of the town hasn't been downgraded though, so I don't know what's going on.
I think what happened is the EF4 rating was kinda like a blanket rating for them. However, they downgraded them after analyzing them a bit more.
 

pohnpei

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Re: Greenfield: there are grounds to believe that the actual near-surface winds were (considerably?) lower than indicated by radar. A recent study suggests this:

Unfortunately, the body of the work itself is behind a paywall, but the critical portion is highlighted below (note that it is supposedly corroborated by other studies):

Now the study does suggest that, in cases where damage aligns with radar, the EF scale underestimates high-end wind speeds near ground level. By the same token, however, if damage suggests weaker winds, then the radar might also overestimate some events. The former would apply to the well-known events that have been discussed here. The latter might apply to events such as Greenfield. Once again, based on DIs, I still do not see much evidence to support considerably higher wind speeds than the survey and DAT currently indicate, unless I am missing something.
Radar would not overestimate some if any event and it's actually quite the opposite at most times.
First of all, tornados winds peaked almost below any types of radar can scan. Second, Radar velocity tend to be the lowest estimation of actual wind flow due to debris, rain, hail centrifugal effect and beam blockage. Third, the wind/pressure gradient within one radar bin can be enormous big, especially for WSR-88Ds, and sometimes even for mobile radars.
I recommend you to reed this for the deep explanation of these things.

Just in terms of Greenfield event, WSR88D only got 70kt vrot from distance away. The much high res radar like DOW6 got at least 155kt vrot(and larger than 220mph velocity) 10km away, then DOW8 got 290mph only 1-2km away. (by the time, DOW6 only got 150-170mph around the time 15km away ). Feature like this could tell you that winds would be always significantly underestimated and smoothed when radar wasn't close enough and high res enough, especially for tornados with tight core.
 
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Maxis_s

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You guy’s know nws Norman, Nashville, and Jackson Mississippi surveyors right? Well now add nws Des-moines to that list of top tier surveyor’s…the greenfield survey is very impressive.
Yeah, even though it's not already complete, the survey is very well done. I'm interested, and somewhat excited, in seeing the final survey. Even if it's only rated 180mph in the end, it's still a very good survey.
 
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I didn't remember until today when reviewing my write-up from that day; but I actually passed through Greenfield on my chase on May 7, 2023. Killed a little time in the Greenfield City Park while waiting for storms to fire in my target area. Was a nice little place. :(



Again, that wind farm seen throughout the video is the same one where some of the turbines got hit.
 
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