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  1. The process is called cyclic tornadogenesis, whereby, in some cases, one tornado weakens and dissipates before or as a new tornado is forming. The original tornado moves away from locations that are favorable for tornado maintenance, often heading more toward the north or northwest for a storm moving east or northeast (so it moves west relative to the storm). In this part of the storm the tornado usually does not have access to buoyant, rotation-rich air suitable to keep it going. Meanwhile, the new tornado usually forms along or near a bulge in the rear-flank gust front, a boundary separating relatively cool westerly outflow air from relatively warm easterly inflow air. Along this boundary, you often have rotation rich air and convergence which are vitally important for tornado formation. Once this new tornado forms, the process may repeat itself again. It is not atypical to have both tornadoes co-existing for a period. It is unusual for both to maintain strong intensity or for them to interact or for there to be three at one time. A third tornado probably requires some additional mechanism or some corollary to the above ideas It is not totally understood exactly why this happens. It has been hypothesized that a tornado that moves in a different direction from the parent storm because of the surrounding flow plays a large part. In other words, if there is weak storm outflow and strong storm inflow, the original tornado can be advected away from the locations suitable for its maintenance where it eventually dissipates and the new one forms. The lack of outflow is largely an internal storm mechanism, however, though anecdotal, storms days like this one where there were several cyclic supercells in a similar area also provide some circumstantial evidence for the idea that there is something unique in the near-storm environment that's important as well.
  2. JMO, but it looks to me that there was a ground circulation there.
  3. I don't think there were. I know CSWR, OU, and TTU were out at various times, but there were no large field projects this year to keep teams out late in the season (PECAN is in 2015). Would have been a good cyclic tornadogenesis dataset and another case of tornado binary interaction (presumably).
  4. It's easy to determine the strength of constructed materials in wind tunnel experiments. It's not easy to test the strength of vegetation of different species in varying soils with varying moisture contents, particularly when the impact of surface-level vertical velocities in the tornado are basically unknown.
  5. You could, but then you'll just reverse the problem by overrating a lot of EF0s. Photogrammetry requires a lot of additional information and for the equipment to be set up properly. EF unknown and EFx+ are options being discussed.
  6. Sometimes local offices aren't aware of mobile radar data because, admittedly, often that radar data isn't looked at for several days or weeks after it's collected. Regardless, I think these debates about select cases and whether they're EF4 or EF5 really lose the forest for the trees. At least on these cases, the rating is right to within 1 EF scale, which is probably good enough for most scientific endeavors (most studies lump all violent tornadoes together anyway). The much bigger problem is the very large number of tornadoes rated EF0 that are off by multiple EF scales.
  7. There's some unanswered questions, such as a. how radar estimated winds at 50 or 100 m height or higher compare to tornado winds where damage is occurring, b. how nearly instantaneous radial wind estimates compare to 3 s gusts used for the EF scale, c. how air velocity (what we're interested in) differs from scatterer velocity, d. the effects of complicated scatterer distributions within a resolution volume on the velocity estimate, and e. the impacts that asymmetric flow and large vertical velocities in tornadoes have on the velocity estimate. However, in all of the above issues except b., the effect is for the radar estimate to be less than the true wind speed. Accounting for all of the above, the radar estimate is very likely to be a lower bound on the true tornado wind speed. Therefore, it should be a reasonable tool to upgrade tornado intensity ratings, but not to downgrade them. That being said, mobile radar data are not a panacea for the intensity climatology. Very few tornadoes get observed by mobile radars, and not nearly enough to even begin to correct for the chronic and severe underrate bias that far surpasses in importance the comparatively minor differences of opinion expressed in this thread.
  8. Agreed. Stop pretending we have the type of accuracy conveyed in the ratings. We do not.
  9. Couldn't agree more. I think the mobile radar data support using three categories: weak, moderate, and strong. Other tornado scientists I respect have said the same, though I don't think anyone's said it publicly.
  10. The concerns about this rating and the desire for an accurate intensity climatology are laudable, but IMO the ship has sailed on the latter. I think the community simply overestimates the precision the system has in determining tornado wind speeds. There's some skill there, but not in consistently getting it correctly into one of six categories. One tornado being rated EF4 vs EF5 has near zero meteorological significance, though I understand there is historical significance.
  11. The decision came from the top and the reasons were not purely scientific, though there are legitimate concerns there as well.
  12. They're based directly on probability.
  13. It's tough to keep everybody happy. I think there were quite a few posters here who actually suggested the "enhanced" wording as a replacement for slight.
  14. Also, I'd suggest reading Wurman et al. (2007), in which the authors use realistic models of violent tornadoes and previous fatality information to estimate worse case scenarios in populated areas. It's a reasonable attempt at an admittedly complex hypothetical scenario, but it shows just how bad tornadoes in urban areas potentially could be. It's just not a pretty situation.