El Reno Oklahoma tornado downgraded to EF3

[Shane Adams]

The Rozel, KS tornado of 5-18-13 is an interesting exception to the above-mentioned criteria; the initial rating was EF2 based on ground surveys, but it was bumped up to an EF4 based on mobile Doppler radar readings of 185mph. It's almost like they call it on a whim, whether or not to use the Doppler measurements.

 

[John Farley]

As I remembered, they went back out and found damage from Rozel that did justify the EF-4 rating, even though it was initially rated EF-2. I did a little searching just now and found the article below, which seems to confirm that. The damage included a 1,000 gallon propane tank that was carried 1/4 of a mile. Here is the article:

http://www.kansas.com/news/article1123440.html

 

[Jeff Duda]

The NCDC version of the reason: http://www.ncdc.noaa.gov/stormevents/eventdetails.jsp?id=457695

 

[Dan Robinson]

I posed this question to Gabe Garfield, and he directed me to this presentation on the Raxpol radar data given at the 2014 Severe Local Storms Conference. At 7:45 in the video, you can see the radar-indicated velocities at the surface are exceeding 110 m/s (246mph) in a swath at least 1/2 mile long, at and south/east of the Reuter/Radio intersection.

https://ams.confex.com/ams/27SLS/videogateway.cgi/id/28356?recordingid=28356

Also of note in this presentation is that the radar-observed windspeeds actually exceeded 300mph in the strongest subvortex, and there is an "unofficial" max reading of 318mph that they are not declaring as of yet. Those are items I hadn't yet heard about.

 

[Dan Robinson]

OK, the saga continues. @Jeff Snyder , if you're around, your input would be golden here!

I paid the AMS fee to access the Snyder/Bluestein paper "Some Considerations for the Use of High-Resolution Mobile Radar Data in Tornado Intensity Determination", and so far am trying to unpack everything.

If I read the paper correctly, the elevation of the scan where the wind speed values in question were obtained is 50m (~164 feet), since the ground scattering effects effectively blocked readings at lower elevations than that.

Wikipedia, citing the paper, says that:

"Revised RaXPol analysis found winds of 302 mph (486 km/h) well above ground level and ≥291 mph (468 km/h) below 10 m (33 ft) with some subvortices moving at 175 mph (282 km/h)............The main funnel is believed to have had radar-estimated EF4 winds, with wind speeds around 185 mph (298 km/h). Radar-estimated EF5 winds were only found aloft and in the smaller vortices that rotated around this funnel at 110 mph (180 km/h)"

I am of course skeptical of a Wikipedia article, by default. Nonetheless, I decided to try and locate where in the paper this writer could have possibly pulled this conclusion from. I'm not seeing it.

I only see the statement in the paper about the boresight-aligned theoretical beam height at <10m AGL and that the subvortex's maimum velocities were sampled at 4-5 degrees elevation. So, what I'm not clear on is if there was an actual 10m measurement at the third deployment site (D3), or if it was only at 50m due to the ground's beam-scattering effects.

In a nutshell I'm trying to determine the following:

1.) Were near-surface EF5 intensity winds measured in a broader swath well outside of the subvortices, meaning that the parent tornado was EF5 intensity, not just the subvortices?

2.) Was there a ~10m reading at D3?

3.) Is 50m considered close enough to "ground level" to estimate windspeeds at the surface?

4.) Is the Wikipedia statement complete rubbish?

 

[Jeff Duda]

1.) Were near-surface EF5 intensity winds measured in a broader swath well outside of the subvortices, meaning that the parent tornado was EF5 intensity, not just the subvortices?

2.) Was there a ~10m reading at D3?

3.) Is 50m considered close enough to "ground level" to estimate windspeeds at the surface?

4.) Is the Wikipedia statement complete rubbish?

Having heard Jeff, Howie, and Kyle talk about this, both in seminars and in private, and from reading extended abstracts, I'm pretty sure the Wikipedia article has it pretty well. There may be spots where conclusions are presented as being over-confident, but the numbers are probably consistent with what's in the research.

This tornado was so large and complex that I'm not sure you can separate the main tornadic circulation from the subvortices without doing a spectral decomposition of the full wind field, which may not be possible unless those in the research institutions who obtained these data were willing to share them so as to get a dual-Doppler analysis to estimate the wind field. Perhaps they are doing that, but I just don't know. Even then, I'm not sure anyone has really derived a technique to do such a decomposition. There is a technique called Ground-Based Velocity-Track Detection which attempts to separate the noise in a rotating fluid from the smaller scale turbulence, but this tornado may have had such large subvortices as to eliminate the scale separation between the large and small scales needed to use that technique. I don't think anyone ever tries to separate the strength of suction vortices from the strength of the entire tornado - it is all considered to be one. Thus your first question really can't be answered fully. However, I'm pretty sure the EF5 winds were measured only within the subvortices.

I believe the requirements for EF-scale rating is a 3-second gust at 10-m. THerefore, readings at 50-m are too high to be considered legitimate for an EF-scale rating. I believe laboratory studies as well as some observations (look into the Goshen County, WY tornado) suggest that the strongest winds in a tornado occur just above the surface, like around 50 m AGL. Tornado wind speeds are not constant with height at those levels. Thus I don't think you can equate measured 50-m winds with those at 10-m AGL. There may be a statistical relationship that can be used to derive what the 10-m winds would be for a given 50-m AGL measured wind, but I don't know what it is.

 

[Jonathan Beeson]

The Rozel, KS tornado of 5-18-13 is an interesting exception to the above-mentioned criteria; the initial rating was EF2 based on ground surveys, but it was bumped up to an EF4 based on mobile Doppler radar readings of 185mph. It's almost like they call it on a whim, whether or not to use the Doppler measurements.

The problem is a lack of consistency of rating criteria between offices. Instead of a set standard for rating damage across the board for the WFO's located in tornado prone areas, each office appears to follow its own set of rules when surveying damage. Dodge City, for example, seems to go high on its ratings (Rozel and Greensburg comes to mind) while Little Rock and Oklahoma City tend to go low with their ratings. (Notably Vilonia and Chickasha)

 

[Pete Kustra]

I think one of the biggest problems with the downgrade is that the El Reno tornado's winds peaked at nearly double those of the Rozel tornado, but it is rated lower. In fact, it doesn't seem that there was any EF4-level damage from the Rozel tornado at all - so if the NWS offices want to be objective and consistent, rate the tornadoes EF2 and EF3 or EF4 and EF5.