EF-5 upgrade based on mobile Doppler radar data

The El Reno / Piedmont / Guthrie tornado was upgraded to EF-5* this afternoon, based in part on measured RaXPol Doppler velocities of over 210 mph.

Here’s the relevant portion of the NWS Public Information Statement:

EVENT DATE: MAY 24, 2011

I’m not certain if this is the first time mobile radar data have been used to upgrade a tornado rating, but it’s certainly an unusual occurrence. (If you know of such an instance, please post a comment!) EF-5 tornadoes are extremely rare events, mobile radar data collection in them, even rarer, and crucial near-surface wind measurements, rarer still. The Doppler velocities in the upgraded EF-5 tornado were collected at 60 m AGL, according to my former officemate and Ph.D. candidate, Jeff Snyder. Since RaXPol is such a new radar, he and other members of Howie’s team have been double- and triple-checking their measurements throughout the past week. So far, I’m told, the data are of reliable quality. But, the data will still have to be subjected to the scientific peer-review process in more formal studies yet to be composed.

Doppler radar cross-section of the Greensburg tornado

Pseudo-RHI of (top) uncalibrated reflectivity factor and (bottom) Doppler velocities collected in the 4 May 2007 Greensburg, Kansas tornado. Note the weak-echo column down the center of the funnel, indicative of centrifuging of hydrometeors and debris. Also note that no data were collected at altitudes below 1.2 km AGL. From my Ph.D. dissertation. Data collected by UMass X-Pol.

For comparison, on 3 May 1999, a DOW measured winds over 300 mph in the Moore/Bridge Creek, OK F-5 tornado. In a 2002 paper about that data set, it was noted that lofted/centrifuged debris could actually contaminate the velocity measurements near the surface. In the Greensburg, KS, EF-5 tornado, which I studied as part of my dissertation research, Doppler velocities exceeded 180 mph, but only well above the surface. (We deployed too far away from the Greensburg tornado to collect data in that crucial near-surface layer – see the figure at right.)

Remember that the EF scale is not a wind scale. The wind speeds are estimates based on damage (which is the only evidence tornadoes consistently leave behind for us to study), rather than the other way around. For this reason, there may be forthcoming disagreements as to whether Doppler radar measurements can even be used to make an EF-scale determination. Stay tuned…

* An explanation of the EF scale (and how it differs from the original Fujita scale) can be found here.

Correction: The Mulhall, OK tornado was F-4, not F-5, and the 300+ mph measurement was in the Moore/Bridge Creek, OK tornado. Thanks to Roger Edwards and Mike Coniglio for the corrections!

7 thoughts on “EF-5 upgrade based on mobile Doppler radar data

  1. IIRC – There was a tornado pre-EF scale in (I think!) Baylor County Texas that was given an F3 rating based on DOW data. The tornado was huge, but damaged little to nothing because of it’s location in the middle of nowhere. I know it occurred when I was in Minnesota so I’m thinking it was 2006 or so…

  2. Robin — I think that some DOW velocities were used in the Goshen County assessment back on 6/5/09. As you remember fondly, I am sure, that tornado essentially hit nothing other than tress and perhaps an outbuilding or two. If I recall correctly, radar observations were used to “justify” the rating. It certainly doesn’t happen often, largely because it’s not common for us to have radar data near enough to the surface (in this case, <100 m) and with high enough resolution to have much confidence in the winds that were likely occurring at the surface. In this case, even with a reduction from 60-100 m AGL to the surface using a 5-10-15% reductions noted in some of the DOW data, that winds still would have been above the EF5 threshold.

  3. Karen is correct – we rated a tornado in north Texas based on DOW data around 2006. It was not well-received by some…

  4. While I understand what they are trying to do, I think that rating tornadoes based off observed wind speed is a bit unfortunate. In doing so they are polluting a dataset with outside information and making it less useful in general.

    As meteorologists move away from working to predict the weather and instead move to predicting societal impacts from the weather things that are now viewed as quirks in a dataset, such as the fact that if a tornado causes no damage it can be hard to accurately assess the wind speeds, will be viewed as incredibly useful information.

    A model prediction that a tornado with 200+ mph wind speeds will cause only EF-1 damage can easily help forecasters prioritize where they should be spending their time. And historical reforecasts made with such a model will undoubtedly have their performance measured against the database of recorded tornadoes. This means that tornadoes rated based on wind speeds instead of damage will have to be filtered from the dataset in order to ensure accurate results.

    To me, we should evolve with the technology and preserve the EF scale ratings to be based off observed damage only. And given the increasing number of mobile (and otherwise) radars scanning tornadoes very close to the ground, not to mention the increased number of observing stations that are being impacted by tornadoes should lead to the addition of another piece of information in the tornado database: max observed wind speed.

    Future scientists will thank us all if we push for something like this to happen. Of course if we let them down many will end up muttering our names intertwined with a wide variety of curse words.

  5. Don’t forget Spencer, SD 31 May 1998. The DOW measured 246mph and the tornado received an F4 Rating. This was used to confirm the F4 damage, rather than upgrade it.

  6. “In doing so they are polluting a dataset with outside information and making it less useful in general.”

    Eh, our tornado climatology/record is pretty much crap so I’m don’t see how we’ll pollute it by incorporating radar data. Just make sure the actual wind speed is included besides the damage rating (as you mentioned0.

  7. Jeff- do you know where the 5-10-15% reductions come from? Are you just saying that “even if you reduce the winds 15% they’re still EF-5”? Or instead are they log wind profile based? Or maybe from a large sample size of points averaged into a mean near-sfc profile? In the TC community a quick search of “mean boundary layer wind profile in a TC” returns all kinds of interesting dropsonde results- some showing that near-surface (60-100 m) winds actually *decrease* from the surface, but most showing that winds increase aloft. As you know they now have 10+ years of GPS dropsonde data in a range of TC intensities (most not more than 50 m/s though, but some approaching 70 m/s) and still can’t agree on a reasonable reduction factor within the surface layer…. hence my curiousity where 5-10-15 came from! (Hopefully not straight log wind profile as that’s for a neutrally stable column, and the non-linear terms that get introduced when the column isn’t neutral have large uncertainties, in my opinion…)

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