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:
STORM 2... BINGER-EL RENO-PIEDMONT-GUTHRIE

PRELIMINARY DATA...
EVENT DATE: MAY 24, 2011
EVENT TYPE: TORNADO
EF RATING: EF-5
ESTIMATED PEAK WINDS (MPH): GREATER THAN 210 MPH
INJURIES/FATALITIES: UNKNOWN/9
EVENT START LOCATION AND TIME: 8 WNW BINGER 3:30 PM CDT
EVENT END LOCATION AND TIME: 4 NE GUTHRIE 5:35 PM CDT
DAMAGE PATH LENGTH (IN MILES): 75 MILES
DAMAGE WIDTH: UNKNOWN
NOTE: RATING BASED ON UNIVERSITY OF OKLAHOMA MOBILE DOPPLER RADAR MEASUREMENTS.

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!