170 A Possible Case of Mistaken Identity: An Anticyclonic Tornado That (Probably) Never Was

Thursday, 10 November 2016
Broadway Rooms (Hilton Portland )
Jeffrey C. Snyder, OU/CIMMS and NOAA/OAR/NSSL, Norman, OK; and Z. B. Wienhoff, L. J. Wicker, H. B. Bluestein, and D. W. Reif

Several tornadic supercells moved across parts of central and eastern Oklahoma and far northeastern Texas on the afternoon and evening of 9 May 2016. One of the first supercells, while producing a long-lived cyclonic tornado in south-central Oklahoma, appeared to produce a long-lived anticyclonic tornado that originated northeast of the cyclonic tornado and moved northeastward into the primary radar echo “core” – there was an indication of a strong anticyclonic vortex signature in data from several WSR-88D radars, and a post-storm damage survey found a narrow but relatively long swath of damage nearby. Current understanding of supercell structure and the limited observations of cyclonic-anticyclonic tornado pairs in supercells indicates that the storm-relative location and track of this apparent anticyclonic tornado would be very unusual. Whereas the nearest WSR88Ds were ~100 km from the parent supercell, RaXPol – a mobile, rapid-scan, polarimetric weather radar – was much closer [i.e., O(10 km)].

Given the unusual location and evolution of this anticyclonic “feature” and having examined the higher-resolution mobile radar data, we provide an alternative hypothesis for this feature that is consistent with the radar observations, previous observations of cyclonic-anticyclonic tornado pairs, and current understanding of supercell structure. The higher spatiotemporal resolution of the nearby mobile radar data suggests that what appears to have been an anticyclonic tornado or mesoanticyclone in the WSR88D data may instead have been a series of storm-scale, quasi-horizontal (i.e., with a non-zero tilt) vortices; a few transient, northward-moving anticyclonic couplets were observed in the RaXPol data, but they were generally short-lived and did not follow the damage swath. It is shown that there were at least two northward-moving “waves” of such features, perhaps associated with boundary layer convective rolls, that propagated northward into the forward-flank downdraft region of the supercell. Although the exact cause of the damage that was produced is still unknown, the fact that there was little evidence of convergence in the wind field implied by the damage indicates that it, even though the shape of the swath resembles one produced by many tornadoes, may not have been associated with a long-lived anticyclonic tornado as first though.

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