Moore, Oklahoma Tornado: What Turned It Violent?

 http://www.weather.com/news/tornado-central/moore-tornado-what-made-it-violent-ef5-20131019

 

Is it possible the Moore, Okla. tornado on May 20, 2013 turned violent thanks to adjacent, non-severe thundershowers?

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This was the question posed by Ryan Wade, a University of Alabama-Huntsville (UAH) severe weather researcher, at the annual meeting of the National Weather Association in North Charleston, S.C. on Oct. 15.
Wade was presenting findings from an investigation of supercell mergers and other storm-scale interactions in recent tornado outbreaks with fellow UAH researchers Todd Murphy and Dr. Kevin Knupp.
In the radar loop above from the National Weather Service in Norman, Okla. (hereafter, NWS-Norman), you can see the Moore supercell with a pronounced hook echo first developing as it passed northwest of the town of Newcastle.
Highlighted by the yellow arrow is a decaying cluster of weak thundershowers moving northeast toward the supercell.
Wade noted an outflow surge — a surge of stronger winds associated with the supercell's rear-flank downdraft — occurred just after supercell merged with the decaying thundershowers. The supercell's hook echo then became more pronounced, with a distinct debris ball signature (circular area of red/purple reflectivity headed toward Moore), indicative of wind-lofted tornado debris.
"Combining damage surveys with the radar characteristics of the Moore supercell indicate the Moore tornado intensified after the cell mergers," said Wade.
Specifically, Wade noted that EF4 and EF5 damage, shown by red and purple contours in the map below, occurred after this cell merger and subsequent outflow surge.