Radar loop of May 20, 2013 Moore, Okla. tornadic supercell from 3:25 p.m. to 4:34 p.m. CDT. The location of Moore, Okla. is denoted by red box near the center of the loop. The location of the tornado is denoted by the debris ball of higher reflectivity headed toward Moore. The yellow arrow denotes the decaying thundershowers tracking toward the Moore supercell. (Credit: National Weather Service - Norman, Okla.)
Is it possible the Moore, Okla. tornado on May 20, 2013 turned violent thanks to adjacent, non-severe thundershowers?
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.
Damage swath from May 20, 2013 tornado. Color contours denote boundaries of EF0 (light blue), EF1 (light green), EF2 (yellow), EF3 (orange), EF4 (red) and EF5 (purple) damage. (Credit: National Weather Service - Norman, Okla./Google Maps)
Recent studies by Ryan Hastings, a research assistant at Penn State University, and Josh Wurman, founder of the Center for Severe Weather Research, have laid out cases of supercell and other storm-scale interactions leading to tornadogenesis.
"The outflow surge observed after the Moore supercell single-cell merger appears to be consistent with numerical simulations by Hastings," said Wade. "Outflow surges have been identified by Hastings as a key component to mesocyclone intensifications observed after cell mergers."
In simple terms, the theory goes like this. A gust front marks the leading edge of a thunderstorm's outflow winds, featuring winds spinning about a horizontal axis, like rolling a pencil across your desk. When a thunderstorm's updraft interacts with this gust front, the gust front's horizontal spin can be tilted into the vertical and stretched. Like the oft-cited example of a spinning skater pulling in their arms, stretching a vortex makes it spin faster.
In the Moore tornado example, that dying thunderstorm cluster may have triggered an outflow surge in the supercell, providing additional low-level spin to intensify the Moore tornado.
Wade did caution these are simply initial observations, made also by others through Twitter. A more in-depth examination of the data is needed to come to more concrete conclusions. "I'm sure the folks in Oklahoma are already working on detailed analyses of this case since it significantly impacted their community."
To be fair, NOAA's Storm Prediction Center mentioned the "potential for strong tornadoes" two days prior to the event. Given the magnitude of potential energy and wind shear, particularly in the lowest levels of the atmosphere, it's possible this supercell would have produced a tornado, perhaps even a violent one, without the interaction with the dying thundershowers.
"Cell mergers observed in the Moore supercell and violent tornado cases from the April 27, 2011 and March 2, 2012 outbreaks highlight the need to identify and analyze other cells in the vicinity of supercells for their possible impacts after merger occurrence," says Wade.
Adjacent, weaker thunderstorms may be the instigators of another destructive future tornado.Follow @wxjerdman
MORE: Moore Tornado Damage Photos
A tornado moves past homes in Moore, Okla. on Monday, May 20, 2013. (AP Photo/Alonzo Adams)