Senior meteorologist at The Weather Channel. Proud to be a weather-obsessed weather geek. Would be a DJ if not a meteorologist.
By: Stu Ostro , 10:03 PM GMT on February 04, 2013
My first year at The Weather Channel was 1989, and I was on duty the hectic afternoon and evening of November 15. Upon hearing news that Huntsville, Alabama had been hit hard by an F4 tornado, I reviewed the radar imagery, and saw that it had occurred during the process of a line of thunderstorms merging with a supercell ahead of it.
Image credit: Steven Goodman and Kevin Knupp, "Tornadogenesis via Squall Line and Supercell Interaction: The November 15, 1989, Huntsville, Alabama, Tornado" from The Tornado: Its Structure, Dynamics, Prediction, and Hazards
Déjà vu last week, as the high-end-EF 3 Adairsville, Georgia tornado appeared to have also occurred during a cell/line merger, and the more I looked into the evolution of the parent thunderstorm, the more interesting it became.
Its origin can be traced all the way back to southern Alabama near Mobile, more than five hours prior.
This is a long radar loop which I played on the computer screen and made a video of with my cellphone cam and then edited/annotated.
Radar image credit: UCAR
Viewing other radar imagery suggests that the cluster of showers/tstorms didn't start rotating until it reached northeast Alabama. It still didn't produce a tornado yet, but as the primary line caught up with it, the rotation aloft intensified in northwest Georgia, and damage surveys indicate the tornado formed just southwest of Adairsville.
Base reflectivity and storm-relative velocity, 1615 UTC 30 January 2013, three minutes after the tornado is estimated to have formed. Image credit: Gibson Ridge
There's a spectrum of parent convection types from which tornadoes can be produced, ranging from isolated discrete supercells, standing totally alone and far away from anything else, to spin-ups embedded within a quasi-linear convective system (QLCS), the latter being the case with many of the tornadoes during the January 29-30 outbreak. Within that broad continuum are cell/line mergers.
In the recent paper "Observations of Mergers between Squall Lines and Isolated Supercell Thunderstorms" by Adam French and Matthew Parker, they said this in their conclusion (my italics for highlights):
"A number of past studies have examined cases where the types of mergers discussed in this paper appear to lead to tornadogenesis. For the present cases, it would appear that the importance of the merger in tornado formation may be linked to the background environment. A larger fraction of tornado reports occurred during or just after the merger in the WF [weak synoptic forcing, weak to moderate shear] environment, whereas the peak in the SF [strong synoptic forcing, strong shear] environment occurred with the premerger supercells.
"It is important to note that while there appeared to be a link between the merger and tornado production in the WF cases, tornadoes only occurred in 50% of these cases overall. Given the overall rarity of tornadoes in general (e.g., Brooks et al. 2003), this is not all that surprising, but it does underscore that the merger alone is likely insufficient to favor tornadogenesis. Rather, we speculate that in cases where conditions may be favorable for tornado formation, the merger may in some way serve as an instigator. This speculation is lent further credence by our observations of enhanced low-level rotation following the merger for a number of cases in both the WF and SF environments (e.g., Figs. 11 and 14), suggesting that some aspect of the merger appears to favor the development of low-level vertical vorticity."
Previous to that, they note: "Across these two environments, a spectrum of convective evolutions were observed, generally leading to the development of bow echo structures following the merger."
So, cell/line mergers produce a range of outcomes, and there aren't panaceas for the many challenges of forecasting various types of weather phenomena. Nevertheless, these mergers can sometimes facilitate tornado development and there is scientific basis for possible mechanisms involved, and it's noteworthy that such a process may have played a role in the strongest tornado in last week's outbreak (more study would be necessary to try to determine to what extent and how) and did so in that catastrophic one in '89 not far away in northern Alabama; the more we can learn about the meteorology involved and apply to real-time analyses and predictions in the future, the better.
Daiki Corporation facility in Adairsville. Image credit: NWS Peachtree City
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