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The Adairsville tornado

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

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

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7. WunderAlertBot (Admin)
4:41 AM GMT on February 08, 2013
stuostro has created a new entry.
6. Barefootontherocks
6:08 PM GMT on February 06, 2013
Hi stuostro,
Ditto what beell said about the weather content with new bloggers, and please add my thanks.

Doubt I'll ever be able to think these events out in the way you two are, but I am enjoying the discussion.

SPC event archive for January 30, 2013 is posted now and contains selected mesoscale data back to 1200 gmt. The mesoscale parameters used for MCD 97 graphic might be of help. Also, perhaps pertinent words from SPC mesoscale discussion 97 issued a moment before the tornado:

I tend to think EF3-plus rides on "the LLJ effect" that puts extra "super" into supercells, line or no. Related the Adairsville case study, simplistic, probably.

Thanks for the great discussion, Stu and bl.
Member Since: December 31, 1969 Posts: Comments:
5. beell
1:28 PM GMT on February 06, 2013
It (the cellphone capture) worked just fine. Quick, relatively easy, and effective. I appreciate your blog and other WC staff (and I have to include Professor Lee) that have added great weather content and great value to this site.

From French and Parker link (pgs 255-256):

"...Goodman and Knupp (1993) investigated a case from November 1989 wherein a merger between a squall line and an isolated supercell coincided with the development of a tornado rated F4 on the Fujita scale that struck Huntsville, Alabama...As the squall line approached the supercell,forward progress of its gust front slowed in the vicinity of the merger and accelerated south of the merger location, effectively appearing to "wrap around" the supercell's mesocyclone."

Could this (inbound velocity) be what we're looking at here?

Have a good 'un.
Member Since: December 31, 1969 Posts: Comments:
4. Stu Ostro , Senior Meteorologist
1:31 AM GMT on February 06, 2013
Ahh, the radar site not the website! Yes, FFC.

And in regard to the video, it would have been time-prohibitive to process a gazillion images and make an animated GIF so I gave the cellphone/mouse a shot and it seemed to work. :)
Member Since: December 31, 1969 Posts: Comments:
3. beell
3:13 AM GMT on February 05, 2013
Thanks, Stu. I don't see much sign of a surface low on the UCAR surface you posted. I made a quick look there but it only went back to the 31st. Probably just the tail-end of the surface trough plotted as a low by the HPC computers.

I do see a fairly decent dp boundary from Chattanooga to the NE, along the TN/NC border. But really, more confluence rather than convergence along the squall line if anything.

And I guess I should have been clearer-What radar site was used to generate the GR images (FFC, HTX, etc)? Just to pin down a point of reference to understand the SRV image a little better. I'm guessing FFC by the RF pattern.

TIA, and another thanks for the link to the French/Parker paper. That should keep me busy for a while.

ps. excellent and steady eye-hand coordination with the cellphone and mouse pointer.

Member Since: December 31, 1969 Posts: Comments:
2. Stu Ostro , Senior Meteorologist
2:15 AM GMT on February 05, 2013
Yeah, beell, a colleague at TWC was wondering about that too, and perhaps there was some sort of boundary which helped trigger the increased rotation, but the surface analysis below doesn't seem to show a convergent warm front at Rome GA that is noticeable, at least on this scale.

For retrieving archived radar data that can be plotted via GR, http://mesonet-nexrad.agron.iastate.edu/level2/ra w/ has it going back a few days; for times prior to that there's the NCDC radar data inventory.

Member Since: December 31, 1969 Posts: Comments:
1. beell
1:22 AM GMT on February 05, 2013
Thanks for the post and the effort required, Mr. Stu.
Just out of curiosity and with an eye towards the reflectivity and SRV images, which radar site provided the data for the GR images?

I wish I had a good source for archived mesoscale data. SPC Meso Page Archive only goes back approx 72hrs. Might be helpful in deciding how much of a factor a plotted surface low just west of Adairsville played in low level convergence. And a stub of a stationary WF boundary on the 18Z surface. HPC occasionally plays peek-a-boo with surface lows.

click for full image
Member Since: December 31, 1969 Posts: Comments:

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Proud to be a weather-obsessed weather geek! \m/ Senior meteorologist at The Weather Channel. If not a meteorologist, would be a DJ ♫

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