Friday, March 2, 2012

How does the West Liberty, KY tornado compare with other big tornadoes?

Today, a tremendous tornado outbreak from Indiana south to Alabama produced a spectacularly intense tornadic supercell in eastern Kentucky, of all places.  The tornado struck the town of West Liberty causing devastating damage and unfortunately there were many casualties.  The intensity of the damage and the size of the storm's circulation impressed me enough that I became interested to see how it compared to other spectacularly big tornadoes in recent history that I either seen, surveyed or know about in general.  So this post will be a tour of my hand selected big events.

I picked a single image from each event where the tornado just reached maturity and appeared to be near maximum intensity based on the lowest scan velocity from the nearest WSR-88D.  For the West Liberty, KY storm, that time occurred when the tornado was well west of the town where the tornado was still well embedded in its large parent circulation (figure 1).  The Jackson, KY radar showed what almost seemed like two channels of inflow feeding the tornado.  The appearance is somewhat an artifact because the radar was only detecting radial winds, and not the full two-dimensional wind field.  However, even considering that, I believe there were still two channels of enhanced inflow, one coming from the forward flank core in the form of the storm's outflow accelerating into the vortex core.  The other one was the accelerated inflow from the environmental air ahead of the storm.

The strength of the tornado as observed by radar was taken by simply adding the absolute magnitudes of the individual range gates containing the maximum and minimum velocity as long as it appeared meteorologically reasonable. I chose the gates on either side of what is called a Tornado Vortex Signature (TVS) if the vortex diameter was smaller than a few beam widths.  Since the radar sampled the storm in super-resolution mode, the effective beam width (~1.1 deg) is wider than the individual azimuth spacing (0.5 deg) so a TVS could still have peak velocities a couple azimuths apart.

For the West Liberty, KY storm, that value was a velocity difference (Delta-V) of 188 kts.  The distance from the Jackson, KY radar to the storm was roughly 20 nm. That's an impressive value reflecting the impressive nature of the tornado.

Let's see how it compares with other big ones.  Before I go on, note that each panel of Figure 1 is 21 nm wide, and the imagery is courtesy of GR2analyst.
Figure 1.  The supercell west of West Liberty, KY as the tornado reached maturity on 2012 March 02, 2243 UTC.  The 0.5 deg reflectivity is in the upper left, 0.5 degree storm-relative velocity in the upper right, 0.5 deg azimuthal shear in the lower right (called NROT), and the volume-based maximum expected hail size is in the lower left.

The Tuscaloosa, AL tornado from the super tornado outbreak of April 27, 2011, was one of the bigger events of that day.  Through Tuscaloosa, it produced a lot of EF3, some EF4 damage, and one area where it reached in the upper echelons of EF4.  That's the time that the radar from KBMX sampled the storm shown in figure 2.   At 2215 UTC, the tornado was reaching near maximum intensity and was relatively wide too.  The reflectivity shows an intense debris ball, a dismal reminder of the destruction wrought on the city of Tuscaloosa.  The velocity appearance  was quite similar to that of the Kentucky storm in that it showed a channel of strong accelerating flow emanating from the forward flank precipitation core into the vortex core.  The delta-V of this TVS was 199 kts.  Honestly I didn't check later on in the tornado's life to look for higher values when the storm approached Concord, AL or the north Birmingham suburbs.  Perhaps there was a higher delta-V there.  When I sampled it, the tornado was 38 nm from KBMX at 2215 UTC.  So at a farther range, the Tuscaloosa tornado was stronger than the Kentucky storm.  The precipitation shield of the supercell was also bigger but the estimated hail size was smaller than for the West Liberty storm at the time of the respective images.

Let's go on and look at other big tornadoes.
Figure 2.  Like figure 1 except for the Tuscaloosa, AL tornado of 2011-04-27 2215 UTC sampled by KBMX. 

Last year in 2011 was filled with big tornado days.  Perhaps one of the biggest tornadoes of 2011 occurred on May 24 where it started west of El Reno and continued northeast well past Piedmont, OK.  It was on the ground for almost two hours which was a little longer than the Tuscaloosa tornado but not as long as the Hackleburg to Madison, AL tornado on April 27.  The most intense phase of this tornado started south of I-40 west of El Reno where a mobile research radar (RaxPol operated by Howie Bluestein and Jeff Snyder) observed winds in excess of 120 m/s.  That radar observed EF5 winds as the tornado crossed I-40 and approached where I was watching, along with four REU (Research Experiences for Undergraduates) students.  None of the REU students have seen anything like it, even though it was partially rain obscured.  The tornado at this time was a wedge and it struck an oil well drilling platform.  Despite its weight of over 1 million pounds, the tornado had no trouble catapulting it outside the property boundaries of the site.  The tornado narrowed passing north of El Reno, and then seemed to become swallowed into a new mesocyclone.  It was quite likely the new mesocyclone vortex core produced a separate tornado north of this tornado and then both vortices merged to help create a new wedge tornado east of El Reno.

This was the time that I sampled the delta-V in the TVS below in figure 3.  The value reached 198 kts, similar in strength to Tuscaloosa.  The damage from here to north of Piedmont reached similar intensity to the earlier strong phase west of El Reno.  There was more intense damage on the ground than with Tuscaloosa.  But like Tuscaloosa, the radar depicted what appeared to be strong inflow from the main precipitation core of the supercell and a separate pre-storm ground-relative inflow reaching 50 kts.

Speaking of the precipitation core, it was huge.  By far, it was the largest of any supercell core for any of the cases I show here.  Likewise, the circulation and tornado appeared larger and stronger than the Kentucky storm, and even the Tuscaloosa tornado.
Figure 3.  Like figure 1 except for the El Reno to Piedmont tornado of 2011-05-24 2140 UTC sampled by KTLX.

Just two days before El Reno, was the Joplin, MO tornado, a massive tornado that unfortunately struck a small city of around 40 thousand people.  The results, as we know, were devastating.  No one believed that we would see another 100 person fatality producing tornado since none occurred since Flint, MI in 1957.  After all, the NWS and partners had modernized the integrated warning system.  But then again, perhaps we haven't seen a violent wedge producing tornado of this size strike such a densely populated region.

Did this tornado set the benchmark for intensity?  Like the El Reno tornado, it was rated EF5 and EF3 like damage occurred over a wide swath.  I sampled the tornado at its peak intensity and got a delta-V of 210 kts as seen in figure 4.  The size of the circulation appeared as large as the El Reno tornado but in this case, the radar was 58 nm away as opposed to 38 nm for El Reno.  This storm's appearance on radar reflects in every way the size of the tornado on the ground, similar in width to the El Reno tornado when it crossed I-40.  However, note that the size of the precipitation core was not as large as that in El Reno.  Also, this tornado quickly weakened after crossing south of I-44 and had a track of only 12 to 15 miles.  Even though it was moving slowly, the tornado didn't last nearly as long as El Reno or Tuscaloosa (only 38 minutes).  So perhaps the time integrated kinetic energy expenditure was not as large as the previous two tornadoes.  Even the Kentucky tornado likely lasted longer.  But at peak intensity, it ranked right near the top, unfortunately expending its energy into destroying a wide swath of Joplin.
Figure 4.  Like figure 1 except for the Joplin tornado of 2011-05-24 2140 UTC sampled by KTLX.
Joplin was big enough so that the tornado didn't wipe the city off the map.  On the other hand, Greensburg, KS was smaller and just about wiped off the map when it was unfortunately afflicted by a tornado of similar size to Joplin.  The Greensburg tornado was absolutely huge!  It laid down a path similarly wide to Joplin, and perhaps a bit wider when it was south of town.  But this tornado lasted well over an hour as it slowly moved to the north at about 15 mph.  I remember how much time I had to watch this monster while closing in on it from the south, every lightning flash illuminating the huge wedge.

As it reached maturity south of Greensburg, I sampled the delta-V at 223 kts at a range of about 38 nm from the KDDC radar (figure 5).  This storm was also sampled when the radar didn't have super-resolution capability.  Considering that handicap, I find the Greensburg delta-V to be the most impressive of the tornadoes I mentioned so far.  Considering that it maintained this strength for more than 50 minutes, the kinetic energy expended by this storm was huge.  The parent supercell went on to produce an even wider tornado after the Greensburg tornado died, and in fact it produced wedges for another 4 hours.  I wouldn't be surprised to see this storm take top spot in total kinetic energy expended if anyone would be up to the task to verify.

Interestingly enough, the supercell precipitation core was smaller than Joplin, El Reno and Tuscaloosa.  It was not much larger than the Kentucky storm at the time I sampled them here.  

There's one more storm to check.
Figure 5.  Like figure 1 except for the Greensburg, KS tornado of 2007-05-05 0229 UTC sampled by KDDC.
I can't leave out Bridge Creek, OK from 1999 May 03.  That day was Oklahoma's tornado outbreak for which all others in the state are compared.  The signature supercell storm of the outbreak was called storm A, and the radar image below was taken when it spawned the biggest, most damaging tornado  of the outbreak.  At the time I sampled it in figure 6, the tornado was at its nastiest and biggest.  The tornado was also on the ground for more than an hour and it also experienced a resurgence in life like that of El Reno.  

I sampled a delta-V of only 147 kts!  That was the most surprising finding, especially considering that the tornado was only about 20 nm from KTLX.  How could that be such a low value?  I'm not sure I have an answer, especially considering that this storm produced the most intense damage of any tornado that I surveyed or seen including the El Reno EF5, the Hackleburg, AL EF5.  It produced damage intensity that even likely exceeded that of Joplin.  The Greensburg tornado barely produced observable EF5 damage though it spent its most intense phase over fields south of town.  Perhaps the delta-V was weighed down by the intense, large debris that was traveling more slowly than the actual winds.  KTLX was also sampling the storm with the legacy resolution and if it had super-resolution data, it would've seen higher winds like with the Kentucky storm.  After all, intense though it was, the tornado was not as wide as Greensburg, Joplin, or even Tuscaloosa.  I guess it goes to show intensity observed at ground is not perfectly correlated with radar-observed intensity.
Figure 6.  Like figure 1 except for the El Reno to Piedmont tornado of 2011-05-24 2140 UTC sampled by KTLX.
This comparison shows that there were certainly bigger tornadoes than the one that struck West Liberty, Kentucky.  I'm also confident that the Greensburg tornado still reigns supreme out of this list, however Joplin comes in close.  However, the West Liberty tornado is amazingly powerful considering that today's storm occurred in a place not known for big tornadoes, and in the foothills of the Appalachians.
The only comparison I have occurred before the WSR-88D network and that was the central Pennsylvania tornado that occurred during the 1985 May 31 outbreak.

I hope you enjoyed this tour of some big tornadoes.  Like I said, there are many candidate monster tornadoes that I didn't sample.  Hacklesburg, AL comes to mind as one I didn't include.  But there are many plains monsters for which nobody will know about except the chasers fortunate enough to witness them.  Any one of those is a candidate for this hall of fame of tornadoes.


  1. Great article Jim! I bookmarked this from the April 27th Tuscaloosa tornado:
    Mike Umscheid (Lead Forecaster DDC, Lead Warning issuer Greensburg) said this from his facbeook.
    Okay... base velocity Delta-V at 2220 UTC... -139 Kts max inboound... +110 max outbound... this beats Greensburg, KS Delta-V and even the Trousdale, KS Delta-V

    There were also reports of 240 to 250 knots of gate to gate shear after the tornado moved east of Tuscaloosa. One can only wonder what the numbers would have been if the Hackleburg-Phil Campbell tornado had zeroed in on more media-friendly cities.

  2. Well done, Jim. My only suggestion for improvement or addition is to compare with the Twin Lakes presentation of the Mulhall supercell phase from 3 May 99--during the twentieth, largest, and longest-lasting tornado from Storm B here:

    While not rated an F5 due to lack of suitable DIs, its SRM signature was huge and extraordinarily intense. Wurman's mobile-radar sampling indicated individual subvortices that were, by themselves, as big as the Spencer SD F4 of 30 May 98.