Significant Tornado Events

[Grand Poo Bah]

Today marks the 20 year anniversary of a tornado outbreak that helped fuel my life long passion for severe weather. 12 tornadoes struck central Iowa and 3 of them were significant.

It was an exceptionally rare outbreak.

The Nov. 12, 2005, tornadoes were Iowa's first November tornadoes in 14 years and the largest November outbreak up to that point in history.

Prior to that day, only 23 November tornadoes had been recorded in the state since 1950.

Of the dozen twisters in this outbreak, three also achieved F2 or greater intensity. Two of those significant tornadoes were on the ground simultaneously, and both struck towns.

Iowa's next November tornadoes would come almost exactly 10 years later on Veterans Day 2015.


Saturday November 12th 2005 started with the Stratford, Iowa F3





At the same time an F2 tornado struck Woodward, Iowa, and this legendary video was recorded. The outbreak was later featured on the Weather Channel show "Storm Stories".

20 years ago: Iowa's deadly Nov. 12, 2005, tornado outbreak

Wednesday marks 20 years since Iowa’s rare November 2005 tornado outbreak, when 12 twisters struck the state, killing one person and damaging dozens of homes.

www.kcci.com

 

[ColdFront]

Plus the thermal boundaries left over. 4/27 was simply as perfect as a tornadic setup we have ever seen. I still think that something above 4/27/11 with like 3,000 CAPE or higher with more extreme kinematics is possible but extraordinarily rare to combine.

This may seem like a hot take, but to me, the most anomalous factor in both super outbreaks isn’t the environment. Don’t get me wrong, those are 99th percentile environments. It’s actually the large number of open warm sector discrete supercells that fired.

A verified high risk day may have 3-4 cells go up and produce the majority of an outbreak’s tornados. On both of those days you had 25+ supercells go up.

While it’s not 100% accurate to the environment that day, I’ve looked at a lot of the reanalysis data sets for ‘74 as well as RAP proximity soundings from ‘11. One thing that I found interesting is that the Xenia, Rainsville, and the two Jackson Co. EF4s weren’t necessarily in an extreme parameter space. Very upper end kinematics, but CAPE was around 1500 IIRC. Almost seems like when you get that many mature discrete supercells, with such a wide geographic warm sector, they’ll drop violent tornados even in the relatively “lesser extreme spaces”.

 

[slenker]

This may seem like a hot take, but to me, the most anomalous factor in both super outbreaks isn’t the environment. Don’t get me wrong, those are 99th percentile environments. It’s actually the large number of open warm sector discrete supercells that fired.

A verified high risk day may have 3-4 cells go up and produce the majority of an outbreak’s tornados. On both of those days you had 25+ supercells go up.

While it’s not 100% accurate to the environment that day, I’ve looked at a lot of the reanalysis data sets for ‘74 as well as RAP proximity soundings from ‘11. One thing that I found interesting is that the Xenia, Rainsville, and the two Jackson Co. EF4s weren’t necessarily in an extreme parameter space. Very upper end kinematics, but CAPE was around 1500 IIRC. Almost seems like when you get that many mature discrete supercells, with such a wide geographic warm sector, they’ll drop violent tornados even in the relatively “lesser extreme spaces”.

I think a combination of the two is what really produces a super outbreak, but the large number of supercells can definitely be considered the main reason why super outbreaks occur. We've seen a lot of instances where mature supercells enter into hostile environments after leaving the area of most favorable ingredients and still produce strong to violent tornadoes - Enderlin, Greensburg, West Liberty, and Cookeville to name a few. The increased amount of discrete supercells on the higher end days simply allows for a higher chance for an increased quantity of mature supercells to leave the primed parameter space but still be steady-state enough to maintain and continue producing, so that definitely allows for higher chances of more strong to violent tornadoes. The eastern tornadoes of 4/27/11 and the northern tornadoes on 4/3/1974 are evidence enough of this.

I don't know if it is necessarily fully comparable because introducing this may lead to issues with the environment of its own, but I imagine if 3/14 had something similar to confluence bands out ahead of the "string of pearls" supercell front it had, we may have had many more violent tornadoes that day, and definitely would have had more strong ones. 3/14 contained an environment that was absolutely comparable to super outbreaks we have had in the past, it just didn't have as many discrete supercells. (Also, MEG was surveying some of the damage. Bakersfield was an EF4).

 

[ColdFront]

I don't know if it is necessarily fully comparable because introducing this may lead to issues with the environment of its own, but I imagine if 3/14 had something similar to confluence bands out ahead of the "string of pearls" supercell front it had, we may have had many more violent tornadoes that day, and definitely would have had more strong ones. 3/14 contained an environment that was absolutely comparable to super outbreaks we have had in the past, it just didn't have as many discrete supercells. (Also, MEG was surveying some of the damage. Bakersfield was an EF4).

I would concur with that. Especially in the southern Ark/Mississippi area. Imagine if the warm nose wasn’t as stubborn as it was. There were plenty of convective initiation attempts that night. We could’ve had a string of pearls all the way from southern Missouri to southern Mississippi. Then couple that with a few confluence bands out into Tennessee. Really not hard to imagine a super outbreak scenario.

Re: 74. I’ve always found convective band 2, which was responsible for the vast majority of violent tornados that day, the most intriguing. It seems like there were “mini bands” within band #2.

The northern Kentucky/Central & Southern Indiana storms in one mini band. This band seemed to have initiated on a confluence band induced by channeling of winds by the Mississippi River valley coupled with ascent from the trough.

The Central and Southern Kentucky & Central and Northern Tennessee storms in the next mini band. I theorize this may have been the same confluence band as above, or one formed from similar mechanisms. Storms took a while longer to fire on this band due to a more stubborn cap in this area. However, they were primed to go. I watched an interview from a central TN meteorologist on staff that day who stated once the temperature in TN hit 80 degrees, cloud tops hit 60000 feet in minutes.

And finally the southern Tennessee and infamous Alabama storms in the third. Here is a surface map from Corfidi’s 1974 paper from 2100 UTC. Very clear areas of confluence in northeastern Mississippi and central Mississippi.

Seems like you need very wide swaths of open warm sector lift to produce a high numbers of supercells like you said.

 

[andyhb]

The overall aerial expanse of the warm sector/favorable parameter space is one of the main things that sets apart 4/3/1974 and 4/27/2011 from other events. You need a wide warm sector so storms have time to mature without running out of instability, and it does need to be far enough north so that enough storms stay within the moist air. 3/14 this year was a bit of a "just in time" moisture scenario where the warm sector was not quite as large/dewpoints as high.

12/10/2021 was another event that had a very large warm sector and yielded a historic event for the time of year. With that said, there weren't as many supercells with that event because the northern mode was primarily a QLCS and there wasn't quite enough forcing for storms further south into LA and MS. Also the fact that it was December (less time for heating, just cooler in general) did generally limit CAPE to the 1000-2000 J/kg range vs. the 3000+ present in some of the big April events.