Violent Tornado Outbreaks by Upper Trough Tilt

[Fred Gossage]

I am posting this over here for archiving and reference purposes, given the significant La Nina that we are dealing with, and what that often means in Dixie Alley during the winter into spring months. I am not trying to solicit responses, and I wouldn't be offended if this topic doesn't receive any replies. I only wanted it brought over to the new forum, given the severe weather issues we will likely face in the coming months. I also decided to post the article in this sub-forum because, demographically, this is the most suitable place. While Alabama is geographically a Southeastern state, without argument, the combination of the East Coast demographic being oriented in such a way that most of their Southeastern posters come from Georgia and the Carolinas... and the fact that the vast majority of the convective posters reside here... makes this a much better place for a topic such as this.

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This is something I've been meaning to do for a little while now, but only recently have had the data resources to accomplish it. For several years now, at least since the April 7, 2006 tornado event and possibly before then, we have often talked about the tilt of the upper-level trough during severe weather events over here in the Southeast and how that plays on forcing over the warm sector and subsequent convective coverage and storm mode... two things which greatly affect the tornado risk with a particular system. We have speculated that a neutral to positively tilted trough may sometimes be more favorable for a widespread tornado outbreak because it often means more spaced, subtle forcing over a warm sector... a warm sector that is often weakly capped in this part of the country, compared to the Plains... and that would generally be more favorable for spaced, cellular convection.

In order to test this theory, I have selected a pretty strict criteria for "violent tornado outbreak" days in Alabama from 1891 to 2010, and then used the 6-Hourly 20thC V2 Reanalysis Data Composites from NOAA's Earth System Research Laboratory to separate each violent tornado outbreak into three categories, based on the tilt of the 500-mb trough closest to the time of the main action in Alabama.

For this study, in order for a tornado event to be classified as a "violent tornado outbreak", it must meet one of the three requirements below. This was done to hone in on the violent tornado episodes that are of the highest impact to both property and lives, and will be remembered for the longest time. An isolated occurence of F4/EF4 damage isn't that hard to come by with the right set of freak mesoscale ingredients in what would otherwise be a low-end or marginal tornado day. The above mentioned requirements are as follows:

1.) At least one instance of F5/EF5 damage occuring during a tornado event.

2.) At least two instances of F4/EF4 damage occuring during a tornado event.

3.) At least one occurence of F4/EF4 damage, accompanied by at least three F3/EF3 tornadoes.

Using this criteria, I have come across fourteen "violent tornado outbreaks" in Alabama state history from 1891 to 2010 (1891 is when the ESRL reanalysis data begins). They are separated by trough tilt below.

Negatively Tilted Troughs

April 24, 1908

April 20, 1920

February 12, 1945

May 1, 1953

March 1, 2007

April 24, 2010

Positively Tilted Troughs

May 27, 1917

March 21, 1932

April 3, 1974¹

March 27, 1994

May 18, 1995²

April 8, 1998

Neutrally Tilted Troughs

April 4, 1977

February 5-6, 2008 (Super Tuesday)

Footnotes:

1.) Although the upper-level trough started out negatively tilted on the morning of April 3, 1974; due to the way the trough evolved during the day, it was positively tilted during the time of the F4-F5 tornado damage across northern and central Alabama.

2.) This outbreak is included because the second F4 of the event happened less than a county away from the Alabama state line, with the same cluster of supercells that spawned the Anderson Hills, AL F4 in Madison County.

After dividing the outbreaks up by trough type, as shown above, these are the results.

There is a completely equal split between the number of these events associated with negatively tilted troughs and positively tilted troughs. This shows that, while it's important not to ignore the textbook classic negative tilt configuration, events of this caliber are just as likely with a positively tilted trough... that's long been thought to be unfavorable for significant tornado events. This shows the grave importance of ALL severe weather forecasts being grounded in an ingredients-based approach, rather than "rules" created decades ago, many of which are long outdated based on numerous recent meteorological studies as observational technology has grown. One interesting thing to note, is that NONE of the negatively tilted trough episodes occurred with a sharp, negatively tilted trough, but rather with low-amplitude, broader based systems. See a previous article I've written about the amplitude of troughs during severe weather events here. The fact that none of these negatively tilted trough events in the study were sharp, highly amplified troughs... but were rather, low-amplitude, broad-based systems... supports the theory that more subtle, spaced out forcing is a key synoptic player in the ferocity of our most venomous tornado events in this part of the country.

[baroclinic_instability]

Interesting information. It should be mentioned that while the negative tilt troughs were broad in origin, they often feature intense "ejecting" speed maxes which are eseentially which act much in the same way as sharp and/or low amplitude but intense shortwaves. It is a broad categorization, but it reminds me of both Greensburg outbreak May 2007 and Oklahoma May 1999. Either way, doesn't invalidate what you said...and I agree, all potential storm events need to be approached with care.