Discussing the Super Outbreaks of 1974 and 2011: Was one more “super” than the other?

April is the most volatile month for tornadoes. Plenty of years feature not a ton of activity, but there is also seemingly unlimited high-end outbreak potential. Two events since the modern record began in 1950 stand out above all the others in this category: The Super Outbreaks of April 3-4, 1974 and April 25-28, 2011.

Spatial comparison of the 1974 (red) and 2011 (blue) outbreaks. Map by Kathryn Prociv

Spatial comparison of the 1974 (red) and 2011 (blue) outbreaks. Map by Kathryn Prociv

Related: April 2011 Super Outbreak | April 1974 Super Outbreak

Event Similarities

Outside tornado alley:

Both events were similar in that they did not occur over the storied Plains-centric Tornado Alley, but rather throughout the Midwest, Lower Mississippi Valley, and Southeast.

Tennessee:

Another similarity between the two events was that the state of Tennessee experienced the most tornadoes in both events.  Yes, both!

Charts showing the breakdown of tornadoes by state for the two events. Notice Tennessee has the highest number of tornadoes both in 1974 and 2011. Charts by Kathryn Prociv

Charts showing the breakdown of tornadoes by state for the two events. Notice Tennessee has the highest number of tornadoes both in 1974 and 2011. Charts by Kathryn Prociv

Even though the April 1974 outbreak had a more northerly bias spanning across the Midwest, and the April 2011 outbreak a more southern bias, Tennessee was the geographic center for both.

Event Differences

Duration:

The April 2011 outbreak lasted much longer at three days and seven hours as opposed to just 18 hours during the April 1974 outbreak.

Total number of tornadoes:

The April 1974 outbreak featured 148 total tornadoes and 350 dropped in 2011.  However due to the fact that 2011 was a continuous three-day outbreak, it should be no surprise it triggered two-in-a-half times as many tornadoes as 1974! It’s also more than likely that tornadoes were missed in 1974 that are counted today.

2011 had more than twice as many tornadoes as 1974, but it is important to remember it occurred over a much longer time period. Chart by Kathryn Prociv using infogr.am©

2011 had more than twice as many tornadoes as 1974, but it is important to remember it occurred over a much longer time period. Chart by Kathryn Prociv using infogr.am©

Number of major tornadoes (F/EF3-F/EF5):

Where 2011 outscored 1974 in terms of total number of tornadoes, 1974 made up for with the number of what I’ll call “major” tornadoes, or F/EF3-F/EF5.  You may remember the next graph from an article earlier this month for the anniversary of the 1974 outbreak:

Comparing the number of significant tornadoes between 1974 and 2011. Chart by Kathryn Prociv

Comparing the number of significant tornadoes between 1974 and 2011. Chart by Kathryn Prociv

And here is a map of only F/EF 3-F/EF5 tornadoes from the two events:

Map of only F/EF3-F/EF5 tornadoes for both events.

Map of only F/EF3-F/EF5 tornadoes for both events.

If Tennessee is the hardest state overall, then Alabama comes in second as the hardest hit state with the most total F/EF 3+ tornadoes from both events combined.

1974 featured 65 tornadoes of F3 strength or higher, meaning these tornadoes made up 44% of the total number of tornadoes.  2011 had 37 tornadoes of EF3 strength or higher, making up just 11% of the total number of tornadoes in the significant category.  A big difference! Partly due to the huge number of EF0/EF1 tornadoes in April 2011.

Pie charts showing the breakdown of tornadoes per category per event. The significant tornadoes are highlighted in black. Left: 1974, right: 2011. Charts by Kathryn Prociv using infogr.am©

Pie charts showing the breakdown of tornadoes per category per event. The significant tornadoes are highlighted in black. Left: 1974, right: 2011. Charts by Kathryn Prociv using infogr.am©

Geographic epicenter:

At the beginning of the article it was discussed that while both events took place across relatively the same broad regions of the Midwest, lower Mississippi valley, and Southeast.  However, the April 3-4, 1974 had a much more northerly bias to the event, whereas the highest concentration of tornadoes occurred farther south during the April 25-28, 2011 outbreak.  One way to visualize this is by looking at the overall tornado track maps for both events (map shown above).

Another way is to visualize is using a density, or heat map.  Making a heat map for each event, then overlaying them on top of one another allowed for the visualization of where the epicenters for each event were centered:

Overlay of the two density maps for each one. Notice the difference in epicenters between the two events. Map by Kathryn Prociv

Overlay of the two density maps for each one. Notice the difference in epicenters between the two events. Map by Kathryn Prociv

Analyzing the map it is easy to spot the different epicenters of the two events.  It is also evident that Tennessee experienced a high density of tornadoes during both events.

Conclusion

When discussing the largest tornado outbreaks in United States history, the April 3-4, 1974 and April 25-28, 2011 are at the top of the list.  These two events stick out in terms of sheer number of tornadoes that occurred, the intensity of the tornadoes that occurred, and the similarity that neither one occurred in “Tornado Alley.”

Both events are “must-study” case studies for all meteorologists including forecasters and students alike.  In comparing and contrasting the two events, perhaps it is worth debating which event was in fact more impressive?  The quick answer is 2011 simply due to the staggering number of 358 tornadoes.  However, keep in mind that 1974 had almost double the amount of significant F/EF 3-F/EF5 tornadoes and over a much shorter time period.

Speaking of time period, some basic calculations reveal that while the 2011 outbreak had an average of 4.4 tornadoes per hour, the 1974 outbreak produced double that at 8.2 tornadoes per hour.

Which Super Outbreak do you think is more…. “Super?”

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B.A. and M.S. at Virginia Tech in geography with an emphasis in geospatial technology and meteorology. Meteorologist and contributor for the Washington Post’s Capital Weather Gang. See full bio.

11 thoughts on “Discussing the Super Outbreaks of 1974 and 2011: Was one more “super” than the other?”

  1. Derek says:

    Very interesting! Thank for sharing this!

    And your piece has me wondering if there are ways to compare the impact of these outbreaks in other relevant ways. Given the significant human toll in each event, I wonder if other statistics could be examined that would reflect the level of impact on the people who were there at ‘ground zero’ that go above and beyond basic casualty figures. ie. just how much suffering was shared by those who were there in during those terrifying hours?

    Perhaps this fundamentally different point of view would be best served by making comparisons between the two outbreaks in terms of 1. ‘percentage of total distance travelled by tornadoes that occured in urban areas versus rural areas’ , b. ‘Death & Injuries per mile of tornado track.’, and c. Total number of tornadoes (or tornado miles?) where there was at least one serious injury.

    Perhaps the first could give us a sense of how much suffering occured in locations where there would have been more people who would have directly witnessed the event and/or been exposed to the emotional ‘fallout’ in the panic-filled minutes immediately after the destruction had occured (sort of a ‘shared-suffering index’). The second could give us a sense of witnesses’ perceptions of the ‘casualty rate’ or ‘survival chances’ in each event, and the third would remove any tornadoes that would have not been responded to by emergency medical professionals or required protracted search & rescue efforts.

    Even if my suggested calculations don’t seem all that useful, I still believe that finding a means to compare the amounts of shared physical & emotional suffering from the viewpoints of those who were part of the story ‘in the moment’ would shed a different light on how these two outbreaks compare.

    1. Kathryn says:

      Hi Derek,

      Thanks so much for your thoughts! I agree in that looking at the non-meteorological factors could shed some real light on the severity and impact on the two events. There are some studies that have looked at the financial/damage loss aspects of the two events but I really like your idea of comparing rural vs. urban impacts.

  2. Jim Dahlem says:

    Nice post, I love maps and the track maps are neat. Regardless of whether or not one was more “super”, the major tornadoes in eastern TN are amazing to me, there are two just west of the NC state line in 2011 on your map and I know that is very mountainous terrain with steep gorges. Were those EF3, 4, or 5? They look to be short-lived and probably were effected by the terrain, but still amazing they could even form and become EF3 or greater in that topography.

    Based on your number, 1974 seems more “super” to me. EF0 or even EF1, while surely ABLE to cause destruction and loss of life, generally to me seem like more nussiance events that cause minor damage to homes and businesses. The winds of an EF0 are like that of a severe thunderstorm but only in a small very narrow location whereas a severe thunderstorm affects a wide area. So an outbreak of EF0s seem like nothing more noteworthy than a few severe thunderstorms. Significant to the few affected, but not really earth-shattering news stories. The large numbers of EF3s and above in 1974 seem to me to make it a huge story. (Of course that’s not to minimize the large tornadoes in 2011 and loss of life and property for all those affected, it was still a big story as I remember the news very clearly from that.)

    1. Kathryn says:

      Jim all great points. I agree with you in that the 1974 outbreak seemed more intense with the higher number of strong and violent tornadoes. When you mentioned the few tornadoes that seemed to cross over mountainous and rugged terrain, I actually studied the effect of topography on supercell thunderstorms for my M.S. and found some interesting trends.

      While mountains have an overall mitigating influence on storms due to higher stability, on the tiny scale that tornadoes exist at the topography can actually enhance wind flow that could cause rapid (albeit weak) spin-ups into brief tornadoes!

  3. Kathryn Prociv says:

    Jim I just realized you’re Jim as in Jimmytrain21! You’re already familiar with my research… Sorry about the repeat!!

  4. Michael says:

    Great article that really put these 2 “super” outbreaks in context. Its really hard to discern which was more super because I have seen studies that say some of the tornadoes of the 74 outbreak would be rated lower going by the EF scale we use today. Either way my county in Kentucky was hit by 3 of these tornadoes. I was not alive but I know all about it as my parents lived through it, Pulaski County in Kentucky. These two events were amazing and catastrophic at the same time. Anyways I just wanted to tell you that I loved the article and keep up the great work.

    1. Kathryn Prociv says:

      Hi Michael! Thanks so much for checking out the site, and I’m glad to hear you enjoy the articles!

      It is definitely amazing how many tornadoes both Kentucky and Tennessee received during the 1974 outbreak, especially since they represent two states NOT in “tornado alley!”

  5. serwis klimatyzacja warszawa says:

    Hi there! This is my first comment here so I just wanted
    to give a quick shout out and tell you I truly enjoy reading through your articles.

    Can you suggest any other blogs/websites/forums that go over the
    same subjects? Thanks for your time!

  6. Kathryn says:

    Thank you for reading! Glad you enjoy our articles. Another great site is the Washington Post’s Capital Weather Gang page. While most articles are catered to Washington, D.C. area weather, there are also great write-ups on other weather topics!
    http://www.washingtonpost.com/blogs/capital-weather-gang/

  7. Jeremy Kappell says:

    In my opinion, there were likely MANY MORE weaker tornadoes (EF-0 and EF-1) than were reported during the ’74 Super Outbreak. You see, the way we detect and measure tornadoes has changed drastically over the last 3 or 4 decades. Now, because of advanced radar systems, increased number of spotters, chasers and a larger general population just about EVERY tornado is detected if not videoed. That was not the case in 1974. A truer measuring stick between these two outbreaks would be the comparison of strictly strong tornadoes (EF-3-EF-5), like the one above, because these were not likely to be missed. Also, another point to note, is that an “outbreak” is considered to be a continuous event. The April 25 – 28th event was not. It is considered a tornado “sequence”. Which is a series of tornado outbreaks, not a singular event. So that too artificially inflates the numbers of the 2011 storm.

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