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.

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!

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.

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:

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

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:

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?”