Going to go on a bit of a rant here because I love this topic and I feel like talking about it, lol. I wouldn't know what other thread to put this convo in.
A bit of a warning: A lot of what I'm about to say could potentially fuel weenie-like forecasting behavior. I promise this isn't that and there's at least something I can point to to defend it (statistical mechanics) If anyone notices that I've misrepresented something somehow, please let me know.
Statistically speaking, throughout all of Earth's entire geologic history (referring to times where the Earth's temperature, geological background, and atmospheric composition were drastically different than they were today, which includes times and areas on Earth where favorable tornado environments were more/less common) there has certainly been years, maybe even years that include human history, that contained multiple super outbreaks as we would define them today. Based on what we currently understand, the Earth has been around for a little over 4.5 billion years. This next part is practically pure conjecture by me, my numbers could be incorrect, but the idea still stands. I'm guessing significant severe weather would have been possible from the moment the jet streams became well-established (and hell, maybe even before that), which I'm guessing is whenever the Earth's surface cooled to an acceptable temperature difference between the poles and the equator for their formation, such that the resultant pressure differentials in the atmosphere would be satisfied with the movement of wind. From what I understand, this occurred for, at least the
first time, close to ~4.2 billion years ago (before the
late heavy bombardment) and would have definitely been well-established by the time the late heavy bombardment ended. Even if the jet streams didn't get well-established for billions of years for whatever reason (which is highly unlikely to me) then we would still have hundreds of millions of years at the very least of possible significant severe weather occurring at any given time.
The outbreaks I would consider "super" that are somewhat documented would be 1884, 1896, and 1932 (I could be forgetting a few). 1965, 1974, and 2011 were the other "super" outbreaks that I would consider to be well-documented. On average, these events occur about once every ~26 years if we refer to the years I presented here, so we should expect another super outbreak to happen sometime around 2037 if we're going purely statistically. So we can call these super outbreaks, all of which are quite comparable to one another, a 1-in-25 year event, roughly speaking. What about 1-in-1000 year tornado events? Or, what about a 1-in-100 000 year tornado event? At this point, the atmosphere is not marginally changing in its characteristics anymore, so now you've got to take that into account if you're going to try to define something like that.
But, the point still stands: We've been accurately documenting these things for only about ~60 years, and getting some semblance of documentation for at least 400 years. That is an infinitesimal blip compared to the history of the Earth. The most extreme events you can get here are likely much more intense events than even 2011 could have fostered. There's also likely been times where a "tornado alley" didn't exist on Earth for an appreciable time period, or times on Earth where there was a "tornado alley" that dwarfed ours today. We simply don't know, but statistically speaking, I believe these ideas are quite plausible.
We discuss the 2011 super outbreak as "everything coming together in perfect harmony," which is pretty much true. But what about events where every parameter and possibility for a super outbreak comes into harmony better than 2011 did? To say that such an event never existed would be almost laughably wrong to me. I'm sure you could come up with a hypothetical trough ejection that is geometrically more favorable, with an even wider warm sector and more extreme kinematics, with favorable parameters moving with the storms and timed similarly with peak daytime heating. If it's remotely realistic and/or possible, it has likely occurred or come very close to actually occurring.
For example: Imagine a slightly more geometrically favorable 4/3/74 trough swinging through the same areas with better thermos, like it coming through at the beginning of May rather than the beginning of April. Or, better yet, what if there was an extremely unseasonable example of a mega-trough occurring in June-July and swinging through the US with summer-like CAPE values overspreading 4/27/2011 like kinematics over an even wider area than the original 4/3/74 outbreak? We cannot say such a thing is impossible, the Earth's atmosphere is extremely chaotic and things like this can and will eventually occur over an infinite time period. Billions of years is definitely long enough for such a thing to end up happening at least once, I believe. Such an extreme event would dwarf the magnitude of 4/27/2011 and maybe is genuinely impossible, but we cannot say that for certain, not yet.
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