This is what always floors me about major severe weather outbreaks. The same thing was true of the Super Outbreak, and probably of other known major severe weather outbreaks (I only know this fact about the Super Outbreak due to this paper). It's sort of frightening to realize that such a major severe weather event can be spawned from a seemingly innocent looking synoptic setup. Granted, anyone with experience watching synoptic scale patterns associated with major tornado outbreaks would've been able to recognize the potential this day had when looking at model forecasts a few days out. But really, the same ingredients that were in this event are in many severe weather events, even those that aren't major:Sounds about right. There wasn't anything terribly unique about this setup from what I can recall. Was a combination of a number of very strong parameters coming together. Think the key to the day may have been storm spacing and the isolation that was had by most supercells. I've seen some amazing parameters before "go to waste" due to the seeding of the storms and their effective competition with each other. Not the case with this outbreak.
-a moderate amplitude trough aloft with neutral to negative tilt that was propagating east/northeast
-moderately unstable air mass with mid-upper 60s dewpoints in the warm sector and 80-90 F surface temperatures
-Strong low-level and deep layer shear
-Moderate low-level instability
-A surface boundary or synoptic scale lift to trigger storms
Think about it, how often do you see these features with other severe weather setups? Almost each one appears regularly. Yes, the degree of deep layer and low-level shear was on the extreme end of the statistical distribution, but I have seen such high levels of shear and helicity associated with instability and forcing that did not result in a major tornado outbreak in other cases. There are likely a few smaller ingredients that came together to make these storms spin like tops and drop violent tornadoes left and right. These are the ingredients that projects like VORTEX2 are trying to discover.
Let me reiterate: from a synoptic standpoint, there was little about this setup that was unusual, uncommon, or difficult for the atmosphere to achieve.