Time for a post-mortem? Time for a post-mortem! Kind of. Once again, any input/correction is welcomed as I love delving into the science of events and figuring out why things happened (or didn't happen) at least until Convective Chronicles makes a superior version in the next couple days. This is all just speculation plus a bit of what others far smarter than me have brought up so yes please add or debate stuff
The cool thing is Jackson took soundings every three hours the whole event so we have a lot of observed data to work with
Here's what we had at the time of the Rolling Fork tornado on the RAP
Very narrow tongue of mid 60s dews streaming up along central MS based on surface observations; rather dry low level air to the east but the cell was primed to interact with this plume of deeper moisture rather perfectly; instability was pretty decent if nothing spectacular on the arrival of an intense low level jet
This low level jet was no doubt heavily enhanced by a surface low near Memphis, offering low level backing and a dramatic strengthening of the wind profile as the jet max pushed through; a very quick improvement in wind profiles as well as a good aid for rapid advection. However only a couple supercells managed to develop, the Rolling Fork one and the one that moved up to near Tupelo - literally everything else to the south and east struggled and died until the linear conglomeration later. Why is that? Maybe soundings will help
Here's soundings from JAN every three hours from 12z Friday to 06z Saturday, showing a dramatic and fascinating change in the profile; we start with a very significant EML just above the surface, with very dry air and a significant inversion. By 18z, an interesting evolution; widespread sun had allowed surface temps to dramatically rise, steepening lowest level lapse rates tremendously but drying out the surface quite a bit as the dry air from the EML mixed down. Another very important feature here - a very evident warm nose at 3km. This of course is something that tends to cause lapse rates in that layer to suffer, really killing vertical growth had storms been trying to form then; this continues to be evident for hours, only very slowly modifying. Note the surface temperature and dew spreads; we get moistening of the low levels as advection picks up, but the dry air above the surface also quickly vanishes as deep moisture becomes more evident all the way to 3km or so. You really want to see a somewhat drier layer aloft to get steep lapse rates there; a super saturated column tends to have pretty poor lapse rates in southeastern events and there's pretty marginal vertical growth evident in the warm sector here despite slight improvement by 00z. The cell near Jackson really struggled through that environment and was never really able to root and take off. With little forcing and some pretty marginal lase rates, it looks like cells didn't really have enough vertical acceleration to take off and take advantage of this environment. Note also the gradual improvement in wind profiles as the hodograph becomes more impressive, the LLJ and surface low along with the arriving upper air support helping to bring that up. The storm is nearby at 03z and that profile is probably the most representative aside from the ones taken by researchers, but it's important to note that the supercell did not develop anywhere near this environment; it had been growing for quite a while
The cell developed south of Alexandria, LA as a struggling messy shower with lots of mergers along a confluence band and tracked for about four hours across Louisiana before finally taking root crossing into Mississippi, hooking right, and immediately dropping a violent tornado
Here's the Lake Charles profile at 18z, lapse rates are better here but still with a bit of a warm layer between 1-3km that probably held cells back a bit; no shortage of low level moisture however.
A confluence band is an area that can focus convection and act as a lifting mechanism; when these are present, it can negate poor lapse rates somewhat and help cells to grow. The radar evolution combined with the profile seems to tell the story of what happened: the meager vertical acceleration across Mississippi with no real focusing mechanism really made it hard for cells to grow in the open warm sector, leading to not much development there aside from that one cell near Jackson that struggled and eventually died, but the confluence bands across Louisiana managed to get some updrafts going by focusing lift.
These struggled along for a while, but research has shown that multiple cell mergers can help shore up an updraft in a weak lapse rate enviro and eventually build a robust updraft even in the absence of good lapse rates, and it certainly looks like the storm of the day went through several mergers as it tried to take root, helping negate weak vertical acceleration somewhat. Once it neared Mississippi and the edge of the deep moisture streaming in, at about the same time that the LLJ really started screaming in slightly backed surface flow with the low near Memphis and jet support moving in, it was able to really take advantage of the better environment thanks to having an established updraft with multiple mergers before reaching the area where updrafts just couldn't get going. Winds being backed at this point S of the surface low and not veering strongly and supporting a more QLCS setup for a couple more hours when the low was well to the NE made the timing absolutely perfect to maintain discrete structure.
Thus, it
developed in an area where the confluence bands helped get stuff going and
moved into an environment that was super volatile but new cells were unable to develop, just in time for the vicious wind profiles to start peaking and the tongue of deeper moisture aligning perfectly with its path; many factors coming together to put it in the perfect spot, while stifling new convective growth and keeping it all to itself with unimpeded inflow for a long time
- Confluence bands focusing lift in weak lapse rate environment allowed cell formation
- Multiple cell mergers helped establish an updraft where otherwise would be unable to
- Moved right into rapidly improving dynamics/low level moisture plume along LLJ and S of surface low at the perfect time to take advantage
- Weak lapse rates and no real focusing mechanism just yet prevented other cells from developing/impeding
I feel like that's an excessive number of mentions of lapse rates in one post but hey who's counting 
