There seems to be, as a few have already mentioned, a bifocal aspect to Monday's threat. While there seems to be an apex in the parameter space over the area where the SPC highlighted, threat longevity is likely to be highest further south into the Tennessee Valley and Southeast. The kinematics overnight across the Deep South will be very strong, and even in the absence of diurnal heating, strongly-forced storms will have no problem sustaining themselves and mesocyclones, thanks to our 850 mb LLJ, in very classic Deep South fashion. I wouldn't be surprised if the southern portion of this event looked a bit like 4/28/14 with regard to the MS/AL area. Interestingly, 06Z GFS hints at weak, secondary low development on Wednesday afternoon across the Southeast, with the possibility of a
second round of severe storms following the first that comes through late Monday night into Tuesday morning. It doesn't have a classic tornado look by any means, but strong instability will be in place to support robust convection.
Overall, I think both the southern and northern modes have their own conditional factors, and, despite an impressive parameter space, these areas of interest aren't slam-dunks as far as forecasting goes. As
@ColdFront mentioned, kinematics are an important part of this equation. The main conditional factor with the Missouri and Ohio Valley region is time - a relatively short convective window before the front sweeps through and takes kinematics and thermodynamics with it, and likelihood that things will grow upscale rapidly. One one hand, with this kind of setup, you can have a 3/14/25 situation with a line of supercells, or you can have convection that never really gets going before it goes QLCS. On the southern end of things, in the Lower Mississippi Valley and Tennessee Valley, it becomes more of a question of areal coverage and convective evolution. There, favorable kinematics will be in place for an extended period, all the way through the morning on Tuesday. What forms and where will be a deciding factor in the ultimate outcome of this system over the Deep South. It's very possible little to nothing occurs in the Southeast until well into the overnight as well, but the LLJ will be supporting convective development at that point.
