As of 1:00 PM/ 18Z... 12Z GFS model indicating a hole of no instability existing in North Mississippi, also where a very strong cap over the atmosphere will exist, preventing storms from developing. While a watch is possible for that area, I anticipate that, in the event some storms develop, they will have only some downdraft (thunderstorm cooled winds) instability to work with. 12Z GFS does indicate some potentially strong storms are to break that cap and introduce some severe weather into the area. There is potential for heavy rain, with precipitable water (PWAT) values exceeding 2.5 inches. This means that if all the air in the atmosphere were condensed, there would be 2.5 inches of water in that column of space.
For the Dakotas, instability will remain not too impressive, with readings coming out at 1000-1750 j/kg. However SE North Dakota is expected to break the 2000 j/kg mark for instability, and that could be the strong point of the storms tonight. PWAT values will exceed 1 inch, and could flirt with 1.5 inches in some areas. SFC (Surface) winds will be converging along a front, so there is tornado potential to the north and south of the front.