Surface analysis indicates low pressure is present in the Northern Plains. With a warm front nearby, the atmosphere should begin to destabilize along the front. Pop up thunderstorms could begin with rotation due to the influence of the low pressure's wind field and the proximity to the 'Ring of Fire'. Soundings validate the presence of a capping inversion in the lower leves of the atmosphere, but with instability prominent in the mid levels of the atmosphere.
Sounding analysis across the Ohio Valley and Mid Atlantic indicates a capping inversion is in place, with nearly 1000 j/kg of CINH in place. This should hold down storm development until the afternoon. There is some indication that an EML (Elevated Mixed Layer) has overspread parts of the Mid Atlantic, and this could also hinder storm development for that area. (An Elevated Mixed Layer, in a nutshell, is something that hinders storm development.)
Expectation is for scattered yet strong storms to develop in both areas of severe weather risk area per RAP model guidance. The areas are so big due to the scattered-ness of the storms and a general 'safety risk' to cover all areas that have a potential for storms that could turn severe, rather than covering small, individual areas that the models say will get struck.