Coincidental with the onset of deep convection near Waco was the passage of the MCS-generated gravity wave. Having originated in Arkansas, the central Texas portion of this feature moved southwestward in-step with the surface wave until around 2100 UTC, at which time the cold front overtook the dry line near Austin. The advance of the gravity wave and its motion relative to the developing storms is readily apparent in the animated satellite imagery shown in Figure 6.
As previously noted, while the lower tropospheric environment over central Texas on the afternoon of 27 May 1997 was very unstable, such instability is certainly not unprecedented. The arrangement and behavior of mesoscale surface features was also not especially noteworthy: slowly-moving dry line/frontal mergers of the type observed occur in that region several times a year. Readily apparent gravity waves are also not uncommon in the area. Nevertheless, the fact that supercells continued to form over central Texas through much of the afternoon despite the presence of weak vertical shear suggests that some unusual circumstances were indeed coming into play. While a more conclusive answer must await careful modelling studies, it is the author's opinion that the fortuitous orientation and timing of the MCS-induced gravity wave, relative to both the slowly-moving surface boundaries and to the diurnal heating cycle, were critical in realizing and maintaining supercellular convection.
I would consider a gravity wave to be a invisible gravitational force creating a disturbance in the atmosphere across a flat area. Not a mountain/hill creating a wave in air flow like this:
