The question of what VGP means was brought up in the NOW thread for 04/26/11.
VGP = Vorticity Generation Parameter/Potential. There are a number of websites that give a short blurb about it (usually the same one that is copied from SPC's mesoanalysis page). However, Rasmussen and Blanchard (1998) gives a very scientific explanation of it. Essentially, VGP = Shear*sqrt(CAPE) and gives an expression for how well horizontally generated vorticity will be tilted into the vertical. It has been shown to discriminate non-tornadic from weakly-tornadic from strongly-tornadic supercells.
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