Generalization of the Heuristic Drift SOL model for finite collisionality and effect on flow shearing rate vs. interchange growth rate

•This paper generalizes the highly successful Heuristic Drift model of the power scrape-off width to include collisional effects relevant at high density, as well as finite target temperature effects relevant at very low density.•Reasonable agreement is found with recent power-scrape-off width measurements made on ASDEX-Upgrade.•The flow shearing rates calculated for the SOL of H-Mode plasmas substantially exceed their interchange growth rates, which could explain the low turbulence level of these plasmas.•Furthermore, the generalized HD (GHD) model predicts that this flow shear is reduced at higher densities, leading ultimately to the transition back from H-Mode to L-Mode at the point where the shearing rate equals the interchange growth rate.•This is found to be consistent with ASDEX-Upgrade high-density H-mode limit studies.•ITER H-Modes are predicted to have extremely high shearing rates compared with interchange growth rates, perhaps indicating that exceptionally high quality H-modes may be accessible.