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Results from a study by Jon Davies on the distribution of low-level CAPE and tornadic vs. non-tornadic supercells.
This graphic (from research by Jon Davies (displayed at http://members.cox.net/jondavies1/Llbuoyprimer/cape3rnv2.gif), indicates that low-level CAPE values of greater than 100 J/kg may be a useful cut-off for tornadic vs. non-tornadic supercells. However, Davies emphasizes that model forecasts of low-level CAPE are very sensitive to forecasts of low-level moisture, and are not always reliable. He recommends that low-level CAPE be evaluated in conjunction with other parameters, such as convective inhibition, to more fully evaluate the low-level thermodynamic environment. The bottom line is that tornadogenesis is favored in areas with ample low-level instability, which enhances low-level upward acceleration of parcels and vorticity development due to vertical stretching.