The basic premise behind the EHI (Energy-Helicity Index) is that storm rotation should be maximized when CAPE is large and SRH is large. 0-1-km EHI values greater than 1-2 have been associated with significant tornadoes in supercells.

References:
Rasmussen, E. N., 2003: Refined supercell and tornado forecast parameters. Wea. Forecasting,, 18, 530-535.

Thompson, R. L., R. Edwards, J. A. Hart, K. L. Elmore, and P. Markowski, 2003: Close proximity soundings within supercell environments obtained from the Rapid Updage Cycle. Wea. Forecasting, 18, 1243-1261.

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