Energy Helicity Index
(or EHI) - An index that incorporates vertical shear and instability, designed for the purpose of forecasting supercell thunderstorms
. It is related directly to storm-relative helicity in the lowest 2 km (SRH, in m2
) and CAPE (in j/kg) as follows:
EHI = (CAPE x SRH)/160,000.
Thus, higher values indicate unstable conditions and/or strong vertical shear. Since both parameters are important for severe weather development, higher values generally indicate a greater potential for severe weather. Values of 1 or more are said to indicate a heightened threat of tornadoes; values of 5 or more are rarely observed, and are said to indicate potential for violent tornadoes.