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/s2) 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. However, there are no magic numbers or critical threshold values to confirm or predict the occurrence of tornadoes of a particular intensity.
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