The Energy-Helicity Index, or EHI, is a dimensionless value formed by multiplying CAPE by storm-relative helicity and dividing by 160000, so a single number captures the combined thermodynamic and low-level rotation potential of an environment. The normalization constant was chosen so typical significant tornado environments give values on the order of one or greater.
What the formula is telling you
The product of CAPE and SRH grows quickly when both are large and collapses when either is small. That structure encodes the ingredients-based idea directly. A strongly unstable airmass with weak low-level rotation gives modest EHI. A strongly sheared low-level flow with only marginal instability gives modest EHI. Both together give a large number.
Layer choice matters. A 0 to 1 km EHI uses low-level SRH and is more closely tied to tornado potential when the low-level curve is favorable. A 0 to 3 km EHI uses a deeper helicity layer and behaves more like a general supercell diagnostic. Effective-layer variants are also computed and are less sensitive to elevated storms feeding on air above a shallow surface layer.
How forecasters use it
EHI is typically inspected as a field on a mesoanalysis map, alongside SCP and STP, to pick out regions where both energy and helicity are present at the same time. Point values in soundings extend that with more detail.
As a rough scale, 0 to 3 km EHI values above two or three are typically associated with an increased threat of tornadoes in supercell environments, though the mapping is not a strict rule. Negative values indicate environments where the storm-relative flow favors left-moving anticyclonic supercells, which are far less common tornado producers.
Where it misleads
EHI does not know which ingredient carried the value. A high number driven mostly by extreme CAPE over marginal SRH describes a very different day than a high number driven mostly by strong low-level shear over modest CAPE. The individual fields have to be inspected.
The composite also depends on the storm motion assumption used to compute SRH. If real storms move differently from the assumed motion, the ingested helicity is not what the composite implied. And like every environmental composite, EHI describes the setup, not the storm. Storms still have to fire, and they still have to organize into rotating updrafts before any tornado potential is realized.