METEOROLOGIST JEFF HABY
1. What is EHI?
The Energy Helicity Index (EHI) is a combination of two indexes. By itself, it is the best index available for
tornado prediction since it combines both
Helicity. The CAPE is the amount of pure instability
present from a parcel of air that rises from the lower
PBL. Helicity is
the product of low level shearing (known as streamwise vorticity) and storm inflow directly into the streamwise
vorticity. The Helicity is storm relative which means the Helicity is calculated from the storm's frame of
2. How is EHI determined?:
EHI = (CAPE * SR HEL) / 160,000
The sounding at the bottom shows an EHI value of about 6. The EHI value has no units.
Here is how the value was determine:
CAPE = 4,385 J/kg
SR HEL = 220 m^2/s^2
EHI = (4,385 * 220) / 160,000 = 6
3. Operational significance of EHI:
|EHI > 1 ||Supercell potential||1 to 5 ||up to F2, F3 tornadoes possible
||5+ ||up to F4, F5 tornadoes possible
a. Tornadoes are still possible in cases with a low EHI in cases where the CAPE or the
shear is very large. This is
especially true when the CAPE is low based and when mesoscale boundaries enhance the Helicity.
b. HEL is especially variable on the mesoscale-- thus EHI may be much higher in particular areas.
c. Make sure storms will develop in the first place in the high EHI environment
(i.e. cap will break).