The Estofex "NCEP GFS Experimental Hail Parameter" was created by Pieter Groenemeijer.

The calculation is done by the following formula:

SHEAR_SFC_EL	= Bulk wind shear [m/s] between 10m and half the equilibrium level
MU_LI_10	= Buoyancy of the most unstable parcel at the -10 °C level in kelvin
MR_BL	= Mixing ratio of the boundary layer [kg/kg]
T_MU_LCL	= Temperature at lcl of the most unstable parcel

UPDRAFT	= (1.0 + (SHEAR_SFC_EL / 20.0)) * (-MU_LI_10)
ICE	= 1.0 - (1.0 / (1.0 + (10.0 * UPDRAFT / (T_MU_LCL * T_MU_LCL))))

If T_MU_LCL is < 0.0 the Max. Hail-Size is set to 0.
If UPDRAFT is < 0.0 the Max. Hail-Size is set to 0.

Max. Hail-Size = ICE * MR_BL * 1000.0 * 0.5 * (1.0 + sqrt(UPDRAFT / 10.0))  [cm]

If the Max. Hail-Size is < 0.5cm it is set to 0.5cm.