The index is formulated as follows:

STP = (mlCAPE/1500 J kg^-1) * ((2000-mlLCL)/1500 m) * (ESRH/150 m² s^-2) * (ESHEAR/20 m s^-1) * ((200+mlCIN)/150 J kg^-1)

When the mlLCL is less than 1000 m AGL, the mlLCL term is set to one, and when the mlCIN is greater than -50 J kg^-1, the mlCIN term is set to one. Lastly, the ESHEAR term is capped at a value of 1.5, and set to zero when ESHEAR is less than 12.5 m s^-1. A majority of significant tornadoes (F2 or greater damage) have been associated with STP values greater than 1, while most non-tornadic supercells have been associated with values less than 1 in a large sample of RUC analysis proximity soundings.