SSI   > 2:   no thunderstorms.
SSI =< 2:   thunderstorms.

(*) To find 500 hPa parcel temperature: construct LCL for parcel at 850 hPa. From this point ascend saturated adiabatically to the 500 hPa level and read off.

This Index doesn't work well if a frontal surface or inversion is present between 850 hPa and 500 hPa.

Formula used: SSI = T - Tparcel  (both T at 500 hPa)