A= 0 when T850-T500 >25
A= 2 when T850-T500 22-25
A= 6 when T850-T500 <22

(T = temperature at 850 or 500 hPa)

SI    <39:        no  thunderstorms (89%).
SI    41 to 45: thunderstorms possible (42%)
SI    >46:        thunderstorms likely (75%)

SI is used to forecast thunderstorm coverage and severity. Derived from the TT Index, this index adds 700 hPa moisture and a variable parameter based on the VT (Vertical Totals). The additions of 700 hPa moisture tailors the index for sections of Europe since low level heating is usually less intense then in the States. It is useful from April to September.  

Formula used: SI = TT - (T700-Td700) - A