CT  < 18        weak potential for thunderstorms.
CT 18 to19    moderate potential for thunderstorms.
CT 20 to 21   strong potential for thunderstorms.
CT 22 to 23   weak potential for severe thunderstorms.
CT 24 to 25   moderate potential for severe thunderstorms.
CT >25          strong potential for severe thunderstorms.
 

Tests on the field (in the States) showed there is a good correlation between severe weather events and CT =>24.

Formula used: CT = Td850 - T500