TTI < 44:       thunderstorms unlikely.
TTI 44 to 45: isolated thunderstorms.
TTI 46 to 47: scattered thunderstorms.
TTI 48 to 49: scattered thunderstorms, isolated severe thunderstorms.
TTI 50 to 51: few thunderstorms, scattered severe thunderstorms, isolated tornadoes.
TTI 52 to 55: few to numerous thunderstorms, scattered to few severe thunderstorms, isolated to scattered tornadoes.
TTI > 56:       numerous thunderstorms, few severe thunderstorms, scattered tornadoes.
 

The definition of the coverage terms is as follows:

Isolated: expect condition to occur in the forecast area but only expect conditions to affect about 10% of the area. (90% of the are won't have the condition). 
Widely scattered:10- 20% of the area will be affected by the condition.
Scattered: 20 - 50% of the area will be affected by the condition.
Few or likely: 50 - 70 % of the area will be affected by the condition.
Numerous or just the condition: more than 70 % of the area will be affected by the condition.
 

TTI is used to forecast thunderstorm coverage and severity. This index is quite good with cold air aloft. It may overcast severe weather when sufficient low-level moisture is not available and with cold upper air. TTI values work best in the flatter lower elevations east of the Rocky Mountains. For operational purposes, TTI correlations need to be adjusted by region in mountainous terrain. Generally speaking the lower TTI values can indicate more severe thunderstorms in the higher terrain region.

Tests on the field (in the States) showed there is a good correlation between severe weather events and TTI values >48.

Formula used: TT = (T850 - T500) + (Td850 - T500) or TT=T850 + Td850 - (2 x T500).