TTI < 44: thunderstorms unlikely.
TTI 44 to 45: isolated thunderstorms.
TTI 46 to 47: scattered thunderstorms.
TTI 48 to 49: scattered thunderstorms, isolated severe
TTI 50 to 51: few thunderstorms, scattered severe thunderstorms,
TTI 52 to 55: few to numerous thunderstorms, scattered to few
severe thunderstorms, isolated to scattered tornadoes.
TTI > 56: numerous thunderstorms, few severe thunderstorms,
The definition of the coverage terms is as
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
Scattered: 20 - 50% of the area will be affected by the
Few or likely: 50 - 70 % of the area will be affected by the
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
Tests on the field (in the States) showed there
is a good correlation between severe weather events and TTI
Formula used: TT = (T850 - T500) + (Td850 -
T500) or TT=T850 + Td850 - (2 x T500).