2. Parameters

Stability Assessment » Total Totals Index (TT) Definition, Strengths and Limitations

Definition: Total Totals Index

The Total Totals index (TT) is yet another severe weather index. It is computed using the temperature and dewpoint at 850 hPa and the temperature at 500 hPa. The higher the 850 hPa dewpoint and temperature and the lower the 500 hPa temperature, the greater the instability and the resulting TT value.

TT values are empirically related to severe weather likelihood as follows:

TT Event
44 Thunderstorms
50 Severe thunderstorms possible
55 or greater Severe thunderstorms likely;
possible tornadoes

Strengths and Limitations

TT is a widely-used severe weather index that is very easy to compute. However, it is limited in that it uses data from only two mandatory levels (850 and 500 hPa) and thus does not account for intervening inversions or moist or dry layers that may occur below or between these levels. In addition, it does not work for areas in the western Great Plains or the Rocky Mountains, where 850 hPa is near the surface or below ground. Last, like several other severe weather indexes, it does not take into account wind shear, which is a critical factor in many severe convective environments.