CAPE

(Convective Available Potential Energy)
The chance of severe thunderstorms can be estimated by looking at atmoshperic conditions.
The activity of a thunderstorm is caused and supported by atmospheric energy, so it seems to make sense to look at the energy available in the atmoshphere when considering the potential for severe weather.
Three ingredients are required for convective storm development: moisture, instability, and a triggering mechanism.
One of the indicators that may be used is CAPE, which is an estimation of the energy available for convective activity, in Joules/Kg of air, and relates to instability. Of course, the higher this number, the greater the chance of severe weather.
A CAPE of 1000 indicates a good chance of storms forming, 2000 to 3000 and above indicate a good chance of severe weather.

CAPE, however, is not the only influence on vertical motion within a convective storm. Intense updrafts can develop with weak to moderate CAPE in sufficiently sheared environments, where dynamical effects (i.e., lifting forces associated with the interaction of an updraft with the vertically sheared environment) can enhance the strength of an updraft substantially.