The convective available potential energy (given in joules per kilogram air) is the amount of energy a parcel of air would have if lifted a certain distance vertically through the atmosphere. Any value greater than 0 J/kg indicates instability and an increasing probability of thunderstorms and hail.
Below is a chart containing the CAPE values on the 18th of August 2017 when a severe thunderstorm hit Switzerland in the evening. One can clearly make out the increasing instability of the atmosphere.
The lifted index (given in Kelvin) is the difference between the temperature of the environment and that of an air parcel lifted adiabatically to a given pressure height in the troposphere, usually 500 hPa. The atmosphere at the given height is stable for positive values of the lifted index and unstable for negative values. When the value falls below -2, thunderstorms are anticipated. and for lower values, especially those below -6, severe weather is expected.
Below is a chart containing the lifted index values on the 18th of August, 2017, when a severe thunderstorm hit Switzerland in the evening. We observe the rapidly decreasing values around 18h when the storm formed.