Lifted Index (LI) A simple measure of thermal instability. The lifted index (LI) is the temperature difference between the environment surface and an air parcel theoretically lifted adiabatically to 500 mb. The temperature difference is measured in Celsius.
When the value is positive, the atmosphere (at the respective height) is stable and when the value is negative, the atmosphere is unstable.
LI is between 0 and – 2 : Slightly Unstable, Thunderstorms Possible.
LI is between -2 and -6 : Unstable, Thunderstorms Likely, Some Severe
LI is less than -6: Very Unstable, Severe Thunderstorms Likely
A negative LI doesn’t necessarily imply thunderstorms; you also need a triggering mechanism.
(excerpted from Wikipedia)