+3 to 0
Stable...Weak Convection Possible With Stong Lifting
0 to -3
Marginally Unstable...Scattered Thunderstorms
-3 to -6
Moderately Unstable...Thunderstorms Likely, Some Severe
-6 to -9
Very Unstable...Severe Weather Likely
Less Than -9
Extremely Unstable...Dangerous Thunderstorms

The lifted index is the temperature difference between the 500 mb temperature and the temperature of a parcel lifted to 500 mb. Negative values denote unstable conditions. LI is more of a measure of actual "instability" than CAPE because it represents the potential buoyancy of a parcel at a level, whereas CAPE is integrated through the depth of the troposphere.