The Lifted Index is a thermodynamic parameter that compares the theoretical 500 mb parcel temperature (temperature of a parcel of air lifted from the lower PBL to the 500 mb level) to the actual (environmental) temperature at 500 millibars. The Lifted Index is plotted with the relative humidity on model progs and is available at:
Regions with an unstable LI are hatched and partially hide the RH colors. Contours of LI values are also drawn in white.
A lifted Index value that is positive represents stability of the troposphere with respect to boundary layer convection. A negative LI indicates instability. A LI of 0 is neutral. LI should only be used when forecasting in the warm season or in the warm sector of a mid-latitude cyclone. Parcels of air will NOT rise from the lower PBL on the colder side of a frontal boundary or within dense PBL polar air. The LI is best used when the troposphere has the potential to produce boundary layer based thunderstorms. The more negative the LI, the more potential acceleration an air parcel has if lifted to the Level of Free Convection (LFC). LI values from -1 to -3 are unstable, -4 to -6 are very unstable, -7 or less are extremely unstable. CAPE and LI are related in that as a general rule: when the LI decreases, CAPE increases.
The LI value says nothing about if storms will occur. It gives a forecaster a general idea of convective forcing if thunderstorms do develop. Why is LI plotted with RH? Because unstable LI values combined with a high RH indicates the troposphere is near saturation and has instability. A "trigger mechanism" such as a front will be able to produce thunderstorms and heavy rain in this high RH low LI environment. Again, LI is not of much use in the winter because the PBL tends to be dry (low dewpoints) and cold (stable). "Elevated convection", "dynamic forcing without thermodynamic forcing" and "isentropic lifting" do not mesh well with using the LI. The LI can be very stable but the troposphere produces precipitation when the three terms in parenthesis above occur.