LI = the temp of the environmental air at the height of the 500hpa pressure level (about 18,500 feet up) - the temp of an air parcel if it was lifted to that height (nominally from the surface).

So for example, if the temp of an air parcel that rose to the 500hpa height was -10C and the surrounding air at that height was -15C, the LI would be -5.

In other words, negative LI values indicate that the air parcel is still staying warmer than the surrounding air at that height and therefore will want to keep rising (which are needed to form thunderstorms). The more negative an LI value, the more unstable the air (generally speaking).

BUT just like other indices that measure instability, it relies on various assumptions and has limitations. For example, if the forecast moisture or temp of the air parcel that rises from the surface doesn't reflect reality, the forecast LI value also won't be realistic. Furthermore, parcels of air often don't rise just from the surface on sunny warm days when the lower atmosphere is thoroughly mixed (instead, they rise from that mixed layer that can be thousands of feet deep) so the surface LI values you see on BSCH charts aren't always representative of true instability. And lastly, LI only uses a single fixed height (500hpa) so there can be situations where the temp difference between a rising air parcel and the surrounding environmental air is different at other heights which in turn affects the true instability. CAPE partly overcomes this limitation because it integrates the temp difference through all heights where the parcel stays warmer than the surrounding air but it too, relies on assumptions about the air parcel.

This is why it's a lot better to check forecast soundings (side-on profiles of the atmosphere at any given point) to get an idea of what the instability is like through the whole atmosphere but it can be daunting for people to try and interpret if they're not used to it so LI's still do give a rough idea of where there may be instability.

Any afternoon/evening thunderstorm activity that occurs tomorrow looks somewhat isolated in nature to me, and mainly over inland and southern parts of our region.