One of the great mysteries in weather forecasting is predicting the exact location a thunderstorm will form. What causes a storm to form at one location and not another just down the road? Why do some of the thunderstorms that are near each other become stronger than others? Will we ever be able to predict the locations individual thunderstorms will develop and move?
These are all great questions that today's technology can not yet answer fully. However, we do understand why storms form in one place and not in another. With today's technology, forecasters know a region in which thunderstorms or severe thunderstorms "watch boxes" are likely to develop but not over which counties they will develop.
For warm season thunderstorms "air mass thunderstorms" the two important known factors which determine where a storm will form are the cap and boundary layer conditions (assuming mid-levels of atmosphere are unstable). With one Skew-T sounding the cap is only known for that one point. In every direction from the sounding the cap strength will be different. This is synonymous with rain gauges. Spread rain gauges out over a county and each one will record a different rainfall amount (some more than average and some less than average). This same idea is true of the cap; it is stronger in some locations than others, even over small distances. What makes it even more complicated is that these maximums and minimums in the cap are in motion.
The second factor is boundary conditions (region from surface to the bottom of the cap). The best tool to assess boundary layer stability or instability is Theta-E and zones of small scale convergence. Theta-E, as you know, combines temperature and moisture. Theta-E increases (boundary layer more unstable) as temperature and/or moisture content increase. Theta-E ridges represent areas which are potentially more buoyant than others if the air is allowed to rise.
Putting these two ideas together, air mass thunderstorms will first develop at locations that have a combination of a low cap and high Theta-E and boundary layer convergence. Today, this can be done operationally on the synoptic and the medium to large mesoscale, but not yet at a scale small enough to predict over which county a storm will develop. Exceptions to this occur on small time scales. With a wind analysis (areas of convergence and divergence) and a cap, theta-E analysis, thunderstorms can be predicted just before they form (less than an hour before they form) if a mesoscale network is in place such as Oklahoma's MESONET. But the scale is too large and atmosphere too chaotic to be able to forecast more than 6 hours in advance the exact location a storm will form. For now we will have to stick with probabilities (percentage chance of rain, scatteredness of storms).
Of course there are other factors such as topography and dynamical lifting that make this discussion even more complicated. The primary point to make is that every indice value on a Skew-T varies across the forecast region.