"Isolated thunderstorms" means that indicators predict only one line of storms affecting 10 to 20 percent of the area, according to Aerostorms. Isolated thunderstorms have nothing to do with the intensity of the storm.
When isolated thunderstorms are predicted, there is typically only one storm complex in the area, and the area covered is less than when scattered thunderstorms are predicted. Scattered thunderstorms may also have more than one line of storms in the area that have the potential to affect a larger portion of the map. The squall line or storm of an isolated thunderstorm may be small and cover less, or it may be large and affect many.