Here's the answer I found when I googled it. I love Google.
The National Weather Service has very specific definitions for the terms "scattered" and "isolated". These terms are used to describe the amount of area covered by thunderstorm activity. For example, forecasting isolated thunderstorms implies that less than 30 percent of the forecast area should experience a thunderstorm. If scattered thunderstorms are in the forecast, 30 percent to 50 percent of the forecast area should experience a thunderstorm.