An isolated T-storm in a weather report means that a small percentage, typically between 10 percent and 20 percent of the affected area may see a thunderstorm. The clouds are usually a part of a squall line, which usually proceeds a cold front.
A thunderstorm does not produce much rain or lightning, and it does not last long. Sometimes there can be two storms that follow each other quickly as the squall line passes over. However, they usually move quickly, and the skies are clear soon after. Sometimes an isolated thunderstorm can produce some high winds depending on the size of the squall line following it.