The storm in this radar image is moving northeast toward the photographer.  The line of weak reflectivity (light blue) to the east of the storm represents outflow of rain-cooled air from the storm.  This outflow is what led to the shelf cloud in the photograph.  An additional outflow boundary can be seen oriented southwest-northeast across Adams county.  This outflow boundary was leftover from a weakening thunderstorm seen over Highland County.  As the Brown County storm crossed this outflow boundary, it moved into an area of rain-cooled air from the earlier storm and lost its source of warm buoyant inflow.  Therefore the storm weakened by the time it reached the photographer.