A control system and method is provided for controlling the operational configuration of a spout of a harvester being used to perform unload on the go operations with an associated transport vehicle. The velocities of the harvester and transport vehicle and the lateral and longitudinal distances between the harvester and transport vehicle are used to predict future lateral and longitudinal distances between the harvester and transport vehicle. A control signal is issued to a harvester spout control system if either of the predicted lateral distance or longitudinal distance is outside of an associated acceptable range. The harvester spout control system then either shuts off the spout or changes the orientation of the spout in response to the control signal.