Final answer:
The stage of a middle latitude cyclone that involves a disturbance along the polar front that initiates convergence is the incipient stage.
Step-by-step explanation:
The stage of a middle latitude cyclone that involves a disturbance along the polar front that initiates convergence is known as the incipient stage. During this stage, a temperature contrast between the warm air moving poleward and the cold air moving equatorward creates a disturbance along the polar front. As the warm air rises and the cold air sinks, a low-pressure center begins to form, initiating convergence of air masses.