How does fast moving cold air create thunderstorms when combined with warm air

To create a thunderstorm you need to have three things, the first of which is a mass of unstable air, which is the result of warm moist air in the lower atmosphere and cold dry air in the upper atmosphere. The second thing needed for a thunderstorm to develop is the force, which mixes the warm and cold air causing the instability. This force can be wind from mountains, fronts or in air convergence like that found where bodies of water and land meet. The warm air rises from the lower atmosphere causing an updraft, which mixes with the heavier cold air. The third factor that must be present for the creation of a thunderstorm is moisture in the atmosphere. This happens when the moisture in the air turns into water droplets and forms clouds. These three things together make up the cumulus stage or formation of a thunderstorm.
 
The second phase of a thunderstorms life cycle is the mature thunderstorm phase which is when the moisture formed in the clouds is released when it becomes to heavy to stay up in the atmosphere and falls to earth in the form rain, snow, sleet or hail. During this phase there is an updraft of the lighter moist warm air from the ground, which keeps the thunderstorm from dissipating. While the warm air is rising there is cold dry air that is causing a down draft causing the heavy water droplets to fall. This is also the period when there is thunder and lightning, which is one of the more spectacular aspects of thunderstorm.