What atmospheric condition is typically present during the formation of an air mass thunderstorm?

During the formation of an air mass thunderstorm, local instability is a critical atmospheric condition. This instability occurs when warm, moist air near the surface rises into cooler air above it, creating a situation conducive to the development of thunderstorms. The warm air's buoyancy is key; as it rises, it cools and condenses, forming cumulonimbus clouds that can lead to thunderstorms.

While high-pressure systems are generally associated with calm and clear weather, local instability provides the necessary lift for the air, which can trigger convection and lead to the development of storm clouds. Overcast skies do not specifically indicate the presence of air mass thunderstorms, as such conditions can occur under various cloud formations not necessarily linked to instability. Stable air masses typically resist vertical movement, making them unfavorable for thunderstorm development. Thus, the presence of local instability is what sets the stage for air mass thunderstorms.