Loaded gun scenario
On a day of instability through a great depth of the troposphere, and high values of CAPE (q.v.), rising surface temperatures will at some point ensure that convection parcels leave the surface, the condensation level will be reached, cloud will grow (given sufficient moisture), and a heavy shower, or even a thunderstorm will result. It sometimes happens though that although the atmosphere is markedly unstable above, say, 2 km a 'lid' opposing surface-based convection exists at or below this level, due often to a layer of warm/dry air that has become entrained in the airflow from some source. This means that surface temperatures must become very high to overcome this 'lid', often requiring additional triggers, such as low-level convergence or release of medium level potential instability by a mid-level trough, thus lifting the whole column and releasing the 'pent-up' energy in a sudden burst ... and the 'loaded gun' will be 'fired', perhaps leading to a severe storm/supercell event. (see 'Spanish plume').
