Have you felt like conditions were ripe for storm development, only to have nothing materialize? Or seen cumulus clouds start to show greater height, only to stall out? What might be occurring is a “capped inversion” or “cap”. This is a layer of relatively warm air aloft, usually several thousand feet above the surface. Rising air is relatively cooler than this layer, limiting its ability to continue to rise. This in turn, slows or prevents the development of thunderstorms, even when the air mass is extremely unstable. If the cap weakens, or is displaced, however, you can see rapid thunderstorm development. It can go from clear skies to severe storms in a matter of hours.
Mary Knapp, Weather Data Library