A storm usually is preceded by an __________ shift in the direction of the wind and by an _________ circulation of air.

A storm is often characterized by specific atmospheric conditions, particularly related to wind direction and air circulation. The correct choice highlights an "abrupt" shift in the direction of the wind, which is a common precursor to storm activity. This shift can indicate the arrival of a weather front, where contrasting air masses meet, leading to the formation of storms.

Additionally, the mention of "unstable" air circulation is crucial. Unstable air refers to a condition where warm air rises rapidly and leads to the development of strong updrafts, which can create severe weather conditions associated with storms such as thunderstorms and tornadoes.

In contrast, other options either suggest conditions that do not align with typical storm formation—like stable air, which promotes clear, calm weather— or imply a gradual shift, which does not produce the sudden atmospheric changes usually required for storms. Thus, the characterization of wind and air circulation in this context is key to understanding storm development.