Which atmospheric phenomenon typically precedes a tornado?

Severe thunderstorms typically precede tornadoes due to the specific atmospheric conditions they create. Tornadoes often form within supercells, which are rotating thunderstorms characterized by a well-defined updraft. The presence of severe thunderstorms indicates significant instability in the atmosphere, as they can produce strong winds, large hail, and heavy rainfall. In addition, these storms can generate the powerful vertical wind shear needed for tornado formation.

The other options represent atmospheric conditions that generally do not lead to tornado occurrences. Fog buildup typically indicates stable air conditions, high pressure systems are generally associated with calm, clear weather, and calm weather conditions do not support the severe instability required for thunderstorms or tornadoes to develop.