Which in-flight advisory would be issued to forecast severe thunderstorms over an area?

The issuance of a Convective SIGMET is specifically designed to warn pilots of hazardous convective weather phenomena, including severe thunderstorms. Convective SIGMETs are issued when there is a forecast of severe weather that includes thunderstorms that exhibit a high probability of producing severe turbulence, surface winds of 50 knots or greater, hail, or tornadoes.

These advisories are critical for flight planning and safety as they inform pilots about areas of potential danger due to severe thunderstorms. A Convective SIGMET is issued for significant convective activity and, therefore, is crucial for understanding the dynamics of weather that could affect flight operations.

In contrast, the other options serve different purposes: Cumulus SIGMET relates to cloud formations and their potential impacts, Turbulence SIGMET deals specifically with turbulence phenomena, and Weather SIGMET encompasses a variety of significant weather but does not specifically target convective threats like thunderstorms. Thus, for the forecasting of severe thunderstorms, the Convective SIGMET is the most appropriate advisory.