A Convective SIGMET is issued for thunderstorms.
Instead of SIGMETs, Convective SIGMETs are issued for thunderstorms in the conterminous U.S. (CONUS). Any Convective SIGMET implies severe or greater turbulence, severe icing, and low-level wind shear. A Convective SIGMET will be issued when any of the following conditions are occurring or, in the judgment of the forecaster, are expected to occur:
• A line of thunderstorms at least 60 miles long with thunderstorms affecting at least 40 percent of its length
• An area of active thunderstorms affecting at least 3,000 square miles covering at least 40 percent of the area concerned and exhibiting a very strong radar reflectivity intensity or a significant satellite or lightning signature
• Embedded or severe thunderstorm(s) expected to occur for more than 30 minutes during the valid period regardless of the size of the area
A special Convective SIGMET may be issued when any of the following criteria are occurring or, in the judgment of the forecaster, are expected to occur for more than 30 minutes of the valid period.
• Tornado, hail greater than or equal to 3⁄4 inch (at the surface), or wind gusts greater than or equal to 50 knots (at the surface) are reported
• Indications of rapidly changing conditions, if in the forecaster’s judgment, they are not sufficiently described in existing Convective SIGMETs
• Special issuance is not required for a valid Convective SIGMET
FAA AC 00-45H Aviation Weather Services pg. 5-7
FAA-H-8083-25B Pilot’s Handbook of Aeronautical Knowledge pg. 13-12