Definition
An in-flight weather advisory issued by the National Weather Service for hazardous convective weather that is significant to the safety of all aircraft. Convective SIGMETs are issued for severe thunderstorms with surface winds of 50 knots or greater, hail at the surface 3/4 inch in diameter or greater, tornadoes, embedded thunderstorms, lines of thunderstorms, or thunderstorms with heavy or greater precipitation that affect 40 percent or more of an area at least 3,000 square miles. They imply severe or greater turbulence, severe icing, and low-level wind shear, and are issued for the contiguous United States.
Plain English
A weather alert pilots receive in flight that warns of dangerous thunderstorm activity ahead — things like tornadoes, severe storms, large hail, or lines of thunderstorms. If one is issued for your route, the weather it describes is bad enough to be a hazard to any aircraft, large or small.
Context Anchor
Pilots encounter Convective Significant Meteorological Information during weather briefings, flight planning, and in-flight weather updates when thunderstorm activity may affect the route.
Derivation
Convective comes from Latin convectus, meaning 'carried together' — referring to the rising and falling movement of air that builds thunderstorms. SIGMET is shortened from 'Significant Meteorological Information.' Together, the term identifies a weather warning specifically about hazardous thunderstorm-type activity, as opposed to a regular SIGMET, which covers other severe non-convective weather.
Why Pilots Care
Pilots must divert around these areas because the storms can produce severe turbulence, icing, and wind shear that risk loss of control.
Analogy
It is like a road warning for a dangerous storm cell on your driving route, except in aviation the safest choice is usually to go around the weather with plenty of space.
Grounding Statement
If a weather briefing shows Convective Significant Meteorological Information along your route, picture an area of thunderstorm weather that deserves active avoidance, not casual monitoring.
Intuition Check
Do not read “significant” as simply “interesting” or “worth noting.” In this FAA weather term, it means the thunderstorm weather is important to flight safety and hazardous to aircraft.
Example Sentence 1
Flight Service advised us of a Convective SIGMET for a line of thunderstorms moving across our route, so we requested a deviation 30 miles south.
Example Sentence 2
During the weather briefing the specialist pointed out an active Convective Significant Meteorological Information that covered the arrival airport due to embedded thunderstorms.