A Convective Significant Meteorological Information, or Convective SIGMET (WST), is a weather advisory issued for all aircraft concerning severe convective activity over the contiguous United States and coastal waters. This advisory serves as a warning system for pilots and air traffic control, highlighting areas where thunderstorms pose a significant hazard to aviation safety. The Aviation Weather Center (AWC) is responsible for issuing these specialized advisories, which provide timely information for the safe rerouting or grounding of aircraft.
The Specific Criteria for Issuance
Convective SIGMETs are issued when specific, objective criteria related to severe thunderstorm activity are met or are expected to occur for more than 30 minutes during the advisory’s valid period. These criteria capture the most dangerous forms of convective weather that severely impact flight operations.
A Convective SIGMET is issued for:
- The presence of a tornado.
- Hail three-quarters of an inch or greater in diameter, posing a risk of structural damage to aircraft.
- Surface wind gusts of 50 knots (approximately 58 miles per hour) or greater, often associated with thunderstorm downbursts.
- A line of thunderstorms at least 60 nautical miles long, with thunderstorms affecting 40 percent or more of that length.
- Thunderstorms affecting 40 percent or more of an area spanning at least 3,000 square miles. This area must exhibit strong radar reflectivity or a significant satellite or lightning signature.
- Embedded thunderstorms, which are obscured by other cloud layers and difficult for pilots to spot visually.
Operational Timing and Validity
Convective SIGMETs follow a predictable, scheduled cycle to ensure pilots and air traffic controllers receive timely updates. Advisories are routinely issued at 55 minutes past the hour (H+55), every hour, for the three designated regions of the contiguous United States: Eastern, Central, and Western.
Each Convective SIGMET is valid for a period of two hours from its time of issuance, or until superseded by a subsequent advisory. If the severe convective conditions persist, a new Convective SIGMET is issued at the next scheduled time. If no conditions meet the criteria in a given region, the AWC issues a “CONVECTIVE SIGMET… NONE” statement to confirm the absence of organized severe convection.
While issuance is primarily scheduled, the AWC forecaster can issue a special, unscheduled Convective SIGMET at any time if conditions rapidly change or if a severe weather event is immediately reported. Conversely, an advisory can be canceled before its two-hour expiration if the severe weather dissipates or falls below the required criteria.
How to Access and Interpret the Information
Pilots and flight planners access Convective SIGMET information through multiple official aviation weather sources. These include the Aviation Weather Center’s website, Flight Service Stations (FSS) during pre-flight briefings, and the Hazardous Inflight Weather Advisory Service (HIWAS), which broadcasts the text over select VHF radio frequencies for airborne pilots.
The textual format uses a standardized structure, beginning with an alphanumeric identifier (e.g., “WST”), a sequence number, and the region (e.g., CONVECTIVE SIGMET 15C). The body specifies the affected area using VOR navigation aids or coordinates to define a polygon. It states the type of hazard, such as “AREA OF EMBEDDED THUNDERSTORMS,” and includes a forecast of the storm’s movement and intensity.
A Convective SIGMET is a forecast of a significant hazard to all aircraft, implying the presence of severe turbulence, severe icing, and low-level wind shear, even if not explicitly listed. The advisory is not a prohibition on flight, but a strong warning requiring pilots to plan a route that avoids the defined area by a safe margin. Graphical products, which overlay the advisory area onto a map, are useful for visualizing the extent of the hazard and making real-time decisions.