The National Weather Service (NWS) issues various aviation weather advisories to ensure safety across the national airspace. These specialized alerts provide meteorological information to pilots and flight dispatchers about conditions that could affect aircraft operations. A Convective Significant Meteorological Information (SIGMET) represents the highest level of in-flight weather warning. It alerts aircraft to highly dangerous, widespread phenomena that pose an immediate and severe threat to flight safety.
Defining the Convective SIGMET
A Convective SIGMET, abbreviated as WST, is a specialized weather bulletin issued exclusively for the conterminous United States (CONUS) to warn of hazardous weather associated with thunderstorms. The purpose of this advisory is to identify and delineate areas of current or forecasted convective activity that could compromise flight safety. Unlike a standard SIGMET (WS), which covers non-thunderstorm-related severe weather like widespread dust storms or severe clear-air turbulence, the Convective SIGMET focuses solely on thunderstorm-related dangers.
The advisory alerts aviation personnel to widespread phenomena, such as severe turbulence, severe icing, and low-level wind shear, which are implied by the presence of a severe thunderstorm. Meteorologists issue these warnings when significant convective weather is occurring or expected. This information allows pilots to make strategic decisions regarding routing and altitude selection to maintain safety.
Criteria for Issuance
The National Weather Service issues a Convective SIGMET only when specific, measurable thresholds of severe weather phenomena pose risks to aviation. These criteria must be met or forecasted to be met for at least 30 minutes to warrant the issuance of an advisory. The presence of a tornado automatically triggers a special issuance Convective SIGMET.
Individual Storm Criteria
An advisory is triggered by the observation or forecast of surface winds greater than or equal to 50 knots (approximately 58 miles per hour) caused by a thunderstorm. Hail at the surface measuring three-quarters of an inch or larger in diameter also meets the necessary threshold. Furthermore, an advisory is issued for embedded thunderstorms, which are completely obscured by layers of clouds, or for any severe thunderstorm expected to persist for more than 30 minutes.
Organized System Criteria
The advisory is also necessary for organized systems. This includes a line of thunderstorms at least 60 miles long, where storms affect 40% or more of that length. Alternatively, a Convective SIGMET is issued for an area of thunderstorms covering at least 40% of a region that is 3,000 square miles or greater. This area must also be exhibiting very strong radar reflectivity or a significant satellite or lightning signature. These criteria identify conditions that produce intense forms of turbulence, icing, and wind shear.
Interpreting the Message
Convective SIGMET information is presented in a highly standardized, coded text bulletin. These advisories are issued by the Aviation Weather Center and cover the conterminous U.S., which is divided into three regions: Eastern, Central, and Western. The bulletin begins with a text header that includes the WST abbreviator, followed by the date and time of posting.
The message includes a sequence number indicating the issuance for that region, such as “CONVECTIVE SIGMET 83C,” where “C” denotes the Central region. This is followed by the valid time, specifying the two-hour period during which the conditions are expected to persist. The affected area is then described using geographical references and navigational aids, such as Very High Frequency Omnidirectional Range (VOR) stations, which define the boundaries of the hazard.
The body of the message details the specific hazard, such as “AREA TS MOV FROM 21035KT,” which translates to an area of thunderstorms moving from 210 degrees at 35 knots. It also includes the maximum cloud tops, for example, “TOPS TO FL300,” indicating the tops of the storms are at 30,000 feet above sea level (Flight Level 300). This specific locational and movement data enables pilots to visualize the danger and plan a route deviation around the affected zone.
Duration and Cancellation
Convective SIGMETs are issued on a scheduled basis, occurring hourly at 55 minutes past the hour, provided the hazardous conditions are present. Each advisory is valid for a maximum duration of two hours, or until it is superseded by the next scheduled or unscheduled issuance. This timeline ensures that pilots receive the most current information possible before and during their flights.
If a region does not meet the specified criteria, a “CONVECTIVE SIGMET… NONE” is issued to confirm the absence of severe thunderstorm hazards. An outlook for severe convective activity covering the following two to six hours is also attached to the end of each regional bulletin. If the severe weather dissipates before the two-hour validity period expires, the advisory may be canceled; if conditions persist, a new bulletin is reissued with an updated sequence number at the next scheduled time.