What Is the Criteria for Issuing a Convective SIGMET?

Active thunderstorms must meet specific conditions before a WST is released.

Wednesday, March 06, 2024 at 01:43 PM ET

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When convective weather is plentiful, expect delays when departing or arriving high-impact airports. [Courtesy: Scott Dennstaedt]

Key Takeaways:

Convective SIGMETs (WSTs) are "NOWcasts" issued by the Aviation Weather Center (AWC) to warn pilots of organized, significant en route convective hazards, distinguishing them from easily avoidable scattered thunderstorms.
Forecasters use specific minimum criteria for issuance: a line of thunderstorms at least 60 miles long (affecting >= 40% of its length), an area of active thunderstorms covering at least 3,000 square miles (>= 40% coverage with strong radar/satellite/lightning signatures), or embedded/severe thunderstorms expected for over 30 minutes.
Special convective SIGMETs may be issued for reported tornadoes, large hail (>= 3/4 inch), or wind gusts (>= 50 knots), or rapidly changing conditions. These advisories are typically issued when criteria have been met for at least 30 minutes and are valid for no more than two hours.

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Question: What is the criteria used by forecasters for issuing a convective SIGMET?

Answer: During the warm season, convective weather has a huge impact on the National Airspace System (NAS). As the amount of usable airspace diminishes on any given day, this ultimately engenders delays in the system. A departure within busy airspace usually means a delay. In the worst-case scenario, ground stops may be levied depending on route of flight and destination airport. Nevertheless, forecasters at the Aviation Weather Center (AWC) are busy at work issuing advisories to warn pilots of these dangerous convective areas.