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 to warn pilots of organized, significant, and dangerous en route convective hazards (e.g., embedded, severe, or dense thunderstorms) that are difficult or dangerous to penetrate.
Forecasters issue WSTs based on specific minimum criteria, including lines of thunderstorms (at least 60 miles long, affecting >= 40% of length), areas of active thunderstorms (>= 3,000 sq miles, >= 40% coverage with strong signatures), or embedded/severe thunderstorms expected for more than 30 minutes.
Special WSTs may be issued for reported tornadoes, large hail, or strong wind gusts; however, routine advisories generally require convection to persist and meet criteria for at least 30 minutes before the scheduled 55-minute-past-the-hour issuance.

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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.