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 (WST) are "NOWcasts" issued by the Aviation Weather Center (AWC) to alert pilots to significant and organized en route thunderstorm hazards that are challenging or dangerous to penetrate.
Routine convective SIGMETs require specific minimum criteria, including 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 with 40% coverage and strong radar/satellite/lightning signatures; or embedded/severe thunderstorms expected for over 30 minutes.
Special convective SIGMETs may be issued for reported tornadoes, hail (≥ 3/4 inch), or wind gusts (≥ 50 knots), or for rapidly changing conditions not adequately described in existing SIGMETs.
Issuance typically occurs after convection has persisted and met criteria for at least 30 minutes, with routine advisories issued at 55 minutes past the hour and valid for up to 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.