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 are "NOWcasts" issued by the Aviation Weather Center to warn pilots of organized, dangerous convection that poses a significant en route hazard to aviation, not for widely scattered thunderstorms.
Forecasters issue a convective SIGMET when specific minimum criteria are met, including: a line of thunderstorms at least 60 miles long affecting at least 40 percent of its length; an area of active thunderstorms affecting at least 3,000 square miles covering at least 40 percent of the area with strong intensity; or embedded/severe thunderstorms expected for more than 30 minutes.
Special convective SIGMETs can be issued for reported tornadoes, large hail (>= 0.75 inches), or wind gusts (>= 50 knots), but routine advisories typically require convection to persist 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.