Sunday, November 1, 2026 · Captain's Challenge
The Captain's Challenge.
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The meteorological watch office at Montréal (CWUL) issues a SIGMET at 1400Z. The phenomenon is severe icing in freezing rain associated with an approaching warm front. What is the maximum valid period for this SIGMET, and what type code applies?
The answer
3Valid up to 4 hours (expires by 1800Z); type code WS
Explanation
A SIGMET for severe icing uses the WS header and is valid for a maximum of 4 hr. TC AIM MET 6.6: "The period of validity of a SIGMET is 4 hr ... There is an exception for volcanic ash and tropical storm SIGMETs which are valid for 6 hr." Freezing rain from a warm front is neither, so the 4 hr maximum applies. WC is the tropical cyclone header and WV the volcanic ash header.
Memory tip: WS (turbulence, icing, mountain wave, thunderstorms, wind shear, dust) = 4 hours. Only WV (volcanic ash) and WC (tropical cyclone) = 6 hours.
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