SIGMET & AIRMET message lifecycle
SIGMET and AIRMET: in-flight hazard warnings from a meteorological watch office (MWO) when weather turns worse than the GFA showed. Split is severity. Both amend the GFA.
| Message | Code | Validity | Issued ahead | Update | Cancel code |
|---|---|---|---|---|---|
| AIRMET | - | 4 hours | up to 4 hours before validity starts | at least every 4 hours, replaces previous of same letter | CNL |
| SIGMET | WS | 4 hours | up to 4 hours before phenomenon starts | at least every 4 hours | CNCL |
| SIGMET volcanic ash | WV | 6 hours | up to 12 hours before phenomenon starts | at least every 6 hours | CNCL |
| SIGMET tropical cyclone | WC | 6 hours | up to 12 hours before phenomenon starts | at least every 6 hours | CNCL |
- Update sooner if needed.
- Forecast phenomenon: validity begins at expected start.
- Forecast AIRMET or SIGMET issued only for a phenomenon's first appearance in Canadian airspace.
AIRMET cancelled when:
- Phenomenon stops or is no longer expected.
- Strengthens enough to need a SIGMET instead.
- New GFA issue now covers it.
Cancellation message:
- Neither self-cancels at validity end.
- MWO issues it (code in table).
Trap: waiting for validity to end. MWO must issue a cancellation.
References
- TC AIM MET 5.0 (AIRMET)
- TC AIM MET 6.0 (SIGMET)