SIGMET & AIRMET issuance criteria
Each phenomenon has one threshold: cross it, message issued; stay under, none. Lifecycle (validity, updates, cancellation): sigmet-airmet. This lesson: trigger tables.
AIRMET (MET 5.1, 5.2):
- Hazards up to and including 24,000 ft (FL 240)
- Only if phenomenon not already forecast in the GFA and SIGMET not warranted
| Hazard | AIRMET code | Threshold |
|---|---|---|
| Surface wind | SFC WSPD |
above 30 kt |
| Surface visibility | SFC VIS |
less than 3 SM (5,000 m) |
| Broken/overcast cloud | BKN CLD / OVC CLD |
base below 1,000 ft AGL |
| Turbulence | MOD TURB |
moderate, except turbulence in convective clouds |
| Icing | MOD ICE |
moderate, except icing in convective clouds |
| Mountain wave | MOD MTW |
moderate |
AIRMET thunderstorm/towering-cumulus ladder (11 codes):
- TS:
ISOL TS,OCNL TS - TSGR:
ISOL TSGR,OCNL TSGR - TCU:
ISOL TCU,OCNL TCU,FRQ TCU - TCU + TS:
OCNL TCU ISOL TS,FRQ TCU ISOL TS - TCU + TSGR:
OCNL TCU ISOL TS GR,FRQ TCU ISOL TSGR - All 11 stay AIRMET: none is a squall line or frequent thunderstorm, the SIGMET triggers.
- Coverage words (ISOL, OCNL, FRQ): see
gfa-clouds-weather-symbols.
SIGMET (MET 6.2):
| Hazard | SIGMET code |
|---|---|
| Turbulence | SEV TURB |
| Icing | SEV ICE (or SEV ICE [FZRA] for freezing rain) |
| Mountain wave | SEV MTW |
| Low-level wind shear | L LVL WS |
| Heavy dust storm | HVY DS |
| Heavy sandstorm | HVY SS |
| Radioactive cloud | RDOACT CLD |
| Volcanic ash | VA |
| Tropical cyclone | TC |
SIGMET thunderstorm ladder (8 codes), built only from frequent thunderstorm activity or a squall line:
| Trigger | Codes |
|---|---|
| Frequent | FRQ TS, FRQ TSGR, FRQ TSGR POSS +FC, FRQ TSGR +FC |
| Squall line | SQLN TS, SQLN TSGR, SQLN TSGR POSS +FC, SQLN TSGR +FC |
+FC= tornado/waterspout.- Squall line = thunderstorms along a line, little or no space between clouds (MET 6.2 Note 1).
Rules:
- One criterion per message: each message covers one criterion; several = separate messages (MET 5.2, MET 6.2 Note 4).
- Exception: thunderstorm SIGMET implies severe icing and turbulence; neither gets own SIGMET (MET 6.2 Note 3).
- SEV TURB covers only (MET 6.2 Note 2):
- low-level turbulence with strong surface winds
- rotor streaming
- turbulence (in or out of cloud) near jet streams
Trap: six SIGMET hazards (low-level wind shear, dust storm, sandstorm, radioactive cloud, volcanic ash, tropical cyclone) have no AIRMET tier; no "moderate dust storm" AIRMET exists, so below SIGMET severity nothing is issued.
References
- TC AIM MET 5.1
- TC AIM MET 5.2
- TC AIM MET 6.2