TAF Change-Group Windows
A TAF change group (FM, BECMG, TEMPO, PROB30/PROB40) marks WHEN a forecast shifts. Time formats differ:
| Indicator | Time format | Marks | Rules |
|---|---|---|---|
FM |
DDHHMM (day, hour, minute) |
Single instant | Rapid, permanent change. New complete forecast replaces everything before. |
BECMG |
DDHH/DDHH (day+hour only) |
Window | Gradual, permanent change. Normally 1 to 2 h, never more than 4 h. |
TEMPO |
DDHH/DDHH (day+hour only) |
Window | Recurring fluctuations. Each spell under 1 h; if recurring, total no more than half the forecast period. Longer than 1 h: use FM or BECMG. |
PROB30/PROB40 |
DDHH/DDHH (day+hour only) |
Window | Stated 30%/40% probability. Hazards, including: thunderstorms, freezing precipitation, wind shear at or below 1,500 ft AGL, critical ceiling and visibility. Below 30%: PROB not used. 50% or more: use BECMG, TEMPO, or FM. |
BECMG/TEMPO/PROBwindows: whole UTC hours. OnlyFMhas minutes.
Carry-over:
- Unrestated elements carry over unchanged.
- Two elements always travel together in a change group: sky condition; visibility + weather + obstruction to vision.
- Wind stands alone.
- Layer changes under
BECMG/TEMPO: repeat the whole cloud group, unchanged layers included. - When one group changes, re-list every significant weather group still applying after the change.
FM sets the clock (TC AIM MET 7.3 NOTE):
FMafter the start of a whole hour: laterBECMG/TEMPOapply after theFMtime, not the whole hour.- Example:
TEMPO 2809/2811afterFM280930= temporary change 0930Z to 1100Z, not 0900Z.
Which group governs an hour (TC AIM MET 7.3):
- Latest
FMat or before that hour supersedes all earlier conditions. TEMPOapplies only inside its window; prior conditions continue outside.
BECMG cap is by how much changes, not hours (TC AIM MET 7.3):
- At most two weather groups change; rest unchanged.
- More than two change: use
FMto start a new self-contained part period.
Trap: FM280930 is one instant (hour AND minute), not a window like BECMG/TEMPO; BECMG's "normally 1 to 2 h" is not a hard 2 h ceiling (TC AIM allows 4 h); TEMPO is brief and comes and goes, BECMG is lasting and gradual.
References
- TC AIM MET 7.3