A TAF is not a sentence: it is a timeline
Start with the header and validity period, then place every change group on a timeline. The first block describes initial prevailing conditions. Later groups either replace them or temporarily overlay them.
Reading only the worst conditions, or only the first line, can hide when a hazard is expected. Time and validity are part of the forecast.
Issue time and validity
In TAF LIRX 261100Z 2612/2712, the forecast was issued on day 26 at 11:00 UTC and is valid from day 26 at 12:00 until day 27 at 12:00 UTC. Confirm that the whole flight fits within that window.
AMD identifies an amendment, COR a correction and CNL a cancellation. Always use the current version.
Four groups that must not be confused
| Group | What it does | Timeline |
|---|---|---|
FM | New prevailing conditions at a precise time | Starts a new block and replaces the previous one |
BECMG | Gradual transition within a window | Transition window, then new prevailing conditions |
TEMPO | Temporary fluctuations | Temporary overlay on the prevailing scenario |
PROB30/40 | Explicit probability | Probability and time window, not prevailing conditions |
FM: a new prevailing scenario
FM262000 34006KT CAVOK begins new prevailing conditions on day 26 at 20:00 UTC. They replace the previous block until another change or the end of validity.
BECMG: change within a window
BECMG 2614/2616 18012KT indicates a gradual change between 14:00 and 16:00 UTC. The exact instant is unspecified; by the end of the window, the new conditions should prevail.
TEMPO: an episode, not the entire interval
TEMPO 2616/2620 3000 TSRA BKN015CB overlays temporary conditions between 16:00 and 20:00 UTC. The underlying forecast remains in place outside those episodes. Temporary fluctuations can still be operationally decisive.
PROB30 and PROB40
PROB30 2620/2624 2000 TSRA gives a 30% probability in that window. It is neither certainty nor a reason to ignore the risk. Compare it with the planned flight phase and consequences.
Complete training example
TAF LIRX 261100Z 2612/2712 22008KT 9999 SCT030
BECMG 2614/2616 18012KT
TEMPO 2616/2620 3000 TSRA BKN015CB
FM262000 34006KT CAVOK- From 12:00: wind 220°/8 kt, visibility at least 10 km, SCT at 3,000 ft.
- 14:00–16:00: wind gradually becomes 180°/12 kt.
- 16:00–20:00: temporary thunderstorms, 3 km visibility and BKN CB at 1,500 ft.
- From 20:00: prevailing wind 340°/6 kt and CAVOK.
This fictitious example teaches the method. Retain the original report and verify current official information.