02 · Forecast · TAF decoder
TAF, period by period.
Paste a Terminal Aerodrome Forecast and read each period in plain English. FM, BECMG, TEMPO, and PROB30/40 groups supported. Flight category derived per period.
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Section 01
Decode
Section 02
How a TAF is structured
A TAF (Terminal Aerodrome Forecast) is the 24- or 30-hour airport forecast issued four times a day (06, 12, 18, 00 Z), valid for a window beginning shortly after the issue time and extending across the next operational day. It is the single most useful artefact for pre-flight planning: a paragraph saying "here's what the airport will be doing across this window".
The body is a sequence of periods. The first one (the implicit "initial" group)
covers the validity window from start until the first change marker. After that, every change
marker introduces a new period: FM = instantaneous changeover at a specific time; BECMG = gradual change during a window; TEMPO = temporary fluctuations during a window;
PROB30 / PROB40 = a 30% or 40% probability of the stated conditions during
the window. Within each period the grammar is the same as a METAR's body — wind, visibility, weather,
clouds.
The decoder above shows each period as its own block, with the time range parsed out into day + hour and the conditions rendered in plain English. The flight category badge on each block is derived from that period's specific conditions, so you can scan the TAF for the window when MVFR turns to IFR or VFR comes back.
Section 03
Common pitfalls
- 01TAFs are airport forecasts, not area forecasts. They describe conditions within 5 SM of the airport reference point and are issued only for airports with a forecast service. For en-route weather, use a different product.
- 02BECMG is gradual; TEMPO is fluctuating. A BECMG 1308/1310 RA means the rain becomes the new state during 08–10 Z; a TEMPO 1308/1310 RA means rain might happen sometimes during 08–10 Z.
- 03PROB30 / PROB40 are deliberately uncertain. Treat them as decision-support, not as gospel — they are issued when the forecaster expects the condition but isn't quite confident enough for a BECMG.
- 04TAFs do not include temperature in their body (in US ICAO practice). The TX/TN max-min groups appear in some international TAFs and are out of scope here.
- 05Forecast horizons grow uncertain past ~12 hours — the TAF's accuracy degrades sharply for the back end of the validity window. If the back end looks operationally consequential, wait for the next TAF issue.
Download
Free on the App Store. iOS 18 and up.
The iOS app renders TAFs as a scrubbable timeline — drag a needle along the forecast window and a plain-English readout updates beneath it. Same parser, same period grammar.