How to read a TAF
A TAF (Terminal Aerodrome Forecast) predicts the weather at one airport, typically for the next 24–30 hours. It reads like a METAR, but instead of one observation it is a timeline: a base forecast modified by change groups. The example, decoded by this site's parser:
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Initial 0312 to 0418From 240° (WSW) at 10 kt. Visibility 10 km or more. Scattered at 2,500 ft
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Becoming 0315 to 0318From 270° (W) at 15 kt, gusting 25 kt
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Temporary 0400 to 0409Visibility 4000 m. Rain. Broken at 1,200 ft
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PROB30 Probable 0403 to 0407From 270° (W) at 20 kt, gusting 30 kt
TAF EGLL 031100Z 0312/0418 — header
Report type, station, and issue time (day 03, 1100 UTC) — so far like a METAR. The new element is the validity period 0312/0418: from day 03, 12:00 UTC to day 04, 18:00 UTC. Everything in the TAF lives inside this window. AMD marks an amended TAF, COR a corrected one, and CNL cancels a previous TAF.
24010KT 9999 SCT025 — the base forecast
The conditions expected at the start of the period, in METAR syntax: wind 240° at 10 kt, visibility 10 km+, scattered cloud at 2,500 ft. This holds until a change group modifies it.
FM, BECMG, TEMPO, PROB — change groups
Four ways the forecast evolves:
- FM031800 — from this exact time (day 03, 18:00 UTC), a rapid, complete change: the new conditions replace everything before.
- BECMG 0315/0318 — a gradual, permanent change taking place sometime within the window; after it, the new values apply. In our example the wind becomes 270° at 15 kt gusting 25.
- TEMPO 0400/0409 — temporary fluctuations (each lasting under an hour, covering less than half the window): visibility 4,000 m in rain with broken cloud at 1,200 ft, then back to the base forecast.
- PROB30 TEMPO 0403/0407 — a 30% (or PROB40, 40%) probability of the stated conditions; here, gusts to 30 kt overnight.
Australia and Brazil additionally use INTER for intermittent changes shorter than TEMPO's.
Reading it as a timeline
To know the forecast for a given hour, start from the base forecast and walk the change groups in order, applying every group whose period covers your hour: FM and BECMG permanently update the picture, TEMPO/PROB overlay possible excursions. Worst-case planning uses the most adverse overlay in force.
Extras you'll meet
TX17/0315Z TN09/0405Z — forecast maximum/minimum temperature with their times. WS020/27045KT — low-level wind shear. US military TAFs add altimeter (QNH2992INS), turbulence and icing groups. See the glossary for the full code list.