How to Read a TAF Forecast

Updated: 2026-09-27 · First published: 2026-09-27 · About this site

A METAR tells you what the weather is; a TAF tells you what the forecaster expects it to become. The two share most of their vocabulary, which is why pilots who can read a METAR often assume they can read a TAF — and then misjudge a TEMPO line or miss a PROB40 thunderstorm hidden at the end. This guide decodes the structure that is unique to the TAF: the validity period and the change groups.

What a TAF is — and is not

TAF stands for Terminal Aerodrome Forecast. It is a forecast of the conditions expected at an aerodrome and within about 5 NM of it, written in the same coded language as the METAR and standardised by ICAO Annex 3. It is issued every six hours at most aerodromes (00, 06, 12 and 18 UTC), valid for 24 hours, or 30 hours at the largest international airports. A TAF is amended (TAF AMD) when the forecaster no longer believes it, and corrected (TAF COR) when it contained an error.

What a TAF is not: it is not an area forecast, not a route forecast, and not an observation. When the METAR and the TAF disagree, the METAR is reality and the TAF is the forecaster's expectation of what comes next. Both are shown side by side on every airport page of METAR & TAF 3D, for example EGLL London Heathrow or LTFM Istanbul, with each TAF period decoded and given its own flight category.

The anatomy of a TAF

TAF LTBS 271640Z 2718/2818 VRB02KT 9999 SCT030 BKN120
  TEMPO 2718/2722 -SHRA
  BECMG 2807/2809 35015KT
  BECMG 2816/2818 01018G28KT CAVOK

TAF LTBS — report type and ICAO station.
271640Z — issue time: 27th of the month, 16:40 UTC. Not the start of validity.
2718/2818 — validity: from the 27th at 18:00 UTC until the 28th at 18:00 UTC. Day/hour, day/hour.
VRB02KT 9999 SCT030 BKN120 — the base forecast: wind, visibility, weather, cloud in METAR order. This is what the whole period looks like unless a change group says otherwise.
Everything that follows is a change group, each with its own time window.

The four change groups

FM — from

FM281200 01015KT 9999 SCT025 means that from 12:00 UTC on the 28th the forecast is replaced by what follows. FM is a hard cut: everything before it stops applying, and the new line is a complete new base forecast (all groups are restated). Use FM when the forecaster expects a rapid, decisive change such as a frontal passage.

BECMG — becoming

BECMG 2807/2809 35015KT means that at some point between 07:00 and 09:00 UTC the wind will change to 350° at 15 kt, and it then stays changed. Only the elements listed change; the rest of the base forecast carries on. The window is usually 2 hours and never more than 4. For planning, assume the new condition applies from the start of the window if it is worse, and from the end if it is better.

TEMPO — temporary

TEMPO 2718/2722 -SHRA means light rain showers may occur between 18:00 and 22:00 UTC, each lasting less than an hour, and in total covering less than half of the window. Between showers the base forecast returns. TEMPO is the group that most often hides the operationally important weather: TEMPO 0800 FG BKN002 in an otherwise CAVOK forecast is a LIFR period, and it is that period that decides whether the airport is a legal alternate.

PROB30 / PROB40 — probability

PROB40 2802/2806 TSRA BKN015CB means a 40% chance of thunderstorms with rain in that window. PROB is used alone or in front of TEMPO. ICAO only permits 30 and 40 — a forecaster who believes something is more likely than that writes TEMPO or BECMG instead. Treat PROB as "the forecaster would not be surprised". Many operators require alternates to be selected as if PROB conditions will occur.

Worked example, period by period

Take the Dalaman TAF above and lay it on a timeline:

18:00–22:00 on the 27th — base: variable 2 kt, visibility 10 km+, scattered 3,000 ft, broken 12,000 ft. VFR. Temporarily light rain showers (a TEMPO). Still VFR during showers unless the visibility line in the TEMPO said otherwise — it did not.
22:00–07:00 — base forecast only. Calm night, VFR.
07:00–09:00 on the 28th — wind becomes 350°/15 kt at some point in that window. Everything else unchanged.
09:00–16:00 — 350°/15 kt, VFR.
16:00–18:00 — wind becomes 010°/18 gusting 28 kt and the sky clears to CAVOK. The gusts matter more than the cloud: on Dalaman's runway 01/19 that is a near-direct headwind on 01, so the change group also tells you which runway will be in use. The LTBS page computes exactly that from the live wind.

Traps that catch pilots

Reading the issue time as the start time. 271640Z is when the TAF was written; validity begins at 2718.
Ignoring the day in the validity group. 2718/2818 crosses midnight. "18" after the slash is on the next day.
Treating TEMPO as the base. TEMPO is an excursion; the base returns. But for legal alternate planning the excursion is what counts.
Missing a PROB at the end. The worst weather in a TAF is often the last line. Read to the end before you decide.
Assuming NSW means CAVOK. NSW (no significant weather) only cancels a previously forecast weather phenomenon. Cloud and visibility must still be read from their own groups.
Forgetting the TAF radius. A clear TAF at your destination says nothing about the fog bank 10 NM out on final. Check the neighbouring airports too.

Reading TAFs alongside model forecasts

A TAF is a human forecaster's judgement for one point. Numerical models — ECMWF, GFS, ICON and others — forecast the whole atmosphere but do not know about the sea-breeze front that always arrives at 14:00 local at a particular coastal field. The most reliable picture comes from reading both: the TAF for local knowledge and timing, the model consensus for the trend beyond the TAF horizon. On each airport page the decoded TAF sits directly above a multi-model meteogram so the two can be compared hour by hour.

Frequently asked questions

How long is a TAF valid?

Most aerodromes issue 24-hour TAFs four times a day (00, 06, 12, 18 UTC); major international airports issue 30-hour TAFs. The validity is the second group, e.g. 2718/2818 means from the 27th at 18:00 UTC to the 28th at 18:00 UTC.

What is the difference between BECMG and TEMPO?

BECMG describes a permanent change that happens gradually inside the stated window and then stays. TEMPO describes temporary fluctuations that last less than an hour at a time and, in total, less than half the period — the base conditions return in between.

What does PROB30 mean?

A 30% probability that the stated conditions occur during that window. PROB40 is the highest probability used; anything more likely is written as TEMPO or BECMG instead. PROB groups usually carry the worst weather in the forecast — thunderstorms, fog, freezing precipitation.

Does a TAF cover the whole airport area?

No. A TAF is a forecast for the aerodrome and roughly a 5 nautical mile radius around it. It says nothing about the en-route weather or the approach corridor beyond that circle.

Which TAF period do I use for an alternate?

The worst one that overlaps your arrival window plus a buffer, including any TEMPO or PROB group. Regulations for alternate minima are written around the forecast being at or above minima for a window either side of the ETA, so a single TEMPO line can disqualify an alternate.