TAF and TREND Forecasts

A TAF, formally the aerodrome forecast and in the United States usually called the terminal aerodrome forecast, is a coded forecast of the wind, visibility, weather and cloud expected at an aerodrome over a stated period. It uses the same groups as the METAR, so the weather codes, cloud amounts and CAVOK are read exactly as described in METAR and SPECI. What the TAF adds is time: a validity period, and change and probability groups that say when conditions will change and how sure the forecaster is.

A trend forecast (TREND) is a much shorter landing forecast, valid for two hours, appended to a METAR or SPECI. Together they are the legal basis for deciding whether a destination or alternate can be planned, so a pilot must be able to read not only what a TAF says but how the rules apply each group.

The TAF: purpose and validity

A TAF is prepared by a forecaster at the meteorological office, to the format set in ICAO Annex 3 and the WMO Manual on Codes, applied in the EU through Part-MET. It gives the conditions the forecaster considers most likely at the aerodrome itself. In the United States a TAF covers the area within 5 statute miles of the centre of the runway complex.

Validity. ICAO TAFs are usually valid for 9, 24 or 30 hours: short TAFs for smaller aerodromes and short flights, long ones for the larger aerodromes that long-haul flights need. TAFs valid for less than 12 hours are issued every three hours, and those valid for 12 to 30 hours every six. US TAFs are valid for 24 or 30 hours and are issued four times a day, at 0000, 0600, 1200 and 1800 UTC.

Since the code change of 2008, which introduced the 30-hour TAF, the validity period is written as two day-and-hour groups. In TAF 130500Z 1306/1412, the forecast was issued on the 13th at 0500 UTC and is valid from 0600 UTC on the 13th to 1200 UTC on the 14th. An end time of 24 means midnight at the end of that day.

A TAF contains no RVR, no temperature and dew point, no QNH and no recent weather groups: those belong to observations.

TAF format

The groups follow the METAR order: identifier, location, issue time, validity, wind, visibility, weather, cloud, optional temperature groups, and then the change and probability groups in time order. ICAO examples use the fictitious location indicator YUDO:

TAF YUDO 130500Z 1306/1412 24012KT 9999 SCT030 TX17/1315Z TN07/1405Z BECMG 1310/1312 27018G30KT PROB30 TEMPO 1314/1318 4000 +SHRA BKN012CB FM132000 30008KT 9999 FEW025 PROB40 1404/1408 0500 FG VV002 BECMG 1409/1411 CAVOK

US TAFs follow the same structure with national differences:

View full schematic

Change groups: FM, BECMG and TEMPO

The forecast change indicators divide the validity period whenever the forecaster expects the wind, visibility, weather or cloud to cross the thresholds that matter to operations. The three TAF change groups (FM, TEMPO, BECMG) mean very different things, and the difference decides how each is used for planning.

The FM (from) change group, which syllabi also call the from (FM) change group or the TAF from group (FM), is followed by day, hour and minute, as in FM132000. It marks a rapid change to an entirely new set of conditions. Everything forecast before it lapses, and the group gives every element again. FM starts a self-contained period that runs until the next FM or the end of the TAF, modified only by the BECMG, TEMPO or PROB groups within it.

Becoming (BECMG) is followed by a period, as in BECMG 1310/1312. The change happens at a regular or irregular rate at some unspecified time within the period, and is complete by its end. The period is normally no more than two hours and never more than four. Only the elements that change are given; the others carry on as before. From the end of the period the new conditions prevail.

The temporary change group (TEMPO) is also followed by a period. A TEMPO group forecasts fluctuations that each last less than one hour and, taken together, cover less than half the period. Between them, and after the period, the conditions forecast before the group apply.

When significant weather is forecast to end, the group carries NSW, no significant weather; NSC and CAVOK can be forecast as in a METAR.

Probability groups: PROB30 and PROB40

The probability group (PROB30/PROB40) gives the forecaster's probability, 30 or 40 per cent, that the conditions which follow will occur in the stated period. A probability below 30 per cent is not thought worth forecasting, and one of 50 per cent or more is not a probability group at all but a BECMG, TEMPO or FM group. The percentage refers to the chance of the event, not to the share of the period or of the aerodrome area it affects.

A PROB30 / PROB40 group can be used in two ways:

• alone, with a period, as in PROB40 1404/1408 0500 FG: a 40 % chance that fog sets in during that period;
• before TEMPO, as in PROB30 TEMPO 1314/1318: a 30 % chance of temporary fluctuations.

PROB is never combined with BECMG or FM. In US TAFs, National Weather Service forecasters use PROB30, and never within the first nine hours of the forecast.

Temperature groups: TX and TN

The forecast maximum/minimum temperature (TX/TN) groups give the highest and lowest temperatures expected, each with the day and hour: TX17/1315Z is a maximum of 17 °C at 1500 UTC on the 13th, and TNM03/1406Z a minimum of −3 °C at 0600 UTC on the 14th. They follow the cloud group of the initial forecast. They are optional, used by some states and not others, so many TAFs omit them. Where they appear they help with take-off performance planning and anticipating ground icing.

Amended, corrected and cancelled TAFs

An amended TAF (TAF AMD) is issued when the current forecast no longer describes the expected weather well enough, for example when fog forms that the TAF did not foresee. The amended forecast (AMD) replaces the earlier TAF completely for the rest of its validity; it does not add to it. A METAR that departs clearly from the TAF for the same time is often the first hint that an amendment is coming.

• COR marks a corrected TAF, issued to fix an error in the coding.
• CNL, placed after the validity period in an amended TAF, cancels the forecast altogether.
• NIL means no forecast is available for that aerodrome.

Always check the issue time: the most recent TAF, amended or not, is the one that applies.

The trend forecast (TREND)

A TREND is a landing forecast added to the end of a METAR or SPECI at aerodromes where regional agreement requires one, which in practice means many European aerodromes. It is valid for two hours from the time of the observation and describes only changes to the wind, visibility, weather or cloud that are expected to reach the change criteria.

It uses BECMG or TEMPO, optionally followed by a time introduced by FM (from), TL (until) or AT (at), in hours and minutes:

• BECMG FM1030 TL1130 3000 BR: becoming 3,000 m in mist, starting at 1030 and complete by 1130;
• BECMG AT1100 CAVOK: becoming CAVOK at 1100;
• TEMPO FM1400 TL1500 1500 TSRA: temporary thunderstorms with rain and 1,500 m between 1400 and 1500.

NOSIG, no significant change, replaces the trend when nothing crossing the criteria is expected. A trend never contains PROB groups, and US METARs carry no trend.

Using forecasts in flight planning

A TAF is read for a time window around the ETA, not for a single moment. For each aerodrome, find the last FM or BECMG group that applies at the start of the window, carry forward everything it does not change, and then lay any TEMPO or PROB groups that overlap the window on top.

EASA. Under CAT.OP.MPA.182, the aerodrome selection policy of the fuel/energy scheme in force since 30 October 2022, an operator may select a destination, or an alternate for it, only when the forecasts show the weather at or above the relevant planning minima during a period from 1 hour before to 1 hour after the estimated time of arrival there. Older texts cite CAT.OP.MPA.185, which now covers in-flight fuel management. For alternates the planning minima add margins to the landing minima (see alternate aerodromes). How each group counts is set out in AMC3 CAT.OP.MPA.182 (formerly GM2 CAT.OP.MPA.185), which operators build into their operations manuals. The initial part of the TAF applies until the first FM or BECMG group changes it, and a TEMPO or PROB group may temporarily override it. The rules for the change groups, and above all for TEMPO and PROB forecast groups, are the ones most often examined:

The forecast mean wind should be within limits; gusts may be disregarded.

In the example TAF, YUDO as an alternate with an ETA of 0600 UTC on the 14th gives a window from 0500 to 0700. The PROB40 fog group stands alone and describes persistent weather, so 500 m must be applied, and YUDO cannot be planned as an alternate if its planning minima are higher. For an arrival between 1400 and 1800 on the 13th, by contrast, the PROB30 TEMPO showers could be disregarded.

FAA. Under 14 CFR 91.169 an IFR flight needs an alternate unless, from 1 hour before to 1 hour after the ETA, the destination forecast shows a ceiling of at least 2,000 ft above the airport and a visibility of at least 3 SM, the "1-2-3 rule". An airport can be listed as an alternate only if its forecast for the ETA meets the alternate minimums: unless the approach chart publishes others, 600 ft and 2 SM for a precision approach and 800 ft and 2 SM for a non-precision approach. The rule makes no distinction between TEMPO, PROB and prevailing conditions, and prudent pilots apply a conditional group that falls in the window. Airlines follow the policy on conditional forecasts in their approved procedures.

Whatever the rules allow, a TEMPO or PROB group below minima at the ETA is a warning, and it shapes the choice of alternate and the extra fuel carried (see fuel planning and fuel reserves, thunderstorms and low-visibility operations).

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.