A TAF would be far less useful if it could only describe one state of the weather. What makes it a forecast rather than a snapshot is the set of change groups bolted onto it, the small words that say when the weather will shift, whether the shift lasts, and how sure the forecaster is. Get these four right and a long TAF reads cleanly.
This is general educational information, not operational, legal, or regulatory advice. Rules differ by authority and change over time. Always verify against current official sources and follow your operator's approved procedures.
The four change groups
The TAF code form is defined in ICAO Annex 3 and the WMO Manual on Codes, and there are four change indicators to know:
- FM (from): a rapid, lasting change at a specific time, given to the minute, as in
FM121500for 1500 UTC on the 12th. From that time, the new conditions replace the old ones entirely until the next change group. - BECMG (becoming): a permanent change that arrives gradually during the stated period and then holds. The change period is normally short, not exceeding two hours, and in any case no more than four.
- TEMPO (temporary): temporary fluctuations that come and go, each instance lasting less than one hour and in aggregate covering less than half the period stated.
- PROB (probability): a 30 or 40 per cent probability of the conditions that follow, written
PROB30orPROB40, and often combined with TEMPO.
The probability rule
PROB only ever appears as PROB30 or PROB40. As the FAA AIM and the UK Met Office note, a likelihood below 30 per cent is simply left out, and a likelihood of 50 per cent or more is stated as part of the main forecast rather than hedged as a probability. So PROB40 TEMPO 0820 4000 SHRA means a 40 per cent chance of temporary spells of 4000 metres visibility in rain showers between 0800 and 2000 UTC.
The lasting-versus-temporary distinction
The single most important contrast is BECMG against TEMPO. BECMG is a one-way change: once the new conditions arrive, they stay. TEMPO is a recurring blip: the prevailing conditions remain in force, and the TEMPO conditions interrupt them briefly before fading back. Reading a TEMPO as a permanent change makes the forecast look far worse than it is; reading a BECMG as temporary makes it look far better.
A worked example
Here is a TAF with all four groups at work:
- TAF
- Report type. A Terminal Aerodrome Forecast: the weather expected at the aerodrome over a fixed period.
- EGLL
- Station. The ICAO four-letter location indicator for the aerodrome.
- 181100Z
- Day and time (UTC). The 18th of the month at 1100 UTC. The Z stands for Zulu, which is UTC, not local time.
- 1812/1918
- Validity period. Valid from the 18th at 1200 UTC to the 19th at 1800 UTC. Each part is a day-and-hour pair.
- 24012KT
- Wind. Wind from 240 degrees true at 12 knots.
- 9999
- Visibility. Visibility 10 km or more.
- BKN025
- Cloud. broken (5 to 7 oktas) cloud with a base at 2500 ft above the aerodrome.
- FM181500
- Change group: FM (from). From the 18th at 1500 UTC the conditions that follow replace the previous ones. FM marks a rapid, more or less complete change.
- 27015G25KT
- Wind. Wind from 270 degrees true at 15 knots, gusting 25 knots.
- SCT030
- Cloud. scattered (3 to 4 oktas) cloud with a base at 3000 ft above the aerodrome.
- TEMPO
- Change group: TEMPO (temporary). Temporary fluctuations expected to come and go over the period that follows. In between, conditions return to the prevailing forecast.
- 1818/1822
- Change-group time window. The period over which the preceding change group applies: the 18th at 1800 UTC to the 18th at 2200 UTC.
- 6000
- Visibility. Visibility 6000 metres.
- -RA
- Present weather. Light rain. The intensity prefix is minus for light, plus for heavy, and nothing for moderate.
- BKN012
- Cloud. broken (5 to 7 oktas) cloud with a base at 1200 ft above the aerodrome.
- PROB30
- Change group: PROB30. A 30 per cent probability of the conditions that follow.
- 1900/1904
- Change-group time window. The period over which the preceding change group applies: the 19th at 0000 UTC to the 19th at 0400 UTC.
- 3000
- Visibility. Visibility 3000 metres.
- BR
- Present weather. Mist. The intensity prefix is minus for light, plus for heavy, and nothing for moderate.
TAF EGLL 121100Z 1212/1318 24010KT 9999 SCT035 FM121500 27015G25KT 9999 SCT025 BECMG 1218/1220 02008KT TEMPO 1220/1306 6000 -RA BKN012 PROB30 1300/1306 3000 BR
Read in order:
1212/1318is the validity: from 1200 UTC on the 12th to 1800 UTC on the 13th.- The base forecast is a wind from 240 at 10 knots, more than 10 km visibility, scattered cloud at 3500 feet.
FM121500is a clean break: from 1500 UTC the wind veers and freshens to 270 at 15 gusting 25 knots, cloud lowering to 2500 feet. This now replaces the base.BECMG 1218/1220is a permanent change between 1800 and 2000 UTC: the wind backs to 020 at 8 knots and stays there.TEMPO 1220/1306is temporary: between 2000 UTC and 0600 the visibility may drop to 6000 metres in light rain with broken cloud at 1200 feet, in spells of under an hour, then recover.PROB30 1300/1306adds a 30 per cent chance that, between 0000 and 0600, the visibility falls further to 3000 metres in mist.
If you are arriving at 0400 UTC, your planning case is the BECMG wind plus the TEMPO and PROB conditions overlapping it: light wind, but a real chance of 3000 metres in mist with a low cloud base, which is what your alternate and fuel planning has to cover.
Common pitfalls
- TEMPO is not the main forecast. It interrupts the prevailing conditions; do not plan as if it holds all period, but do plan to meet it.
- PROB is never below 30 or used for a near-certainty. Seeing only PROB30 and PROB40 is correct, not a gap in the forecast.
- All times are UTC. As with the METAR, convert deliberately; an arrival time read in local time against UTC change groups is a classic error.
In Pilot EFB
Pilot EFB shows the decoded TAF alongside the raw text and keeps the raw report, so you can check the plain-language change groups against the original code. A briefing you have already pulled stays readable with no signal; pulling a fresh forecast needs a connection. Pilot EFB is not a certified Electronic Flight Bag, so treat it as a study and planning aid and brief from your official source of record.