You have decoded the METAR and it looks fine. Light wind, good visibility, a few clouds. Then you read the forecast beneath it and see a block of letters that says TEMPO 3111/3115 3000 +TSRA BKN020CB. Is that a reason to cancel the 15:00 circuit, or a line you can skip?
The answer depends on three little words: BECMG, TEMPO and PROB. They tell you how the forecast changes during the day, and they are misread more often than any other part of a TAF. This article explains each one with a worked example and turns the forecast into a simple flying window.
The short answer. A TAF is an aerodrome forecast with a validity period written as two day-and-hour pairs, in UTC. BECMG means the weather changes gradually and stays changed. TEMPO means it comes and goes briefly. PROB30 or PROB40 means a 30 or 40 per cent chance. FM means a change from an exact time. Add three hours to convert UTC to East Africa Time.
How is a TAF laid out?
After the station code and issue time comes the validity period, written DDHH/DDHH: the start and end, in UTC. For example, 3106/3206 runs from 06:00 UTC on the 31st until 06:00 UTC on the next day, which is 09:00 East Africa Time to 09:00 the next morning. After that comes the first, or base, forecast, written like a METAR wind, visibility, weather and cloud. Change groups follow (Met Office, How to decode a TAF).
The example below is one we constructed for this article. It is not a real forecast, and the validity length of real TAFs varies between aerodromes, so always read the validity group in the TAF you are holding.
TAF HKJK 310500Z 3106/3206 09008KT 9999 FEW030
BECMG 3109/3111 14012G22KT SCT035
TEMPO 3111/3115 3000 +TSRA BKN020CB
PROB30 3115/3118 4000 TSRA
BECMG 3120/3122 05005KT FEW040
What do BECMG, TEMPO and PROB mean?
| Group | Meaning | What to do with it |
|---|---|---|
| BECMG | Conditions change gradually during the stated period, then persist | After the end time, treat the new conditions as the base forecast |
| TEMPO | Temporary fluctuations: each spell is short (under an hour) and together the spells cover less than half the period | Plan as if it will happen while you are airborne |
| PROB30 / PROB40 | A 30 or 40 per cent chance of the stated conditions (usually only these two values are used) | Do not ignore it if the conditions are thunderstorms |
| FM | A change from an exact time | Everything before is superseded from that moment |
Sources: Met Office TAF decode guide (BECMG, PROB, FM, validity format; it says usually only 30 or 40 per cent is used). TEMPO limits: the EUMETSAT forecaster training glossary for West Africa (each fluctuation lasts less than one hour) and the US National Weather Service Weather Watch (each occurrence under one hour, total under half the forecast period). Retrieved 30 October 2026. The exact limits come from national rules that follow ICAO Annex 3, so check the Kenya AIP and the TAF you hold.
How do you turn a TAF into a flying window?
This is our recommendation, not a regulation. Draw the forecast as a clock, as in the figure, then work in three steps.
- Mark your own limits first. Write your personal minimums for wind, crosswind, visibility and cloud on the ground, before you read the forecast. Our personal minimums checklist shows how.
- Plot every group in East Africa Time. Convert the validity and each change window, then mark your planned takeoff and landing times.
- Test your window against the worst case. For any time you will be airborne, assume the worse of the base forecast, any TEMPO, and any PROB group that overlaps. If any of them breaks your limits, the flight is not a go for that slot.
In the example, a 10:00 to 12:00 lesson sits wholly inside the calm base forecast. A 13:00 cross-country lies inside the BECMG window, so the gusts to 22 kt could be present by the time you land. Anything after 14:00 sits in the TEMPO thunderstorm period. The honest reading is "fly the morning", which is exactly what the highland daily cycle in our pre-flight weather self-brief predicts.
What does a TAF not tell you?
A TAF covers the aerodrome and a small area around it. It does not tell you what is over the Aberdares or the Rift escarpment, and it cannot see a cell that forms on the ridge at 13:00. Pair it with the current METAR (our guide to reading a METAR explains it), the satellite picture if you have one, and your own eyes. Our article on continued VFR into IMC shows what happens when a pilot trusts the forecast more than the sky.
If your school wants the instructor to see the actual METAR and TAF text next to the student's own weather assessment, AngaBrief can pull the departure and destination reports into the pre-flight brief where a station reports, and the instructor approves, revises or overrides it. Request a demo.
Key takeaways
- Read the validity group first and convert it to East Africa Time.
- BECMG changes the base forecast for good. TEMPO comes and goes. PROB is a chance.
- Plan your slot against the worst group that overlaps it.
- A TAF is a point forecast: it does not describe the terrain between aerodromes.
- Confirm local practice against the Kenya AIP and Kenya Meteorological Department products.
Frequently asked questions
What does BECMG mean in a TAF?
BECMG means conditions change gradually during the stated period and then stay changed. After the end time, treat the new conditions as the base forecast.
What is the difference between TEMPO and BECMG?
BECMG is a lasting change. TEMPO is a temporary fluctuation: each spell lasts less than an hour and together the spells cover less than half of the stated period.
What does PROB30 mean in a TAF?
PROB30 means a 30 per cent probability of the stated conditions. Usually only PROB30 and PROB40 are used. Do not ignore it if the conditions are thunderstorms.
How do you read the validity period of a TAF?
It is written DDHH/DDHH in UTC. 3106/3206 means from 06:00 UTC on the 31st to 06:00 UTC on the 1st, which is 09:00 East Africa Time to 09:00 the next morning.
Is a TAF enough to decide whether to fly?
No. A TAF covers the aerodrome and a small area around it. Pair it with the current METAR, the terrain between aerodromes and your own personal minimums.
AngaBrief is not a dispatch authority. Final go/no-go is the pilot's decision.