TAF Change Groups Explained: BECMG, TEMPO, PROB, FM and What They Mean for Your Flight
A Terminal Aerodrome Forecast (TAF) is one of the most practical tools in pre-flight planning — and one of the most misread. The base weather section is usually straightforward enough: wind, visibility, cloud cover. The part that trips pilots up is the change groups. What exactly does TEMPO mean? How seriously should you take PROB30? When does a BECMG period actually end?
This guide answers those questions in plain language, with worked examples. For the raw decoded TAF data itself, the live TAF decoder at metarandtaf.com displays change groups in colour-coded format alongside the current METAR for any of the 1,300+ airports in the database.
The Structure of a TAF, Quickly
Before getting into change groups, a reminder of what a TAF looks like:TAF EGLL 210500Z 2106/2212 27015KT 9999 FEW025 BECMG 2108/2110 30020KT TEMPO 2114/2120 5000 RADZ BKN015 PROB30 2118/2122 2000 FG BKN005
The first line is the base period — the expected conditions for the majority of the forecast window. Everything that follows modifies that base by specifying when and how conditions will change. Each modifier is a change group.
BECMG — Becoming
What it means: Conditions will gradually change to the values stated within the specified time window. Once the change is complete, the new conditions replace the base and are expected to persist.
Syntax: BECMG DDhh/DDhh followed by the parameters that will change.
Example:BECMG 2108/2110 30020KT
Between 08Z and 10Z on the 21st, wind will gradually shift to 300° at 20 knots. After 10Z, expect 300°/20 KT for the remainder of the forecast — or until another change group modifies it again.
Key points:
The change is expected to begin at the start time and be complete by the end time.
Only the parameters listed in the BECMG group change — all others remain as per the base period or the previous BECMG.
BECMG is cumulative. If a second BECMG appears later, it builds on the result of the first.
When it matters: BECMG groups often signal improving or deteriorating flight category thresholds. If a BECMG introduces BKN010, you need to know when that ceiling comes in — whether that's before or after your planned arrival time changes your alternate-airport calculation entirely.
TEMPO — Temporary
What it means: Conditions will fluctuate temporarily to the stated values during the period, but each individual fluctuation will last less than one hour, and the total time in those conditions will be less than half the TEMPO period.
Syntax: TEMPO DDhh/DDhh followed by the temporary conditions.
Example:TEMPO 2114/2120 5000 RADZ BKN015
Between 14Z and 20Z on the 21st, visibility may temporarily drop to 5,000 metres in rain and drizzle with a broken layer at 1,500 ft — but no single occurrence should last more than an hour, and you shouldn't be in those conditions for more than three hours out of the six-hour window.
Key points:
TEMPO does not replace the base conditions — they return to the base between fluctuations.
The flight category during a TEMPO can drop significantly below the base period category.
A TEMPO containing TSRA (thunderstorm with rain), FG (fog), or FZRA (freezing rain) warrants serious attention regardless of the probability, because those phenomena can develop rapidly.
The practical question pilots ask: "Is this TEMPO likely?" The short answer is that a TEMPO in the forecast means the forecaster considers it likely enough to plan around — typically a greater than 50% probability. If they thought it was less than 50%, they would use PROB (below).
PROB30 and PROB40 — Probability Groups
What they mean: PROB30 indicates a 30–39% probability that conditions will occur during the specified period. PROB40 indicates a 40–49% probability.
Syntax: PROB30 DDhh/DDhh or PROB40 DDhh/DDhh
Example:PROB30 2118/2122 2000 FG BKN005
There is a 30–39% chance that between 18Z and 22Z, visibility will drop to 2,000 metres in fog with a broken layer at 500 ft.
Key points:
PROB50 and above are not used — at that probability, the forecaster uses TEMPO instead.
PROB10 and PROB20 are also not used in standard TAF format — below 30% is not forecast.
The practical range is therefore 30–49%.
ICAO regulations prohibit combining PROB30/40 with TEMPO in the same change group.
How to use PROB groups: PROB30 fog or PROB30 thunderstorm should not be dismissed. At a busy hub with limited alternates, a 30% chance of LIFR conditions is operationally significant. For flight planning purposes, consider a PROB30 with critical phenomena (FG, TS, FZRA) as worth building contingency for.
FM — From
What it means: A rapid, distinct change in conditions beginning at the specified time. From that moment, everything in the base period is replaced by the FM group values.
Syntax: FM DDhhmm followed by the complete new set of conditions.
Example:FM211800 24010KT 9999 SCT030
At 18:00Z on the 21st, conditions change completely to: wind 240°/10 KT, visibility 10 km+, scattered clouds at 3,000 ft. All previous base conditions are superseded.
Key points:
FM is a clean break — it does not carry forward any element from the previous base or BECMG group.
FM groups are used when a frontal passage or other well-defined weather change is expected to produce a sudden shift.
Comparing a BECMG and an FM: BECMG is gradual and the old conditions partially overlap with the new; FM is a hard cut.
AT — At (Less Common)
What it means: Conditions will be as stated at the specified time. Used for a brief snapshot rather than a period.
This change group is less common than the others and not used in all country formats. Where it appears, treat the stated time as a momentary target rather than a sustained period.
CAVOK — Not Exactly a Change Group, But Frequently Misread
CAVOK (Ceiling and Visibility OK) can appear in any section of a TAF and means simultaneously: visibility 10 km or more, no cloud below 5,000 ft or the minimum safe altitude (whichever is higher), no cumulonimbus, and no significant weather. It compresses what would otherwise be three separate fields into a single code.
In a change group context, BECMG 0812/0814 CAVOK means all of those conditions are expected once the change is complete.
Reading a Complete TAF Example
TAF LTBA 210500Z 2106/2206 27012KT 9999 SCT030 BECMG 2110/2112 33018KT SCT015 TEMPO 2114/2118 5000 TSRA BKN015CB PROB30 2115/2119 2000 +TSRA OVC008CB FM212100 30010KT 9999 FEW025
This TAF for Istanbul Airport (LTBA ICAO code) reads as follows:
From 06Z on the 21st: wind 270°/12 KT, visibility 10 km+, scattered at 3,000 ft — good VFR conditions.
Between 10Z and 12Z: wind gradually shifts to 330°/18 KT, base cloud layer drops to 1,500 ft scattered. A METAR during this period may show the intermediate state.
Between 14Z and 18Z: temporary periods of 5,000 m visibility in thunderstorm and rain, broken cumulonimbus at 1,500 ft. This is likely IFR territory during the affected intervals.
Between 15Z and 19Z: 30% probability of visibility dropping to 2,000 m in heavy thunderstorm and rain with overcast cumulonimbus at 800 ft. That is LIFR — below minimums for most approaches.
At 21Z: clean transition to 300°/10 KT, visibility 10 km+, few clouds at 2,500 ft. VFR conditions expected for the rest of the valid period.
For a pilot planning an arrival at 16Z: the base is IFR-marginal during TEMPO, with a 30% chance of LIFR. Alternate airport planning is non-negotiable. Checking the live TAF in the metar taf decoder against the actual METAR as the departure time approaches is exactly the kind of real-time cross-check the tool is designed for.
TAF vs. METAR: Which Takes Priority?
A TAF is a forecast; a METAR is an observation. When they disagree, the METAR reflects what is actually happening right now and takes operational priority. A TAF showing TEMPO TSRA that has not yet appeared in the METAR doesn't mean the thunderstorm isn't coming — it means it hasn't arrived yet.
For pre-flight planning the TAF is indispensable. For real-time decision-making on approach, the current METAR and ATIS are what matter.
Frequently Asked Questions
What is the difference between TEMPO and PROB40 TEMPO? PROB40 TEMPO combines a probability qualifier with temporary conditions. It means there is a 40–49% chance of the described temporary conditions occurring. A standalone TEMPO without a probability prefix implies a higher likelihood — typically greater than 50%.
Does BECMG mean the change will definitely happen? BECMG indicates that the change is expected, but forecasts carry inherent uncertainty. A BECMG introducing deteriorating conditions should be treated as operationally likely and planned for.
How long is a standard TAF valid? Standard TAFs cover a 24 or 30-hour period, issued every 6 hours. Some major airports issue 36-hour TAFs. The validity window is shown in the issuance header — 0606/0712 means valid from day 6 at 06Z to day 7 at 12Z.
What does it mean when a TAF has no change groups? A TAF with no BECMG, TEMPO, FM, or PROB groups means the forecaster expects consistent conditions throughout the entire valid period. This is relatively rare at airports with active weather.
Where can I see a live decoded TAF with change groups highlighted? The metar and taf decoder breaks down each TAF by section, colour-codes the flight category for each period including TEMPO and BECMG intervals, and shows the corresponding current METAR alongside it. The metar taf on the site covers the base forecast structure in detail.