Decoding a TAF and applying change groups
Issue times, validity periods, the difference between FM, BECMG, TEMPO and PROB, and which groups may be used to satisfy a regulatory weather requirement.
Video scheduled
The lesson video for this topic is in production on The Flight Dispatch Room.
Written lesson
The lesson
A TAF is where the release decision is actually made. The regulations that require an alternate, or set alternate minimums, are all evaluated against forecast conditions across a period. Reading the change groups precisely is therefore a compliance skill, not just a comprehension skill.
Scope and timing
- A TAF applies to an area within approximately 5 statute miles of the centre of the airport's runway complex.
- Routine TAFs are issued four times daily, at approximately 0000Z, 0600Z, 1200Z and 1800Z.
- Validity is typically 24 or 30 hours depending on the station.
- United States TAFs express visibility in statute miles, matching the METAR convention.
- All times are Zulu.
The change groups
| Group | Meaning | How to apply it |
|---|---|---|
| FM | From. A rapid change, normally within one hour, to a new set of conditions. | Everything after FM replaces the previous conditions entirely until the next group. |
| BECMG | Becoming. A gradual change over the stated time span. | Only the elements listed change. Unlisted elements carry forward. |
| TEMPO | Temporary fluctuations expected to last less than one hour each, and in total less than half the stated period. | A temporary deviation from the prevailing forecast, not a replacement of it. |
| PROB30 / PROB40 | A 30 or 40 percent probability of the stated conditions. | Indicates uncertainty. Treat as a planning warning. |
Reading a full example
TAF KORD 141130Z 1412/1518 24012KT P6SM SCT035 FM141900 26015G25KT P6SM BKN030 TEMPO 1420/1423 4SM -SHRA BKN015 FM150300 29008KT P6SM SCT040
- Issued on the 14th at 1130Z, valid from the 14th at 1200Z to the 15th at 1800Z.
- Initial conditions: wind 240 at 12 knots, visibility greater than 6 SM, scattered at 3,500 ft.
- From 1900Z on the 14th: wind 260 at 15 gusting 25, visibility greater than 6 SM, broken at 3,000 ft. This replaces the initial line completely.
- Temporarily between 2000Z and 2300Z on the 14th: visibility 4 SM in light rain showers, broken at 1,500 ft. This is a deviation from the FM141900 conditions, which remain the prevailing forecast.
- From 0300Z on the 15th: wind 290 at 8 knots, visibility greater than 6 SM, scattered at 4,000 ft.
Applying a TAF to a regulatory test
When you evaluate the 14 CFR 121.619 exemption or alternate minimums under 121.625, you are asking whether the forecast conditions satisfy a threshold across a defined window. A TEMPO or PROB group forecasting conditions below the threshold within that window means the threshold is not reliably met, and the conservative and expected answer is that the requirement is triggered.
Examination focus
Key exam concepts
- A TAF covers approximately 5 statute miles around the airport's runway complex.
- Routine issue times are approximately 0000Z, 0600Z, 1200Z and 1800Z, with 24 or 30 hour validity.
- FM replaces the whole previous forecast line; BECMG changes only the stated elements.
- TEMPO describes fluctuations lasting under one hour each and under half the stated period in total.
- PROB30 and PROB40 express probability and signal uncertainty rather than a firm forecast.
- TAF AMD supersedes the previous issue.
Watch out
Common exam traps
- Treating BECMG as a full replacement of the previous conditions.
- Assuming a TEMPO group replaces the prevailing forecast for its whole time span.
- Reading a validity period as local time. TAF times are always Zulu.
- Ignoring a TEMPO or PROB group that dips below a regulatory threshold inside the evaluation window.
- Working from a superseded TAF after an amendment has been issued.
Test yourself
Practice questions
Choose an answer to reveal the discussion. Every option is explained, including why the wrong ones are attractive.
- Q01
In a TAF, the group BECMG 1416/1418 indicates that the stated conditions:
- Q02
A TAF shows prevailing visibility P6SM with TEMPO 1802/1806 2SM BR. During 0200Z to 0600Z, the forecast means:
- Q03
The area covered by a TAF is approximately:
Verify
Official FAA references
- AC 00-45Aviation Weather Services — TAF format and change groups.
- FAA-H-8083-28Aviation Weather Handbook.
- 14 CFR 121.619Alternate airport for destination: Domestic operations.
- 14 CFR 121.625Alternate airport weather minimums.
Current regulation text is published on eCFR. Always confirm against the current version before relying on any citation.
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