TAF.AERO01 · Build the forecast timeline
A TAF is an aerodrome forecast for a stated validity window. Its initial group establishes prevailing conditions. Later groups replace the prevailing state, modify selected elements, or temporarily overlay it. Mastering TAF means knowing which conditions apply at a chosen time.
TAFs describe an aerodrome, not the whole route or all flight levels. Many last 24 or 30 hours, but issue schedules and validity vary. Read the actual period. Check amendments and compare the latest observation with the forecast.
- Identify station, issue time, version and validity.
- Decode the initial prevailing conditions.
- Apply FM groups as new prevailing states.
- Apply BECMG transitions to changed elements.
- Overlay TEMPO and probability windows.
- Evaluate your planned time and nearby times under applicable operating rules.
02 · Times, validity and versions
| Group | Meaning |
|---|---|
| TAF EGLL 060500Z | Forecast issued day 06 at 05:00 UTC |
| 0606/0712 | Valid day 06 at 06:00 through day 07 at 12:00 UTC: 30 hours |
| 0624 | Day 06 at 24:00, equivalent to day 07 at 00:00; used as a period endpoint |
| FM061430 | New prevailing conditions from day 06 at 14:30 UTC |
| BECMG 0610/0612 | Transition between 10:00 and 12:00 on day 06 |
| TEMPO 0622/0702 | Temporary fluctuations from 22:00 day 06 until 02:00 day 07 |
| AMD / COR | Amended / corrected forecast; use the latest applicable version |
| CNL / NIL | Cancelled / unavailable forecast, not good weather |
| RMK | National remarks, including next issue/amendment availability |
Issue time is not validity start. Month/year come from context; 3123/0106 may cross a month boundary. FMDDHHMM includes minutes and one start instant; DDHH/DDHH gives a window. A 12:00 endpoint is not inclusive of the whole following hour. All times are UTC.
03 · Initial prevailing conditions
TAF EGLL 060500Z 0606/0706 24012KT 9999 SCT020 BKN040
Issued day 06 at 05:00, valid 06:00 day 06 through 06:00 day 07. Initial forecast: 240°/12 kt, visibility ≥10 km, scattered 2,000 ft and broken 4,000 ft. Ceiling 4,000 ft. These are the starting conditions before subsequent prevailing changes.
Core wind, visibility, weather and cloud codes are shared with METAR. Standard TAF bodies do not normally include QNH or temperature/dew point; some international TAFs include TX/TN extremes. A missing weather group in a complete new prevailing group differs from an omitted weather element in a partial change group.
04 · FM replaces the prevailing state
FM061400 starts a new prevailing forecast at 14:00 day 06. Read it as a complete new state rather than retaining superseded weather/cloud groups. It lasts until the next prevailing change or validity end.
24012KT 9999 SCT020 BKN040 FM061400 28018G28KT 4000 SHRA BKN015
Before 14:00 the base applies. From 14:00: 280°/18 kt gust 28, visibility 4,000 m, rain showers, broken 1,500 ft. Do not retain BKN040 as another layer just because it appeared earlier.
FM is a forecast boundary; real weather need not change abruptly at that minute. Changes in expected timing can trigger amendments.
05 · BECMG and inherited conditions
BECMG specifies a change completed within a time interval, not an exact change instant. Before the interval use the old state; after its end use the changed elements. Within the interval consider timing uncertainty.
24012KT 9999 SCT020 BKN040 BECMG 0610/0612 BKN012
Cloud becomes broken 1,200 ft between 10:00 and 12:00. After 12:00, BKN012 is the cloud state. Wind 24012KT and visibility 9999 continue because they were not changed. A new cloud element replaces the previous cloud forecast rather than merely adding a layer.
NSW can explicitly end previously specified significant weather where applicable. International TAFs use BECMG; US NWS TAFs use FM/TEMPO conventions rather than BECMG.
06 · TEMPO is a temporary overlay
International TEMPO describes temporary fluctuations: each episode lasts less than one hour and their combined duration is less than half the indicated period. US NWS guidance uses one hour or less for each episode, with occurrence expected to be more likely than not. It does not forecast those conditions continuously.
28018G28KT 6000 -SHRA BKN020 TEMPO 0614/0618 1500 TSRA BKN008CB
During 14:00–18:00, temporary episodes bring 1,500 m, thunderstorms with rain, broken CB 800 ft. Between episodes, use the prevailing state. No temporary wind change is specified, so the applicable prevailing wind continues.
A temporary cloud element replaces the cloud forecast during the episode. Afterwards revert to the prevailing state in force at that time. If FM changes the baseline inside a temporary window, reassess the underlying state. TEMPO is duration classification, not a percentage probability and not a reason to ignore the conditions.
07 · Probability groups
| Group | Meaning |
|---|---|
| PROB30 0616/0620 1000 TSRA BKN006CB | 30% probability of the stated alternative conditions during the window |
| PROB40 0616/0620 0800 FG | 40% probability of those conditions during the window |
| PROB30 TEMPO 0616/0620 1000 TSRA | 30% probability of temporary fluctuations to those conditions |
Probability is chance of occurrence, not percentage of time. PROB30 TEMPO combines probability with temporary duration. Do not merge the alternative into the certain prevailing forecast.
US NWS use of PROB30 is restricted; it does not use PROB40 or international PROB TEMPO combinations. Whether a probability group affects alternate selection, dispatch or fuel is determined by applicable rules, not the abbreviation alone.
08 · Forecast weather dictionary
Read weather groups in three parts: qualifier, descriptor, phenomenon. Only permitted combinations are used; not every code can be combined with every other code.
| Qualifier | Meaning |
|---|---|
| - | Light intensity |
| No sign | Moderate intensity when intensity applies |
| + | Heavy intensity; +FC is the developed funnel-cloud exception |
| VC | In the vicinity rather than at the observing point; distance conventions depend on the reporting system |
| Code | Meaning |
|---|---|
| MI | Shallow |
| PR | Partial |
| BC | Patches |
| DR | Low drifting |
| BL | Blowing |
| SH | Showers |
| TS | Thunderstorm |
| FZ | Freezing |
| DZ | Drizzle |
| RA | Rain |
| SN | Snow |
| SG | Snow grains |
| IC | Ice crystals / diamond dust |
| PL | Ice pellets |
| GR | Hail |
| GS | Small hail and/or snow pellets |
| UP | Unidentified precipitation (automated reports) |
| BR | Mist |
| FG | Fog |
| FU | Smoke |
| VA | Volcanic ash |
| DU | Widespread dust |
| SA | Sand |
| HZ | Haze |
| PY | Spray (national use) |
| PO | Dust/sand whirls |
| SQ | Squalls |
| FC | Funnel cloud; +FC indicates tornado/waterspout |
| SS | Sandstorm |
| DS | Duststorm |
-SHRASN VCTS FZFG
Light showers of rain and snow; thunderstorms in the vicinity; freezing fog. Read each complete group separately.
Common combinations: TSRA thunderstorm with rain; +TSRAGR thunderstorm with heavy rain and hail; FZRA freezing rain; FZDZ freezing drizzle; SHSN snow showers; BLSN blowing snow; DRSN low drifting snow; MIFG shallow fog; BCFG fog patches; PRFG partial fog.
Fog normally corresponds to visibility below 1,000 m. Mist is not simply another word for fog: standard BR reporting uses visibility at least 1,000 m up to the applicable reporting threshold. Patchy or shallow fog can coexist with better prevailing visibility. Thunderstorm coding without RA does not remove the thunderstorm hazard.
NSW means no significant weather in applicable change groups; it is not a visibility/cloud forecast. CAVOK affects visibility, weather and cloud together. A complete FM supersedes old weather, while partial change groups require inheritance. Some observation-only or national codes in the shared vocabulary are not normally forecast in a TAF.
09 · Cloud, ceiling and CAVOK
| Code | Meaning |
|---|---|
| FEW | 1–2 eighths of sky (oktas) |
| SCT | 3–4 oktas |
| BKN | 5–7 oktas; a ceiling layer |
| OVC | 8 oktas; a ceiling layer |
| VV003 | Sky obscured; vertical visibility 300 ft |
| VV/// | Sky obscured; vertical visibility unavailable |
| SKC | Sky clear (manual/national reporting) |
| CLR | No cloud detected within the automated reporting range; US systems commonly report up to 12,000 ft |
| NSC | No significant cloud under applicable criteria; not necessarily cloudless |
| NCD | No cloud detected by an automated observing system |
| CB / TCU | Cumulonimbus / towering cumulus suffix, e.g. SCT025CB |
| /// | An unavailable element, never zero |
BKN008CB forecasts broken cumulonimbus at 800 ft above aerodrome level. FEW/SCT are not ceilings. VV describes obscuration. Cloud changes replace the cloud element; they do not automatically add to old layers.
CAVOK means ≥10 km visibility, no significant weather, no CB/TCU, and no operationally significant cloud below 5,000 ft or the highest minimum sector altitude, whichever is higher. Higher cloud can remain. NSC, NCD and clear-sky codes have different meanings. A thunderstorm without a CB suffix still carries convective hazards.
10 · Shear, extremes and national remarks
| Group | Meaning |
|---|---|
| WS020/27045KT | Non-convective low-level wind shear: wind at 2,000 ft AGL from 270° at 45 kt; compare with surface wind |
| TX32/0614Z | Maximum +32°C day 06 at 14:00 UTC |
| TNM02/0705Z | Minimum −2°C day 07 at 05:00 UTC |
| RMK NXT FCST BY 00Z | Next forecast expected by 00:00 UTC |
| AMD NOT SKED / AMD LTD TO… | National restrictions on amendment availability or elements |
The 45 kt in WS020/27045KT is wind speed at height, not a 45 kt shear magnitude or surface gust. Absence of WS does not exclude convective shear/microbursts. TX/TN give extremes, not every hour’s temperature. Amendment restrictions matter at aerodromes with limited observation support.
11 · Complete international worked example
TAF EGLL 060500Z 0606/0706 24012KT 9999 SCT020 BKN040 BECMG 0610/0612 BKN012 TEMPO 0612/0616 3000 SHRA BKN008 FM061600 28015KT 9999 SCT025 PROB30 TEMPO 0618/0622 2000 TSRA BKN010CB BECMG 0700/0702 00000KT CAVOK
| UTC interval | State and overlays |
|---|---|
| 06:00–10:00 day 06 | 240°/12 kt, ≥10 km, SCT020 BKN040 |
| 10:00–12:00 | Cloud transitions toward BKN012; wind/visibility unchanged |
| 12:00–16:00 | Prevailing BKN012; temporary 3,000 m SHRA BKN008 |
| From 16:00 | New state: 280°/15 kt, ≥10 km, SCT025 |
| 18:00–22:00 | 30% chance of temporary 2,000 m TSRA BKN010CB |
| 00:00–02:00 day 07 | Transition to calm/CAVOK |
| 02:00–06:00 day 07 | Calm/CAVOK; ends at 06:00 |
At 13:00, prevailing ceiling 1,200 ft can temporarily become 800 ft with visibility 3,000 m. At 19:00, the 16:00 FM is baseline with a probability overlay. At 01:00 next day, final improvement may not yet be complete. These are decoding examples, not dispatch decisions.
12 · Complete US worked example
TAF KJFK 061130Z 0612/0718 18010KT P6SM SCT030 FM061800 22015G25KT P6SM BKN025 TEMPO 0620/0624 2SM TSRA BKN008CB FM070200 28012KT P6SM SKC FM070900 30008KT 4SM BR BKN012
Issued 11:30 day 06, valid 12:00 day 06 to 18:00 day 07. Initially 180°/10 kt, visibility greater than 6 SM and scattered 3,000 ft. From 18:00: 220°/15 kt gust 25, broken 2,500 ft. Temporary 20:00–midnight: 2 SM (about 3,219 m), thunderstorm/rain, broken CB 800 ft. From 02:00 day 07: 280°/12 kt, greater than 6 SM and clear. From 09:00: 300°/8 kt, 4 SM (about 6,437 m), mist, broken 1,200 ft. The last FM remains baseline until validity end unless amended.
13 · Common errors
| Mistake | Correction |
|---|---|
| Issue time = start time | Read separate validity. |
| 0624 means 06:24 | In day/hour endpoints it is day 06 at 24:00. |
| Old cloud survives every FM | FM creates a new prevailing forecast. |
| BECMG start = change completed | Transition occurs within the window. |
| TEMPO is continuous or 50% probability | It specifies temporary duration. |
| PROB30 is 30% of time | It specifies occurrence probability. |
| SCT is a ceiling | Use BKN/OVC or vertical visibility. |
| P6SM is exactly 6 miles | It means greater than 6 statute miles. |
| Any valid-period TAF is current | Check amendments, corrections and cancellations. |
| TAF covers the whole route | It is an aerodrome forecast. |
For your arrival, identify the prevailing state and all overlapping change windows. Consider schedule variation and midnight/month rollover. Display categories do not replace approach, alternate, aircraft or operator criteria.
14 · Practise timeline reading
TAF WSSS 060500Z 0606/0712 18008KT 9999 SCT018 TEMPO 0608/0612 2000 TSRA BKN012CB BECMG 0614/0616 24012KT FM070000 00000KT 4000 BR BKN010
What applies at 10:00, 15:00 and 01:00?
10:00 day 06: baseline 180°/8 kt, ≥10 km, SCT018; temporary 2,000 m TSRA BKN012CB possible. 15:00: wind transitioning toward 240°/12 kt; visibility/cloud remain ≥10 km/SCT018. 01:00 day 07: new FM in force, calm, 4,000 m, mist, broken 1,000 ft.
PROB40 0623/0703 0800 FG VV002
Does fog last 40% of four hours?
No. There is a 40% probability of the specified 800 m fog and 200 ft vertical visibility during the midnight-spanning window. Duration and prevailing state are not established by that percentage.
Practise by drawing a UTC timeline, placing prevailing changes, then adding temporary/probability overlays. Compare your reading to the original groups and subsequent METARs to learn both code reading and forecast limitations.
15 · Wind and visibility quick reference
| Forecast group | Read it as |
|---|---|
| 24015G25KT | From 240° true, 15 knots, gusting 25 |
| VRB03KT | Variable direction, 3 knots |
| 00000KT | Calm |
| 18005MPS | From 180°, 5 metres/second; about 9.7 knots |
| 9999 | Visibility at least 10,000 m |
| 4000 | Visibility 4,000 m |
| P6SM | Visibility greater than 6 statute miles; greater than about 9,656 m |
| 1 1/2SM | Visibility 1½ statute miles; about 2,414 m |
| CAVOK | Combined visibility/weather/cloud criteria, not simply clear sky |
A knot is a nautical mile per hour. SM visibility is statute miles: 1 SM = 1,609.344 m, whereas 1 NM = 1,852 m. Cloud heights use hundreds of feet above aerodrome level. Forecast wind is true, while runway numbers generally reflect magnetic direction. Read the unit suffix rather than assuming every numeric wind group is in knots.
Unlike a METAR observation, a TAF does not normally give observed RVR, measured temperature/dew point, QNH or an observation wind-direction variation group. It uses forecast wind/visibility/cloud and change windows. TX/TN and WS are supplementary forecast groups where supported. A wind-favoured runway calculated from a METAR does not establish which runway ATC will use at your future arrival time.
Continue learning
Reference publications: FAA Aviation Weather Handbook and UK Met Office decoding guidance. Examples are original training examples. For rare national groups, use the issuing state’s current meteorological publication.
Reviewed 6 October 2026.