TAF Decoder
Paste a TAF and read it as a timeline: the base forecast and every FM, BECMG, TEMPO and PROB group with its period, wind, visibility, weather, clouds and flight category.
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The TAF decoder reads a terminal aerodrome forecast and lays it out as a timeline. Paste one forecast or several. The result shows the station, the issue time and the valid period with its length in hours, then one row for the base forecast and one row for each change group, with wind, visibility, weather, clouds, flight category and the raw group. All times are UTC, as in the code.
The decoder works on the text you paste and does not fetch live forecasts. It marks each period as VFR, MVFR, IFR or LIFR, so you can see when conditions are forecast to drop below what you plan to fly in. For observed weather, use the METAR decoder.
How to use the TAF decoder
- Copy a TAF from your briefing source and paste it into the box. Several forecasts can be pasted together, separated by blank lines or an equals sign.
- Read the header: station, whether the forecast is amended, corrected or routine, when it was issued, and the valid period.
- Read the Forecast row first. It holds the conditions that apply from the start of the valid period until a change group takes over.
- Work down the rows in time order. Each FM, BECMG, TEMPO, PROB or INTER row shows its own period.
- Check the Category column for the periods you plan to fly in.
- Where a cell says “No change stated”, the group did not repeat that element, so the earlier value still applies.
- Look under the table for decoded maximum and minimum temperatures and wind shear when the forecast carries them.
- Compare the Group column with your original text if you want to see which code produced a row, and use “Copy decoded text” to copy the result.
The five example buttons load a US forecast with a PROB30 group, an amended forecast with fog, a CAVOK forecast with thunderstorms, a forecast with temperatures and an Australian forecast with INTER.
What does TAF stand for?
TAF stands for terminal aerodrome forecast. It describes the expected weather within five statute miles of an airport. Each TAF is valid for 24 or 30 hours and is updated four times a day, at 0000Z, 0600Z, 1200Z and 1800Z. It is a forecast, not an observation.
In TAF KJFK 251720Z 2518/2624 31012G22KT P6SM FEW050 SCT250, KJFK is the station and 251720Z is the issue time, day 25 at 17:20 UTC. The valid period 2518/2624 runs from day 25 at 18:00 UTC to day 26 at 24:00 UTC, which is 30 hours. The wind is from 310 degrees true at 12 knots gusting to 22, visibility is more than six statute miles (P6SM), and there are few clouds at 5,000 ft and scattered clouds at 25,000 ft.
What do FM, BECMG, TEMPO and PROB30 mean?
FM marks a rapid and significant change, usually within an hour, and starts a new set of conditions. BECMG marks a gradual change over its period. TEMPO marks temporary fluctuations expected to last less than one hour. PROB30 gives a 30 percent probability of thunderstorms and precipitation; it is not used for the first six hours of a 24-hour forecast.
| Group | How the decoder applies it |
|---|---|
| FM | Replaces all conditions from its start time |
| BECMG | Changes the prevailing conditions from then on |
| TEMPO | Overlaid on the prevailing conditions for its own period only |
| PROB30, PROB40 | Overlaid for its own period only, with the probability shown |
| INTER | Intermittent changes, overlaid for its own period only |
The KJFK forecast shows how they combine. The base forecast is VFR. FM260000 brings wind from 280 at 8 knots with scattered clouds at 4,000 ft and broken at 10,000 ft, still VFR. TEMPO 2601/2605 adds 4 SM with light rain showers and broken clouds at 2,500 ft, which is MVFR for those four hours. PROB30 2610/2614 forecasts 2 SM in a thunderstorm with rain and broken cumulonimbus at 1,500 ft, which is IFR because visibility is under 3 SM. BECMG 2615/2617 changes only the wind to variable at 3 knots, so visibility and clouds from the FM group continue and the row is VFR.
What does 2518/2624 mean in a TAF?
It is the valid period: day, hour, slash, day, hour, all in UTC. 2518/2624 starts on day 25 at 18:00 and ends on day 26 at 24:00. A period that begins at midnight is written 00 and one that ends at midnight is written 24, so a 24-hour TAF issued at 00Z on the 9th is valid 0900/0924. A period can cross a month end: 3018/0124 is 30 hours.
How is the flight category worked out for each period?
For each row the decoder takes the ceiling (the lowest broken or overcast layer, or vertical visibility) and the visibility, and applies the NWS Aviation Weather Center table: LIFR below 500 ft or under 1 mile, IFR from 500 ft to below 1,000 ft or 1 to under 3 miles, MVFR from 1,000 to 3,000 ft or 3 to 5 miles, VFR above both. Metric visibility is converted at 1,609.344 m to the mile. In the amended EGLL example, BECMG 2518/2521 3000 BR BKN004 gives a 400 ft ceiling and 1.9 SM, so that period is LIFR.
Sources
Format and change groups: FAA Pilot’s Handbook of Aeronautical Knowledge (FAA-H-8083-25C, 2023), chapter 13, terminal aerodrome forecasts. Category thresholds: NWS Aviation Weather Center, GFA help page (aviationweather.gov/gfa/help), read 25 October 2026. The BECMG description follows common usage and is not defined in the handbook. Parsing: the open-source metar-taf-parser library (MIT licence).
Questions from readers
How long is a TAF valid?
A scheduled TAF in the United States is valid for 24 or 30 hours. The decoder shows the length of the valid period you pasted, for example 30 hours for the KJFK sample.
What does AMD mean in a TAF?
AMD marks an amended forecast, issued to replace one that no longer fits the weather. The amended EGLL sample begins TAF AMD EGLL, and the header names it an amended forecast. COR marks a corrected one.
What is the difference between a TAF and a METAR?
A TAF forecasts the weather over a valid period and a METAR reports what was observed at one time. They share the same wind, visibility, weather and cloud groups. To read an observation, use the METAR decoder.
Does the decoder fetch live forecasts?
No. It decodes the text you paste and sends nothing anywhere. Take the forecast from an official source and check it is current.
What does P6SM mean?
Visibility greater than six statute miles. Reports from outside the US write visibility in metres, and 9999 means 10 km or more.
Why does a TEMPO row show a different category from the row above?
TEMPO, PROB and INTER periods are overlaid on the prevailing conditions, so their category can be worse than the surrounding periods. The prevailing conditions return when the period ends.
Is the wind in a TAF true or magnetic?
True, like the wind in a METAR. Runway numbers are magnetic, so adjust by the local magnetic variation before judging a crosswind.
Related tools
- METAR decoder: reads the observation that a forecast is checked against.
- Crosswind calculator: turns a forecast wind into crosswind and headwind for your runway.
- Cloud base calculator: estimates cumulus base from temperature and dew point.
- Zulu time converter: converts the UTC periods of a TAF to local time.
- Density altitude calculator: shows how the forecast temperature affects performance.
- Pressure altitude calculator: finds pressure altitude from the altimeter setting.
- Weather and time tools: the hub for the other weather and time pages.