Free tool
TAF decoder
Paste a terminal forecast and read it as a timeline. Each period shows whether it replaces, amends, or temporarily overlays the forecast.
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Try an example
Paste a forecast above, or try one of the examples. For the weather happening right now rather than later, use the METAR decoder.
Where a TAF comes from
A forecaster at one of the NWS Weather Forecast Offices writes the TAF by hand — it is not generated from a model. The forecaster reads the current METAR, upper-level charts, satellite imagery, and model guidance, then decides how conditions will evolve over the next 24 to 30 hours. The result is a judgment call compressed into code.
An amended TAF (TAF AMD) means a forecaster re-evaluated and decided the current forecast is misleading. Amendments are issued at any time, not on a schedule. A corrected TAF (TAF COR) fixes a formatting error in a previously issued forecast. Both carry the same valid period as the original.
TAF vs METAR at a glance
Both formats use the same codes for wind, visibility, clouds and weather. The difference is what they describe.
| Feature | METAR | TAF |
|---|---|---|
| Purpose | Current observation | Forecast |
| Coverage area | At the airport | Within 5 SM of the airport |
| Time span | A single moment | 24 to 30 hours |
| Update frequency | Hourly (or SPECI) | Four times per day |
| Includes temperature | Yes | Yes (in header only) |
| Includes altimeter | Yes | No |
| Includes flight category | Derived from ceiling/vis | Not applicable |
| Change groups | None (one snapshot) | FM, BECMG, TEMPO, PROB |
TAF report types
The keyword after TAF tells you whether the forecast is routine, amended, corrected, or late.
| Type | Meaning |
|---|---|
| TAF | Routine forecast. The NWS issues it on schedule, usually four times per day. |
| TAF AMD | Amended forecast. The NWS issues an amendment when conditions change enough to make the current forecast misleading. |
| TAF COR | Corrected forecast. Fixes an error in a previously issued TAF, such as a typo in a time or wind group. |
| TAF RTD | Delayed routine forecast. The scheduled TAF was late. The content follows the same format as a routine TAF. |
| TAF CNL | Cancelled forecast. The NWS cancels a TAF when the airport closes, equipment fails, or conditions make a forecast impossible. The valid period ends at the cancellation time. |
How TAF periods work
- A TAF is a layered document
- A TAF does not predict one set of conditions. It predicts a sequence: a base forecast that covers the full valid period, then change groups that modify it at specific times. The forecaster at the Weather Forecast Office writes the base, then adds FM, BECMG, TEMPO and PROB groups where conditions are expected to shift. Reading a TAF means reading the stack — the base plus every modification active at the time you care about.
- FM: everything not mentioned is gone
- FM restates the forecast from scratch. The part people miss: it does not carry anything forward. If the base forecast says BKN040 and the FM group says nothing about clouds, the forecast now has no clouds. It is not "clouds unchanged" — it is "clouds removed." Every other change type inherits from what came before. FM does not.
- BECMG: the change can happen at any point in the window
- BECMG has a start time and an end time. The change can complete at any point inside that window — the beginning, the end, or anywhere in between. The forecaster is saying the shift will happen, but not exactly when. By the end of the window the new conditions are the forecast. Before the start of the window the old conditions still hold. In between is the uncertainty.
- TEMPO: when do you plan for it?
- TEMPO means conditions that come and go, each episode lasting under an hour. The base forecast stays in effect between episodes. In flight planning the question is whether to treat TEMPO conditions as the forecast. For fuel planning and alternate requirements, most operators use the TEMPO ceiling and visibility if the TEMPO conditions would change the flight category. A TEMPO that drops the ceiling from 3,000 to 800 is an IFR planning problem even though the base forecast is VFR.
- Times
- A TAF gives the day of the month and the hour in UTC. This tool shows the times as written. Converting to local time would require knowing the month, which the TAF does not carry.
Change group comparison
Each change type handles the previous forecast differently. This table shows whether the group replaces it or overlays it.
| Type | Replaces previous? | Effect |
|---|---|---|
| FM | Yes — full reset | All previous conditions end. The new forecast states everything from scratch. |
| BECMG | Partial | Only the listed elements change. Unlisted elements carry forward from the previous period. |
| TEMPO | No — overlay | Conditions come and go temporarily. The base forecast remains in effect between episodes. |
| PROB30 / PROB40 | No — probability | A 30% or 40% chance of the listed conditions. Often appears with TEMPO. |
Worked example
TAF KJFK 231130Z 2312/2418 21015G25KT P6SM FEW250 FM231800 31012KT P6SM SCT040 TEMPO 2320/2402 BKN025
- Header
- TAF KJFK 231130Z 2312/2418
- Terminal forecast for JFK. The NWS issued it on the 23rd at 1130Z. It covers the 23rd at 1200Z through the 24th at 1800Z.
- Base forecast
- 21015G25KT P6SM FEW250
- Wind from 210 degrees at 15 knots, gusts to 25. Visibility greater than 6 statute miles. A few clouds at 25,000 ft.
- FM group
- FM231800 31012KT P6SM SCT040
- From the 23rd at 1800Z: wind shifts to 310 degrees at 12 knots. Visibility stays above 6 miles. Scattered clouds at 4,000 ft. This replaces the base forecast entirely.
- TEMPO group
- TEMPO 2320/2402 BKN025
- Between 2000Z on the 23rd and 0200Z on the 24th, clouds may temporarily increase to broken at 2,500 ft. The FM forecast continues between these episodes.
Weather codes in a TAF
A TAF uses the same weather codes as a METAR. Each code is two letters. Codes combine with intensity prefixes: - means light, no prefix means moderate, and + means heavy. Descriptors attach to precipitation: +TSRA is heavy rain with thunderstorm. FZRA is freezing rain. -SN is light snow.
| Code | Meaning | Category |
|---|---|---|
| RA | Rain | Precipitation |
| SN | Snow | Precipitation |
| DZ | Drizzle | Precipitation |
| PL | Ice pellets (sleet) | Precipitation |
| GR | Hail (diameter 1/4 inch or more) | Precipitation |
| GS | Small hail or snow pellets | Precipitation |
| SG | Snow grains | Precipitation |
| IC | Ice crystals (diamond dust) | Precipitation |
| FG | Fog (visibility below 5/8 statute mile) | Obscuration |
| BR | Mist (visibility 5/8 to 6 statute miles) | Obscuration |
| HZ | Haze | Obscuration |
| FU | Smoke | Obscuration |
| DU | Widespread dust | Obscuration |
| VA | Volcanic ash | Obscuration |
| TS | Thunderstorm | Descriptor |
| SH | Showers (intermittent precipitation) | Descriptor |
| FZ | Freezing | Descriptor |
| BL | Blowing (raised to 6 ft or more) | Descriptor |
| DR | Low drifting (below 6 ft) | Descriptor |
| MI | Shallow (ground fog less than 6 ft deep) | Descriptor |
| BC | Patches | Descriptor |
Frequently asked questions
- What is a TAF in aviation?
- TAF stands for Terminal Aerodrome Forecast. It is a weather forecast for the area within 5 statute miles of an airport. The National Weather Service issues TAFs for airports that meet traffic and weather criteria. Each TAF covers 24 or 30 hours.
- How often are TAFs issued?
- The NWS issues most TAFs four times per day: at 0000Z, 0600Z, 1200Z, and 1800Z. The NWS can issue an amended TAF (TAF AMD) at any time when conditions change enough to make the current forecast misleading.
- What is the difference between a TAF and a METAR?
- A METAR is an observation: it reports what the weather is right now. A TAF is a forecast: it predicts what the weather will be over the next 24 to 30 hours. Both use the same coded format for wind, visibility, clouds, and weather phenomena.
- What does PROB40 mean in a TAF?
- PROB40 means there is a 40% probability of the conditions listed. PROB30 means a 30% probability. The NWS does not use PROB values below 30 or above 50. PROB40 TEMPO is a single period: a 40% chance of temporary conditions.
- What does FM mean in a TAF?
- FM stands for "from." It marks a complete change in the forecast. When you reach an FM group, every element it does not mention is gone. It resets the forecast to a new set of conditions at the specified time.
- How do I read the valid period in a TAF?
- The valid period shows as four digits, a slash, and four more digits. The first two digits are the day of the month and the second two are the hour in UTC. For example, 2312/2418 means from the 23rd at 1200Z to the 24th at 1800Z.
- What does BECMG mean in a TAF?
- BECMG stands for "becoming." It describes a gradual change over a time window. Only the listed elements change. Wind, visibility, clouds, or weather not mentioned in the BECMG group carry forward from the previous period.
- What does CAVOK mean in a TAF?
- CAVOK stands for Ceiling And Visibility OK. It replaces the visibility, weather and cloud groups when visibility is 10 km or more, no cloud below 5,000 ft, and no significant weather. US TAFs never use CAVOK. International TAFs use it routinely. The METAR decoder page has the same explanation for the observation side.
- What does the WS group mean in a TAF?
- WS followed by a three-digit height, a slash, and a wind group describes low-level wind shear. WS020/18040KT means wind shear at 2,000 ft above the airport surface, with wind from 180 degrees at 40 knots at that altitude. It warns of a significant change in wind speed or direction below 2,000 ft, common during frontal passages and convective weather.
WS020/18040KT
- What does P6SM or 9999 mean in a TAF?
- P6SM means visibility is greater than 6 statute miles. The P stands for "plus." This is the US convention. International TAFs use 9999, which means visibility is 10 km or more. Both indicate unrestricted visibility. When CAVOK applies, neither appears because CAVOK replaces the visibility group entirely.
Not a substitute for an official pre-flight weather briefing.