Weather & Decoding
Use the TAF decoder below to translate any raw Terminal Aerodrome Forecast (TAF) into plain English. The decoder explains each forecast period, including base conditions, From groups (FM), Temporary fluctuation groups (TEMPO), Becoming groups (BECMG), Probability groups (PROB), and the corresponding aviation flight category.
Usage Guide
The steps below explain how to use the TAF decoder to convert a raw Terminal Aerodrome Forecast (TAF) into a complete plain-English weather briefing.
Copy the complete Terminal Aerodrome Forecast (TAF) from your preferred aviation weather source, such as a flight planning application, electronic flight bag (EFB), airline dispatch system, or an official weather provider. Include the entire forecast, from the station identifier through to the final forecast change group, to ensure every forecast period is decoded.
Paste the raw TAF into the input field and click Decode TAF. The decoder supports both standard ICAO TAF format and US domestic TAF format. Routine forecasts (TAF), amended forecasts (TAF AMD), and corrected forecasts (TAF COR) are recognised and decoded automatically.
The decoder separates the forecast into individual periods, including the base forecast, From groups (FM), Becoming groups (BECMG), Temporary fluctuation groups (TEMPO), and Probability groups (PROB). For each period, wind, visibility, weather phenomena, cloud layers, temperature information (when present), and other forecast elements are translated into plain English.
Review the decoded forecast periods and their associated aviation flight categories to evaluate expected conditions at your departure airport, destination airport, and alternate airport. Compare the forecast with the latest METAR to identify differences between forecast and observed weather conditions and to support operational flight planning decisions.
Definition
A TAF (Terminal Aerodrome Forecast) is an aviation weather forecast that describes the expected meteorological conditions at and around a specific airport during a defined forecast period. It is produced using a standardized international coding format that allows pilots, dispatchers, and air traffic services to interpret weather forecasts consistently worldwide.
TAFs are issued for individual aerodromes and forecast surface weather elements such as wind, visibility, significant weather phenomena, cloud cover, and, where applicable, other operationally relevant conditions. The forecast is intended to support flight planning, fuel planning, alternate airport selection, and operational decision-making before departure.
Most aerodromes issue TAFs four times daily at 0000, 0600, 1200, and 1800 UTC. Standard TAFs typically cover a 24-hour forecast period, while extended TAFs issued for many larger airports cover up to 30 hours.
A TAF is composed of a base forecast and one or more forecast change groups. These change groups include From groups (FM), Becoming groups (BECMG), Temporary fluctuation groups (TEMPO), and Probability groups (PROB), which describe how forecast weather conditions are expected to change during the forecast validity period.
Purpose
A TAF (Terminal Aerodrome Forecast) is used in aviation to support preflight planning and operational decision-making by providing a time-based forecast of expected weather conditions at a specific aerodrome.
Pilots use TAFs to evaluate expected conditions during key phases of flight, including departure, en-route planning, and arrival, with particular focus on wind, visibility, cloud ceiling, and significant weather phenomena. These forecasts are used to determine whether conditions are expected to meet visual flight rules (VFR) or instrument flight rules (IFR) minima during the planned time of operation.
TAFs are also used in flight dispatch and operational planning systems to support route planning, fuel planning, and alternate airport selection. In regulated IFR operations, TAF information is a required input for alternate airport planning requirements under FAA regulations.
Air traffic services and meteorological authorities use TAFs to support traffic flow management, forecast verification, and issuance of amended forecasts (TAF AMD) when observed or expected conditions differ significantly from the original forecast.
Origin
TAFs (Terminal Aerodrome Forecasts) are produced by aviation forecasters working within national meteorological authorities designated under ICAO Annex 3. These forecasting services are responsible for issuing aerodrome-specific weather forecasts in standardized ICAO format.
In the United States, TAFs are issued primarily by National Weather Service (NWS) Weather Forecast Offices (WFOs), which provide official aviation forecasts for designated airports. The Aviation Weather Center (AWC) provides supporting guidance, analysis, and aviation weather products used in forecast preparation.
Forecasters generate TAFs using a combination of numerical weather prediction (NWP) model output, satellite imagery, radar data, surface observations, upper-air soundings, and pilot reports (PIREPs). These inputs are interpreted to forecast expected conditions such as wind, visibility, cloud cover, and significant weather phenomena over a defined time period.
Once prepared, TAFs are encoded in standard ICAO format and disseminated through international aviation communication systems such as the Aeronautical Fixed Telecommunication Network (AFTN) and modern aviation data distribution networks.
TAFs are issued on a regular schedule defined by each State (commonly every 6 hours in many regions) and may be amended at any time when forecast conditions change significantly. Amended TAFs are issued with the prefix TAF AMD and replace all previous versions for the same aerodrome.
Accuracy
TAF accuracy is generally highest in the early part of the forecast period and tends to decrease as the forecast extends further into time. As a result, short-term conditions are typically more reliable, while longer-range forecast elements should be interpreted as trends and possible scenarios rather than precise outcomes.
A TAF is valid for the time period specified in its validity group. Standard TAFs typically cover 24 hours, while extended TAFs may cover up to 30 hours depending on the issuing authority. Each new routine or amended TAF (TAF AMD) supersedes the previous forecast for the same aerodrome. For operational use, pilots must ensure they are referencing the most recent issuance and confirm the validity period before making flight planning decisions.
Because TAFs can be updated at any time, the most recent amendment should always be checked close to departure and arrival times to ensure that no significant forecast changes have been issued after the initial briefing.
Forecast uncertainty is further expressed through probability groups (PROB30 and PROB40). These indicate that a specific set of conditions may occur with lower likelihood during a defined time window. PROB30 represents a lower probability scenario than PROB40 and is used to highlight conditional or less certain forecast outcomes. These groups are intended to provide planning context rather than deterministic outcomes and should be interpreted alongside the main forecast groups.
Comparison
A TAF (Terminal Aerodrome Forecast) is a forecast of expected weather conditions at an airport over a future time period, while a METAR (Meteorological Aerodrome Report) is an observation of the actual weather conditions at that airport at a specific time.
A METAR provides a real-time snapshot of observed surface conditions, typically updated hourly, including wind, visibility, cloud cover, and present weather phenomena. It represents what is happening at the time of observation and is used to confirm current operational conditions at an aerodrome.
A TAF provides a time-based forecast of expected conditions, typically covering 24 to 30 hours depending on the issuing authority. It is used to anticipate changes in weather during departure, en-route, and arrival phases of flight.
Together, TAFs and METARs are used in combination for flight planning. The TAF is used to assess expected conditions during the planned flight window, while the METAR is used to verify current conditions at the aerodrome. Comparing the TAF with the latest METAR also helps evaluate how well the forecast aligns with actual observed weather.
Structurally, a TAF contains a base forecast followed by multiple change groups such as From groups (FM), Becoming groups (BECMG), Temporary groups (TEMPO), and Probability groups (PROB) that describe how conditions are expected to evolve over time, while a METAR contains a single set of observed weather conditions at a specific time.
Decoding Guide
A TAF is decoded sequentially from left to right, starting with the report header and progressing through the base forecast, followed by time-ordered change groups that modify or replace conditions within defined validity windows. The list below explains each component in the exact order it appears in a standard TAF.
Important — FM time format: FM group times are encoded as DDHHmm, meaning the change occurs at a specific minute in UTC. FM100000 means 0000 UTC on the 10th, not 1000 UTC. Misreading this format can lead to significant operational planning errors.
Here’s a TAF example:
TAF KJFK 271720Z 2718/2818 24015KT P6SM FEW035 BKN080 FM280200 27010KT P6SM SCT020 TEMPO 2806/2810 BKN012 FM281200 30015G25KT 6SM -SHRA BKN030
Here is how to read the TAF, section by section:
TAF
TAF identifies this as a routine aerodrome forecast. The report type is always the first element and tells you what kind of document you are reading. If the forecast had been amended after issuance, this would read TAF AMD. A corrected forecast would read TAF COR.
When you see TAF AMD, the forecast has been updated because actual or expected conditions changed significantly from the previous version. The AMD supersedes the entire previous TAF for that aerodrome. When you see TAF COR, a coding or transmission error in the previous TAF has been corrected — the meteorological content may be unchanged.
KJFK
KJFK is the four-letter ICAO code for John F. Kennedy International Airport in New York. The station identifier confirms which aerodrome this forecast covers. In the US, identifiers begin with K for the contiguous 48 states, PA for Alaska, and PH for Hawaii. International identifiers use a different prefix system defined by ICAO region.
A TAF applies within a 5 statute mile radius of the aerodrome reference point. Conditions beyond that radius are not covered by this forecast — pilots operating away from the airport must consult area forecasts for en-route conditions.
271720Z
271720Z means this TAF was produced on the 27th day of the month at 1720 UTC. The format is DDHHmmZ — day of month, hour, minute, followed by Z for Zulu (UTC). All TAF times are in UTC. The issuance time tells you how recently the forecast was prepared and helps you assess whether a more current version may have been issued.
The issuance time will typically be within one hour before the start of the validity period, as forecasters aim to provide the most current guidance possible at the scheduled issue time.
2718/2818
2718/2818 means this TAF is valid from the 27th at 1800 UTC to the 28th at 1800 UTC — a 24-hour forecast window. The format is DDHH/DDHH. The first four digits give the start date and hour; the second four digits give the end date and hour. Minutes are not included in the validity period — TAFs are always valid on the hour.
When a new routine TAF is issued for the same station, it supersedes this one regardless of whether the previous validity period has expired. Always verify you are using the most recently issued TAF before making operational decisions.
24015KT P6SM FEW035 BKN080
The base forecast represents the initial expected conditions from the start of the validity period until the first change group. In this example: wind from 240° TRUE at 15 knots (24015KT), visibility greater than 6 statute miles (P6SM — the P prefix means "plus", i.e. exceeding 6 SM), few clouds at 3,500 feet AGL (FEW035), and broken clouds at 8,000 feet AGL (BKN080). No significant weather is forecast, so no weather group is included.
BKN080 is the ceiling — the lowest broken or overcast layer. A ceiling of 8,000 ft with visibility greater than 6 SM places this period firmly in VFR conditions. The FEW035 layer does not constitute a ceiling since FEW coverage is less than 3 oktas.
FM280200 27010KT P6SM SCT020
FM280200 introduces a complete replacement of all forecast conditions from 0200 UTC on the 28th. Wind shifts to 270° TRUE at 10 knots (27010KT), visibility remains greater than 6 SM (P6SM), and cloud cover changes to scattered at 2,000 feet AGL (SCT020). No ceiling is present — SCT coverage does not constitute a ceiling. VFR conditions continue.
Note that FM groups replace everything — wind, visibility, weather, and cloud from the previous period are all superseded by the values in this group. Only the elements stated in the FM group apply from this time forward, until the next change group or the end of the validity period.
TEMPO 2806/2810 BKN012
TEMPO 2806/2810 indicates temporary fluctuations between 0600 and 1000 UTC on the 28th. During this window, broken cloud at 1,200 feet AGL (BKN012) may temporarily replace the prevailing scattered cloud at 2,000 feet. A broken ceiling at 1,200 ft with visibility greater than 6 SM places this in MVFR conditions during the temporary periods. Each occurrence is expected to last less than 60 minutes, and the total TEMPO duration should not exceed half the 4-hour window.
Only the elements stated in a TEMPO group differ from the prevailing forecast — all other conditions (wind, visibility, any weather) continue from the FM280200 group between TEMPO occurrences. This means the VFR conditions of the FM period return between each temporary deterioration.
FM281200 30015G25KT 6SM -SHRA BKN030
FM281200 introduces a second complete change from 1200 UTC on the 28th. Wind shifts to 300° TRUE at 15 knots gusting to 25 knots (30015G25KT). Visibility drops to 6 statute miles (6SM). Light rain showers are forecast (-SHRA, where - denotes light intensity and SH is the shower descriptor). Broken ceiling at 3,000 feet AGL (BKN030). The 13-knot gust spread (25 - 15 = 10 kt) is at the threshold for reporting gusts and has operational implications for crosswind calculations and approach speed additions.
With a ceiling of 3,000 ft and visibility of 6 SM, conditions in this period are at the lower boundary of VFR — technically VFR, but approaching MVFR thresholds. Pilots should plan for possible deterioration and ensure alternate options are evaluated if conditions worsen beyond the forecast.
Practice
The examples below show complete real-world TAFs broken down into individual forecast periods, with each period decoded and explained in plain language.
TAF KJFK 271720Z 2718/2818 24015KT P6SM FEW035 BKN080 FM280200 27010KT P6SM SCT020 TEMPO 2806/2810 BKN012 FM281200 30015G25KT 6SM -SHRA BKN030
TAF EGLL 221100Z 2212/2318 24012KT 8000 -RA BKN020 FM221600 27015G25KT 9999 SCT025 BKN060 TEMPO 2217/2221 3000 TSRA BKN010CB BECMG 2222/2224 30008KT 9999 NSW FEW030
TAF AMD KDFW 151430Z 1515/1615 18012KT P6SM SCT040 FM151800 21015G25KT 5SM -TSRA BKN025 OVC060 TEMPO 1518/1522 1SM +TSRA BKN010CB FM160000 24010KT P6SM SHRA BKN035 FM160800 27015KT P6SM SCT040
TAF COR KORD 201156Z 2012/2112 35010KT P6SM OVC015 FM201600 33015G25KT P6SM BKN025 OVC060 TEMPO 2020/2024 4SM -SN BKN010 OVC020 FM210000 31010KT P6SM SCT040
Change Indicators
TAF change groups are forecast modifiers that describe when weather conditions are expected to change, how the change occurs, how long it is expected to last, and how certain the forecaster is that the change will occur. Understanding these groups is essential because they determine when forecast conditions begin, end, or temporarily deviate from the prevailing forecast.
The primary change groups used in TAF formatting include:
FM (From) introduces a complete and permanent change in forecast conditions, effective from a specified time. When an FM group becomes active, it replaces all previous forecast elements, including wind, visibility, weather phenomena, and cloud cover.
FM221600 27015G25KT 9999 SCT025 BKN060BECMG (Becoming) indicates a gradual transition from one set of prevailing conditions to another within a specified time window. Only the weather elements explicitly included in the BECMG group change — all other forecast elements continue unchanged from the previous forecast period.
BECMG 2222/2224 30008KT 9999 NSW FEW030TEMPO (Temporary) describes temporary fluctuations from the prevailing forecast conditions. Individual occurrences are expected to last less than one hour at a time and, collectively, occur for less than half of the forecast period covered by the TEMPO group.
TEMPO 2217/2221 3000 TSRA BKN010CBPROB30 and PROB40 indicate forecast scenarios with a 30% or 40% probability of occurring within a specified time window. PROB30 represents a lower likelihood of occurrence than PROB40. These groups are used to highlight potential weather hazards and forecast uncertainty rather than expected prevailing conditions.
PROB40 TEMPO 2222/2302 1500 TSRA BKN008CBINTER (Intermittent) is an ICAO change group used to describe brief intermittent variations from prevailing forecast conditions. Each occurrence is expected to last less than 30 minutes within any one-hour period. INTER is used in some ICAO-format TAFs but is not used in US domestic National Weather Service TAFs.
INTER 0308/0312 31020G40KT 3000 +TSRA BKN010CBUS domestic National Weather Service (NWS) TAFs differ from standard ICAO-format TAFs in the following ways:
5TBBBD and icing groups use 6IBBBD, where BBB is the base height in hundreds of feet and D is the layer depth in thousands of feet.For flight planning purposes, FM and BECMG groups describe changes to the prevailing forecast, while TEMPO, PROB, and INTER groups describe conditional, temporary, or uncertain weather events. Understanding this distinction is critical when evaluating forecast conditions for departure, destination, and alternate airports.
Validity & Amendments
A TAF validity period defines the time window during which a Terminal Aerodrome Forecast is operationally valid for an airport. It specifies when the forecast begins and ends, and determines which forecast should be used for flight planning at any given time. Understanding how TAFs are issued, updated, and superseded is essential to ensure pilots are always using the most current forecast.
TAFs are updated through several standardized message types. The primary TAF update and status types include:
Standard TAF cover a 24-hour forecast window and are issued four times per day at 0000, 0600, 1200, and 1800 UTC. Extended TAF, issued at major international airports, cover a 30-hour forecast window.
A TAF is valid from the start of its validity period until it is superseded by a new routine TAF or an amendment. The validity period is expressed in DDHH/DDHH format at the beginning of the TAF, where DD is the day of the month and HH is the hour in UTC. For example, 2212/2318 means the forecast is valid from the 22nd at 1200 UTC to the 23rd at 1800 UTC.
Because TAF are issued every six hours but valid for 24 or 30 hours, new TAFs overlap with previous ones. The most recent TAF always takes precedence and fully replaces the earlier version.
A TAF AMD (amended TAF) is issued when meteorological conditions differ significantly from the current valid forecast. It replaces the previous TAF for the remaining validity period.
The AMD prefix appears immediately after TAF (e.g. TAF AMD KJFK). Amendments may be issued at any time between scheduled updates and must always be checked before departure.
A TAF COR (corrected TAF) is issued to correct typographical or coding errors in a previously issued TAF. It does not necessarily reflect a change in meteorological conditions.
Like an AMD, a COR supersedes the previous TAF. The COR prefix appears as TAF COR KJFK.
A TAF CNL (cancelled forecast) is issued when a previously published TAF is withdrawn before its validity period expires.
This typically occurs due to aerodrome closure, loss of observing capability, or issuance error. A CNL replaces the full forecast with a cancellation message and contains no meteorological data.
TAF YMML 291200Z 2912/3006 CNLCNL differs from NIL: NIL means no forecast will be issued, while CNL means an existing forecast has been withdrawn.
A TAF RTD (routine delayed forecast) indicates that the TAF was issued later than its scheduled time.
This commonly occurs at part-time observing stations where continuous staffing is not available. RTD appears in the message header, not within the forecast body.
AMD NOT SKED indicates that no forecast amendments will be issued for the station during the validity period.
This is typically used at part-time observing locations without continuous forecasting coverage.
It may include operational time windows:
AFT DDHHmm — observations end after this timeTIL DDHHmm — observations resume at this timeDDHH/DDHH — full period of observation unavailabilityWhen present, pilots must cross-check METARs from nearby stations and PIREPs, as deteriorating conditions may not be reflected in updates.
A TAF NIL indicates that no forecast is currently available for the station.
This may result from equipment failure, staffing issues, or data unavailability. For flight planning, the station must be treated as having no valid TAF, and alternate requirements should be applied accordingly.
US TAF
The Wind Shear (WS) group is a US domestic TAF element that forecasts non-convective low-level wind shear at or below 2,000 feet AGL and is not part of the standard ICAO TAF format, meaning it does not appear in international TAFs. Wind shear refers to a significant change in wind speed and/or direction over a short vertical distance, and the WS group is used to alert pilots to potentially hazardous low-level shear conditions that may affect takeoff, climb, approach, and landing performance. It is important to note that the WS group only applies to non-convective wind shear and does not represent wind shear associated with thunderstorms, which is instead implied by convective weather indicators such as TS or TSRA in the forecast.
The WS group uses the format WShh/dddffKT where:
WS010/31022KT — Non-convective wind shear at 1,000 ft AGL. Wind above the shear level is from 310° TRUE at 22 knots. This group implies a significant wind difference between the surface wind (stated in the main wind group) and the wind at 1,000 ft.
Reference
TAF weather phenomena codes use the same standardized ICAO coding system as METAR, with one addition: NSW (No Significant Weather), which is used exclusively in TAF change groups to signal the end of previously forecast weather. Note that VC (vicinity) in a US domestic TAF means 5 to 10 statute miles from the centre of the runway complex, whereas in an ICAO TAF it means within 8,000 metres of the aerodrome. The tables below list all codes applicable in a TAF context.
Intensity codes describe the expected strength of forecast weather phenomena (e.g. light, moderate, or heavy precipitation).
| Code | Meaning |
|---|---|
| - | Light |
| (none) | Moderate |
| + | Heavy |
| VC | In the vicinity (within 8 km) |
Weather descriptor codes provide additional context about how forecast weather is occurring, such as thunderstorms, showers, or freezing conditions.
| Code | Meaning |
|---|---|
| BC | Patches |
| BL | Blowing (2 metres or more above ground) |
| DR | Low drifting |
| FZ | Freezing |
| MI | Shallow |
| PR | Partial |
| SH | Shower |
| TS | Thunderstorm |
Precipitation codes identify the type of forecast precipitation, such as rain, snow, drizzle, or hail.
| Code | Meaning |
|---|---|
| DZ | Drizzle |
| GR | Hail (≥5 mm) |
| GS | Small hail or snow pellets (<5 mm) |
| IC | Ice crystals |
| PL | Ice pellets |
| RA | Rain |
| SG | Snow grains |
| SN | Snow |
Obscuration codes describe forecast conditions that reduce visibility or obscure the atmosphere, such as fog, mist, haze, or blowing dust.
| Code | Meaning |
|---|---|
| BR | Mist (visibility 1,000–5,000 m) |
| DS | Duststorm |
| DU | Widespread dust |
| FC | Funnel cloud (+FC = tornado over land, waterspout over water) |
| FG | Fog (visibility less than 1,000 m) |
| FU | Smoke |
| HZ | Haze |
| NSW | No significant weather — used in change groups only |
| PO | Dust or sand whirls |
| SA | Sand |
| SQ | Squall |
| SS | Sandstorm |
| VA | Volcanic ash |
Reference
The table below lists all abbreviations used in TAF forecasts, sorted alphabetically for quick reference and interpretation.
IFR Operations
The TAF is the primary forecast used to determine whether an IFR flight requires a filed alternate airport. It is applied using regulatory criteria to assess expected weather conditions at the destination during the arrival window.
Under FAA regulations, an IFR alternate airport is required if, for the period from 1 hour before to 1 hour after the estimated time of arrival (ETA), the destination TAF forecasts a ceiling below 2,000 ft or visibility below 3 statute miles. This is known as the 1-2-3 rule (2,000 ft ceiling, 3 SM visibility, ±1 hour from ETA).
If no TAF is available for the destination, an alternate must be filed.
Forecast changes in FM and BECMG groups are used to determine whether conditions meet the 1-2-3 threshold. TEMPO and PROB groups are not used for regulatory alternate determination and are treated as advisory information only.
The alternate airport must have forecast conditions at or above published alternate minima during the period from 1 hour before to 1 hour after the estimated time of arrival at the alternate.
Typical alternate minima are:
Under ICAO procedures, an alternate is required when forecast conditions at the destination are expected to fall below applicable approach minima plus state-defined margins during the arrival window. Exact thresholds and timing vary by state implementation. Pilots and dispatchers must refer to the applicable State AIP and operational manuals for precise requirements.
TAF-based alternate planning represents the regulatory minimum. It should always be cross-checked against the latest METAR and any issued amendments to verify whether the forecast is tracking current conditions before departure.
Common Questions
FM, TEMPO, BECMG, and PROB are TAF change groups that describe how forecast conditions evolve over time, but they differ in timing, duration, and certainty. FM (From) indicates a rapid and permanent change at a specific time, replacing all previous conditions. BECMG (Becoming) describes a gradual transition to new persistent conditions within a time window. TEMPO (Temporary) indicates short-lived fluctuations lasting less than 60 minutes at a time and covering less than half the period. PROB (Probability) indicates the likelihood (30% or 40%) of specified conditions occurring within a time window and is used for uncertainty rather than expected prevailing conditions.
TAF time groups are decoded using UTC time formats that define when the forecast is issued, valid, and changing. The issuance time is given in DDHHmmZ, the validity period is given as DDHH/DDHH, and change groups use formats such as FM DDHHmm, DDHH/DDHH for BECMG, or embedded time windows in TEMPO and PROB groups. For example, an issuance time of 221130Z means issued on the 22nd at 1130 UTC. A validity period of 2212/2318 means valid from the 22nd at 1200 UTC to the 23rd at 1800 UTC. FM221400 means conditions change from the 22nd at 1400 UTC. TEMPO 2215/2218 means temporary conditions between 1500 and 1800 UTC on the 22nd.
CAVOK in a TAF means Ceiling and Visibility OK and represents excellent forecast conditions. CAVOK is used when all of the following are expected simultaneously: visibility of 10 km or more; no cloud below 5,000 ft AGL or the minimum sector altitude, whichever is higher; no cumulonimbus (CB) or towering cumulus (TCU) at any height; and no significant weather phenomena such as precipitation, thunderstorms, or obscuration. When CAVOK is used, it replaces the separate visibility, cloud, and weather groups in the TAF. It is used in ICAO-format TAFs worldwide but not in US domestic NWS format.
NSW in a TAF means No Significant Weather and indicates the end or absence of previously forecast significant weather phenomena. NSW is commonly used in change groups such as BECMG or FM to show that precipitation, thunderstorms, or other significant weather is expected to cease. NSW is never used in the base (initial) conditions of a TAF — only in change groups, to explicitly indicate the ending of weather that was previously mentioned.
The ceiling in a TAF is the lowest broken (BKN) or overcast (OVC) cloud layer reported in hundreds of feet above ground level (AGL). If multiple layers exist, only the lowest BKN or OVC layer is used to determine the ceiling. Scattered (SCT) and few (FEW) layers are not considered a ceiling for flight planning purposes. If vertical visibility is forecast using the VV prefix, that value acts as the ceiling. For example, a TAF showing SCT015 BKN030 has a forecast ceiling of 3,000 ft — not 1,500 ft.
P6SM in a TAF means greater than 6 statute miles visibility and is used in US domestic forecasts to indicate visibility above the reporting threshold. The P prefix stands for plus, meaning the value exceeds the stated figure. In ICAO-format TAFs, visibility of 10 km or more is reported as 9999. The two codes are similar in that both indicate good visibility well above typical operational minimums, but they use different units and represent slightly different thresholds — P6SM is approximately 9.7 km, while 9999 represents 10 km or more.
VRB in a TAF wind group means variable wind direction and indicates that a mean wind direction cannot be reliably forecast for that period. It is used when wind direction is expected to fluctuate significantly and no single prevailing direction can be assigned. VRB is most commonly associated with light wind conditions, but there is no fixed speed threshold in the ICAO TAF standard — the forecaster applies professional judgement based on expected directional variability during the forecast period.
Flight categories in a TAF are determined using ceiling and visibility thresholds. VFR requires ceiling above 3,000 ft and visibility greater than 5 SM. MVFR applies when ceiling is 1,000 ft to 3,000 ft inclusive, or visibility is 3 SM to 5 SM inclusive. IFR applies when ceiling is 500 ft to less than 1,000 ft, or visibility is 1 SM to less than 3 SM. LIFR applies when ceiling is below 500 ft or visibility is below 1 SM. These thresholds are applied independently — a single element below the threshold for a category is sufficient to assign that lower category.
+TSRA in a TAF means heavy thunderstorm with rain, where + indicates heavy intensity, TS indicates thunderstorm, and RA indicates rain. It represents convective weather with significant precipitation intensity and associated visibility reduction. Weather codes in TAF are constructed by combining an optional intensity prefix, an optional descriptor, and one or more weather phenomena codes in a defined sequence.
US domestic TAFs do not include BECMG groups or temperature forecasts because the National Weather Service uses a simplified domestic format that differs from the full ICAO TAF standard. ICAO-format TAFs, used internationally and at some US international airports, include BECMG, PROB groups, and temperature forecasts using TX and TN groups, while domestic NWS TAFs rely primarily on FM and TEMPO change groups. NWS domestic TAFs also do not use PROB groups in the first nine hours of the forecast.
PROB30 and PROB40 in a TAF indicate forecast conditions with a 30% or 40% probability of occurring within a specified time window. PROB30 represents a lower likelihood of occurrence than PROB40 — conditions described in a PROB30 group are more likely not to occur than to occur. Both groups are used to express forecast uncertainty and highlight potential weather hazards for planning purposes rather than to define expected prevailing conditions.
A TAF can be used for Part 107 drone flight planning as a supporting weather forecast, but it does not replace real-time weather observation requirements. Remote pilots must verify actual conditions at the time of operation, as TAFs provide forecast conditions that may differ from observed surface weather. For Part 107 operations, the TAF is useful for assessing wind, visibility, cloud ceiling, and precipitation trends during the planned operating window, particularly at airports with active TAF coverage.