Weather & Decoding
Paste any raw Terminal Aerodrome Forecast to receive a complete plain-English breakdown of every forecast period — base conditions, FM changes, TEMPO, BECMG, and PROB groups — with flight category assessed for each period.
Paste a raw TAF below — results appear instantly
Usage Guide
Five steps from raw TAF string to a complete period-by-period forecast picture with hoverable token annotations and individual decoded boxes.
Copy the complete raw TAF string from your weather source — aviation weather website, flight planning app, ACARS, or ATC service. Include everything from the TAF identifier through the last change group. Paste it into the input box and click Decode TAF. Single-line and multi-line formats both work. AMD and COR modifiers are recognised automatically.
Each forecast period shows the raw coded TAF text as a row of grey hoverable token chips. Hover any chip to see a tooltip with the field name, its ICAO definition, and operational significance. This lets you verify your interpretation of any code directly against the standard — without leaving the page.
Every field in each period appears as its own labelled box — raw code in bold at the top, field name in small caps, plain-English value below. Wind is shown in degrees TRUE, each cloud layer gets its own box, QNH altimeter and NSW are decoded individually. Three boxes per row, covering all body fields and change group elements.
Each period card shows a VFR, MVFR, IFR, or LIFR badge derived from the forecast ceiling and visibility. Identify immediately which time windows fall below your minimums or IFR approach limits. Use TEMPO badges for contingency awareness, BECMG badges to spot the transition window, and FM badges to see when conditions permanently change — all in a single scrollable view.
Format Reference
A TAF follows a strict sequential structure defined by ICAO Annex 3 §15. The numbered example maps directly to the table below.
Fields 1–10 are the TAF header (ICAO Annex 3 §15). Fields 11–15 are forecast change groups. Change group fields (wind, vis, weather, cloud) follow the same format as the base forecast.
ICAO Annex 3 = Meteorological Service for International Air Navigation (governs fields 1–15). FAA Order JO 7900.5D = US Surface Weather Observations (governs QNH and US-specific format). WMO No. 49 = Technical Regulations (weather codes).
Understanding TAFs
A TAF is not a METAR of the future. It is a probabilistic forecast with defined uncertainty structures. Understanding these structures is what separates a pilot who reads a TAF from one who correctly interprets it.
Under FAA regulations (14 CFR 91.169) and equivalent ICAO provisions, a TAF is the required weather document for IFR alternate planning. A pilot who plans without checking the TAF for the destination and alternate — or who misinterprets a TEMPO as a permanent change — may be legally non-compliant and operationally unprepared.
The TAF is issued by qualified meteorological forecasters (not automated systems) four times per day at 0000, 0600, 1200, and 1800 UTC. An AMD (amended) TAF supersedes the previous version immediately. Always check that you are reading the current valid TAF, not an expired one.
A TAF is most reliable in the first 6 hours. Beyond 12 hours, treat the forecast as indicative rather than precise. This is why pilots review TAFs close to departure time, not just during pre-flight planning the night before. Cross-reference the TAF with the current METAR: if the METAR already differs significantly from the TAF base forecast, the entire TAF may be unreliable.
The forecast skill drops fastest in areas of convective activity, frontal passages, and mountain-influenced weather. In these environments, a SIGMET or AIRMET may be more operationally relevant than the TAF for en-route and near-field conditions.
A common student error is treating TEMPO conditions as the new forecast. TEMPO only
describes temporary fluctuations — each occurrence lasts less than 60 minutes and
the total duration is less than 50% of the period. Between TEMPO occurrences, the
base forecast (or most recent FM group) applies. If you see TEMPO 1215/1218 OVC005,
the ceiling could return to the base forecast value multiple times during that window.
A BECMG group only lists the elements that are changing. Elements not mentioned
continue from the previous forecast. For example, if the base forecast is
24012KT 8000 -RA BKN020
and the BECMG says 30008KT 9999 NSW FEW030,
the wind, visibility, cloud and weather ALL change. NSW explicitly ends the rain.
After the BECMG end time, the full new picture is: 300°/8 kt, vis ≥10 km, few at 3,000 ft, no weather.
PROB40 means the forecaster assessed a 40% probability of those conditions occurring — which means a 60% probability they will NOT occur. PROB30 means 70% likely NOT to happen. Neither should be used to plan primary operations. They exist to flag potential hazards that are operationally significant if they do occur (thunderstorms, very low cloud) but not probable enough to include in the base forecast. Use them for contingency awareness, not primary decision-making.
Exactly as in a METAR, the forecast ceiling is determined by the lowest BKN (broken,
5–7 oktas) or OVC (overcast, 8 oktas) cloud layer. FEW (1–2 oktas) and SCT (3–4 oktas)
layers are not ceilings and do not determine flight category. A TAF showing
SCT005 BKN025
has a forecast ceiling of 2,500 ft — not 500 ft. This distinction is critical for
alternate planning and approach minima assessment.
Both METAR and TAF wind directions are always in degrees TRUE (referenced to geographic north), not magnetic (referenced to the magnetic pole). Runway headings use magnetic north. This means you cannot directly compare a TAF wind direction to a runway heading without applying the local magnetic variation. The crosswind component calculation must account for this: subtract magnetic variation from the TAF wind direction to get the magnetic wind direction before comparing to the runway heading.
If no TAF is available for the destination airport, FAA regulations require an alternate to be filed regardless of actual weather confidence. A TAF is a required planning document, not just a convenience. Similarly, if the destination TAF shows any period within ETA ±1 hour where ceiling is below 2,000 ft or visibility below 3 SM (the 1-2-3 rule), an alternate is required even if conditions at the planned arrival time look acceptable.
IFR Planning
The TAF is the primary document used to determine whether an IFR flight requires an alternate airport and whether a proposed alternate meets minimum requirements.
Under 14 CFR 91.169, an alternate is required if the destination TAF indicates, for the period from 1 hour before to 1 hour after the estimated time of arrival, a ceiling below 2,000 feet or visibility below 3 statute miles. The mnemonic is the 1-2-3 rule: 1 hour either side, 2,000 feet ceiling, 3 SM visibility.
If no TAF is available for the destination, an alternate is required regardless of actual forecast confidence. If the destination has no instrument approach, an alternate is always required.
Destination TAF shows ceiling < 2,000 ft OR visibility < 3 SM during the ETA ±1 hour window — even if only in a TEMPO group.
Once an alternate is required, the alternate airport must have a TAF showing conditions at or above alternate minima during the same ETA ±1 hour window. Standard FAA alternate minima are:
Note: Some airports are designated non-alternate qualified (NA) in chart notes — they cannot be filed as alternates regardless of forecast conditions. Always check the airport diagram and approach plate notes before filing an alternate.
Common Questions
A TAF (Terminal Aerodrome Forecast) is a meteorological forecast for conditions within approximately 5 nautical miles of a specific airport. TAFs are issued four times per day at 0000, 0600, 1200, and 1800 UTC. Standard TAFs cover a 24-hour period; extended TAFs issued at major airports cover 30 hours. Each TAF is valid from the issue time to the end of the forecast period, after which a new TAF supersedes it. The format closely resembles a METAR but represents forecast conditions rather than observed ones, and includes change groups to indicate when and how conditions are expected to evolve.
These are change group indicators that describe how and when forecast conditions are expected to change. FM (From) indicates a complete and permanent change of all conditions beginning at a specified time — everything before the FM group is superseded. BECMG (Becoming) indicates a gradual change to new conditions that will persist, occurring over a 2-hour transition period between two specified times. TEMPO (Temporary) indicates conditions that will last less than 60 minutes in any single occurrence and occur for less than half the total period covered by the group. PROB30 or PROB40 indicates conditions with a 30% or 40% probability of occurring; PROB groups may stand alone or modify a TEMPO group. Understanding these change types is critical because only FM and BECMG changes affect legal alternate planning minima calculations — TEMPO and PROB conditions are planning information only.
TAF time groups use a DDHHmm format for issued time and DDHH/DDHH for validity periods, where DD is the day of the month, HH is the hour (UTC), and mm is the minute. A TAF issued on the 22nd at 1130 UTC is stamped 221130Z. The validity period is given as two DDHH groups separated by a slash: 2112/2218 means the TAF is valid from the 21st at 1200 UTC to the 22nd at 1800 UTC. FM change groups use the DDHHmm format: FM221400 means "from the 22nd at 1400 UTC." TEMPO and BECMG groups use a DDHH/DDHH range: TEMPO 2215/2218 means temporary conditions between 1500 and 1800 UTC on the 22nd.
Under FAA regulations (14 CFR 91.169), a TAF is required for IFR alternate airport planning. If the destination airport does not have an instrument approach, or if the TAF for the destination indicates ceiling below 2,000 feet or visibility below 3 SM for the period 1 hour before to 1 hour after the estimated time of arrival, an alternate must be filed. The alternate airport must itself have a TAF showing conditions at or above alternate minima during that same window. If no TAF is available for the destination, it must be treated as requiring an alternate. ICAO rules for alternate planning also use TAF-based criteria, though the specific thresholds differ by state.
AMD in a TAF header indicates an amended forecast. A TAF is amended when the issuing meteorological office determines that actual or expected conditions differ significantly from the valid TAF. An amended TAF supersedes all previous versions of the TAF for that airport. The amendment indicator is placed before the station identifier: TAF AMD KJFK means this is an amended TAF for JFK. A COR prefix indicates a corrected TAF — one issued to correct a typographical or coding error in the previous TAF — as opposed to an amendment driven by actual forecast changes.
CAVOK (Ceiling and Visibility OK) in a TAF indicates forecast conditions in which all of the following are expected simultaneously: visibility of 10 kilometres or more; no cloud below 5,000 feet AGL or below the minimum sector altitude (whichever is higher); no cumulonimbus or towering cumulus; and no significant weather phenomena. When CAVOK is used, it replaces the separate visibility, cloud, and present weather groups. CAVOK represents excellent forecast conditions and indicates the forecast ceiling is above any operationally relevant limit. It is used in ICAO-format TAFs worldwide but not in US domestic format.
TAF reliability degrades with forecast range. Conditions in the first 6 hours of a TAF are generally reliable. Conditions beyond 12 hours should be treated as indicative rather than precise. PROB30 groups indicate conditions the forecaster assessed as having a 30% probability — meaning they are more likely NOT to occur than to occur. PROB40 conditions have a 40% probability. Neither should be used as the basis for go/no-go decisions without corroborating evidence from PIREPS, SIGMETs, or other weather products. The correct use of PROB groups is to flag potential hazards for situational awareness and contingency planning, not to plan operations around them.
A METAR is an observation — it reports conditions that actually existed at a specific airport at the moment of measurement, and it is valid for one hour. A TAF is a forecast — it predicts conditions expected to occur at the same airport over the next 24 to 30 hours, based on meteorological analysis and numerical weather prediction. METARs are the authoritative source for current conditions; TAFs provide the temporal context for flight planning. A pilot reads the METAR to know what conditions are now and reads the TAF to know what conditions are expected during the planned departure, en route, and arrival windows.
NSW (No Significant Weather) is used in TAF change groups to indicate that significant weather phenomena that were forecast in a preceding group are expected to cease. For example, if the base forecast includes RA (rain) and a later BECMG group includes NSW, it means the rain is forecast to end during the BECMG transition period. 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 — as in a METAR — is the lowest forecast cloud layer at broken (BKN) or overcast (OVC) coverage, reported in hundreds of feet AGL. A TAF reporting SCT015 BKN030 has a forecast ceiling of 3,000 feet (the BKN030 layer), not 1,500 feet. If vertical visibility is forecast (VV prefix), that acts as the ceiling. Scattered (SCT) and few (FEW) layers do not constitute a ceiling for flight planning purposes. For each forecast period in the TAF — base conditions and each change group — the ceiling and visibility determine the applicable flight category, which is why the TAF decoder displays a flight category assessment for every period.