How to Read a TAF: A Group-by-Group Guide

A METAR tells you what the weather was at the top of the hour. A TAF tells you what a forecaster expects at the airport over the next day or so, which is the part you fly into. It is built from the same pieces as a METAR, plus a handful of change groups that split the forecast into time periods. Find those boundaries and the rest reads itself.

What a TAF is, and how it differs from a METAR

A Terminal Aerodrome Forecast is a forecast for the area within 5 statute miles of the center of an airport's runway complex. In the US, National Weather Service offices issue TAFs for nearly 700 airports. Scheduled TAFs go out four times a day, valid from 0000, 0600, 1200 and 1800 UTC, each issued 20 to 40 minutes before its valid period starts. Most cover 24 hours; some major airports get 30. These details come from chapter 27 of the FAA Aviation Weather Handbook (FAA-H-8083-28B).

Product differences are from FAA-H-8083-28B chapters 24 and 27 and the METAR/TAF key in AIM 7-1-28.

The two belong together. The METAR tells you whether the forecast is verifying right now. If the TAF called for 5,000 broken at this hour and the METAR shows 800 overcast, the forecast is already wrong. Treat the rest of it with suspicion until an amendment comes out.

The header line, piece by piece

TAF KRDU 031730Z 0318/0418

• TAF is the report type. You may also see TAF AMD (amended) or TAF COR (corrected). An amended TAF supersedes and cancels the previous one the moment it is issued, even if its valid period starts a little later.
• KRDU is the four-letter ICAO identifier.
• 031730Z is when the forecast was issued: the 3rd day of the month at 1730 UTC. Two digits of day, four digits of time, then Z.
• 0318/0418 is the valid period, in a DDHH/DDHH format: from the 3rd at 1800 UTC to the 4th at 1800 UTC. A period ending at midnight is written as 24, so 0900/0924 means the 9th from 0000 to 2400 UTC.

Every time in a TAF is UTC. On Eastern Daylight Time (UTC minus 4 hours), 0318 is 2:00 p.m. local on the 3rd. Convert before you compare anything to your planned departure, and check the day digits too, because a forecast that crosses midnight UTC changes date in the middle of your afternoon.

Wind, visibility, weather and clouds

After the header comes the forecast for the first period. It uses the same building blocks as a METAR.

Wind. 18012G22KT is wind from 180 degrees true at 12 knots, gusting to 22. Directions in TAFs and METARs are true, not magnetic. VRB04KT is variable at 4 knots, and 00000KT is calm.

Visibility. US TAFs forecast prevailing visibility in statute miles. P6SM means more than 6 statute miles, 3SM is 3 miles, and 1 1/2SM is one and a half. Most of the world uses meters: 9999 means 10 kilometers or more, and 4000 means 4,000 meters (about 2.5 SM). TAFs for some joint civil and military fields in the US are in meters too.

Weather. Same codes as the METAR: -RA light rain, TSRA thunderstorm with rain, BR mist, FG fog, SHRA rain showers. Whenever forecast visibility is 6 SM or less, a weather group explains why. VC (vicinity) means between 5 and 10 SM from the center of the runway complex. In NWS TAFs, NSW (no significant weather) appears only in a TEMPO group, to say that weather from an earlier group is expected to end.

Clouds. SCT060 BKN120 is scattered at 6,000 feet and broken at 12,000 feet. Heights are in hundreds of feet above ground level. The lowest broken (BKN) or overcast (OVC) layer, or a vertical visibility (VV) into an obscuration, is the ceiling under 14 CFR 1.1. VV008 means the sky is obscured and you can see 800 feet up into it, which is an 800-foot ceiling. The only cloud type a TAF names is CB (cumulonimbus), and whenever thunderstorms are forecast, a CB layer is forecast with them.

Wind shear. A group like WS020/27045KT at the end of a period means non-convective low-level wind shear from the surface to 2,000 feet AGL, with the wind at 2,000 feet from 270 degrees at 45 knots. It is the wind at the top of the layer, not the amount of shear.

FM, TEMPO, PROB30 and BECMG change groups

The change groups are where most misreads happen.

FM (from). FM032100 means from the 3rd at 2100 UTC. Everything before an FM group is superseded by what follows it, and every FM group is a complete, self-contained forecast. In NWS TAFs, each FM group starts on a new line.

TEMPO (temporary). TEMPO 0410/0413 describes fluctuations the forecaster considers likely (the handbook says greater than 50 percent), each lasting an hour or less, and in total covering less than half of the stated window. Only the elements that change are listed; anything not mentioned stays as it was in the prevailing forecast.

PROB30. PROB30 0404/0407 is a 30 percent chance of the conditions that follow during that window. NWS forecasters use it only for a thunderstorm or precipitation event and the wind, visibility and sky that come with it. PROB30 is the only probability group the NWS uses. AIM 7-1-28 adds that NWS TAFs do not use PROB in the first 9 hours of the forecast.

BECMG (becoming). NWS TAFs do not use BECMG, but you will see it in US military TAFs and all over the world. BECMG 0401/0403 means the conditions that follow will develop at some point during that window and then persist.

PROB40 and PROB30 TEMPO outside the US

US military and international TAFs may also use PROB40, a 40 percent chance. Some countries attach a probability to a temporary fluctuation, written PROB30 TEMPO or PROB40 TEMPO followed by the time window. Australia's Bureau of Meteorology, for example, decodes PROB40 TEMPO 1102/1113 3000 TSRA BKN040CB as a 40 percent probability of temporary deteriorations to 3,000 meters in thunderstorms between 0200 and 1300 UTC on the 11th (BoM, Aerodrome forecast). Read it as two qualifiers stacked: a chance that, during the window, conditions will temporarily fluctuate to what follows.

A fully decoded example TAF

This TAF was composed for this article to show the format. It is not a real forecast.

TAF KRDU 031730Z 0318/0418 18012G22KT P6SM SCT060 BKN120
     FM032100 25015G25KT 6SM -TSRA BKN060CB
     FM040100 30010KT P6SM SCT080 PROB30 0404/0407 3SM -SHRA BKN040
     FM040900 VRB04KT 3SM BR BKN008
     TEMPO 0410/0413 1SM BR OVC004
     FM041500 33010KT P6SM SCT030

Line by line:

1. Issued on the 3rd at 1730 UTC, valid from the 3rd at 1800 UTC to the 4th at 1800 UTC. From 1800 UTC, wind 180 at 12 gusting 22, visibility more than 6 SM, scattered at 6,000, broken at 12,000. Ceiling 12,000 feet.
2. From 2100 UTC on the 3rd: wind 250 at 15 gusting 25, visibility 6 SM in thunderstorms with light rain, broken cumulonimbus at 6,000. A 6,000-foot ceiling and 6 miles of visibility is a VFR flight category. The thunderstorms are the problem, not the ceiling.
3. From 0100 UTC on the 4th: wind 300 at 10, more than 6 SM, scattered at 8,000. There is a 30 percent chance between 0400 and 0700 UTC of 3 SM in light rain showers with a 4,000-foot broken layer.
4. From 0900 UTC on the 4th: variable at 4 knots, 3 SM in mist, ceiling 800 broken. That is IFR.
5. Temporarily between 1000 and 1300 UTC: 1 SM in mist with a 400-foot overcast. Wind and anything else not listed carry over from the FM040900 group. That is LIFR, in bursts of up to an hour.
6. From 1500 UTC on the 4th: wind 330 at 10, more than 6 SM, scattered at 3,000. No ceiling, good visibility.

On Eastern Daylight Time, the thunderstorms arrive around 5 p.m. local on the 3rd. The low clouds and mist settle in around 5 a.m. on the 4th, with the worst of it between 6 and 9 a.m., clearing by 11 a.m. A pilot planning a 7 a.m. departure reads this TAF as a no-go for that hour; a pilot planning noon reads it as a fine day.

Try it in FlyFrenDecode a live TAFPaste an airport identifier into FlyFren's free airport weather tool to see the current METAR and TAF decoded period by period.

Common TAF misreads

A TEMPO group is not a footnote. It is a forecast that the conditions will happen, just not continuously. If the TEMPO conditions are below your minimums, plan as if you will meet them.

The FM group gives the prevailing conditions, but the TEMPO and PROB30 lines under it belong to the same hours. Read each period as the prevailing group plus everything attached to it, and remember which way the carry-over runs: in a TEMPO or PROB group, anything not mentioned keeps its prevailing value, while an FM group restates everything and nothing carries into it.

FM100000 is 0000 UTC on the 10th, not 1000 UTC. The AIM's TAF key warns about exactly this. The first two digits are always the day.

9999 is not 9,999 miles or 9,999 feet. It is 10 kilometers or more, a little over 6 statute miles, and 4000 is about 2.5 SM, not 4.

A TAF describes the 5 SM around one airport. It says nothing about the ridge, the lake or the frontal band between you and the destination, so use the area forecasts, AIRMETs and PIREPs for the en route picture. The most common METAR and TAF misreads have a post of their own.

Using a TAF for a go or no-go decision

For any flight under IFR or not in the vicinity of an airport, 14 CFR 91.103(a) requires you to become familiar with weather reports and forecasts before you go. The TAF is where that starts.

Work out your estimated time of arrival in UTC, then read every period, TEMPO and PROB group that overlaps a generous window around it. Fronts and fog rarely keep to the minute. Plan for the worst condition in that window, TEMPO groups included. A PROB30 group deserves a plan too: if it contains thunderstorms, know where you would go instead.

Compare against your personal minimums, not only the regulations. If the TEMPO line drops below your minimums during your arrival window, that is a no-go for that time, or a reason to change the time.

Then check the METAR against the forecast. If the observations are already worse than the TAF predicted, the forecast is not verifying; expect an amendment and do not plan on the improvement the TAF promised. If the destination TAF is marginal, the TAFs at nearby airports tell you whether you have somewhere to go.

Re-check before departure. TAFs are amended whenever the forecaster decides they no longer represent the weather, and the one you read at breakfast may not be the current one. The aviationweather.gov TAF page shows the current official text for any US TAF site, and the FAA Aviation Weather Handbook has the full NWS format reference.

This article is educational and is not a substitute for the current regulations, the AIM, official weather briefings, or instruction from a CFI. Check the current text at the linked sources before you rely on any detail.