In meteorology and aviation, TAF is a format for reporting weather forecast information, particularly as it relates to aviation. "TAF" is an acronym of Terminal Aerodrome Forecast or, in some countries, Terminal Area Forecast. TAFs apply to a five statute mile radius from the center of the airport complex. Generally, TAFs can apply to a 9- or 12-hour forecast; some TAFs cover an 18- or 24-hour period; and as of November 5, 2008, TAFs for some major airports cover 30 hours periods. The date/time group reflects the new 30 hour period in Coordinated Universal Time (UTC), as always.
TAFs complement and use similar encoding to METAR reports. They are produced by a human forecaster based on the ground. For this reason there are fewer TAF locations than there are METARs. TAFs can be more accurate than Numerical Weather Forecasts, since they take into account local, small-scale, geographic effects.
In the United States the weather forecaster responsible for a TAF is not usually stationed at the location to which the TAF applies. The forecasters usually work from a centralised location responsible for many TAFs in a state or region, many of which are over one hundred miles from the forecaster's location. In contrast, a TTF (Trend Type Forecast), which is similar to a TAF, is always produced by a person on-site where the TTF applies. In the United Kingdom most TAFs at military airfields are produced locally, however TAFs for civil airfields are produced at the Met Office headquarters in Exeter.
Different countries use different change criteria for their weather groups. In the United Kingdom, TAFs for Military airfields use Colour States as one of the change criteria. Civil airfields in the UK use slightly different criteria.
This TAF example of a 30-hour TAF, released on November 5, 2008 at 1730 UTC:
TAF KXYZ 051730Z 0518/0624 31008KT 3SM -SHRA BKN020 FM052300 30006KT 5SM -SHRA OVC030 PROB30 0604/0606 VRB20G35KT 1SM TSRA BKN015CB FM 060600 250010KT 4SM -SHRA OVC050 TEMPO 0608/0611 2SM -SHRA OVC030= RMK NXT FCST BY 00Z=
The first line contains identification and validity times.
The remainder of the first line and the second line contain the initial forecast conditions. Variations of the codes used for various weather conditions are many—see .
Each line beginning with FM starts a new forecast period.
The remainder of the line has similar formatting to the other forecast lines.
The final line is for errata, comments, and remarks.
TAFs must follow a set of rules that define what must be placed in each line and what criteria require a new line. There are four different lines in a TAF. The first one gives location, valid time and given weather for that time until the next line of the forecast. A BECMG (becoming) line indicates that in the period given, the weather will start to change from the previous line to the next line; an FM (from) line indicates that after the given time, the weather will be what the line states; and a TEMPO line indicates that during the indicated period, temporary fluctuations of the weather are expected. The fluctuations are usually lasting less than one hour, but generally no more than half of the indicated period in total.
A Trend is a truncated version of a TAF giving the expected conditions in a two hour period following the issue of an observation. This short period forecast is appended to the end of a METAR.
METAR EGYM 291350Z 29010KT 8000 -RADZ FEW010 SCT037 OVC043 10/07 Q1008 BLU TEMPO 7000 -RADZ SCT020 WHT=
The Trend reads TEMPO 7000 -RADZ SCT020 WHT i.e Temporary deterioration to 7 km visibility in slight rain or drizzle with scattered at 2000 ft, colour state White.