Products and Forecast Elements

Aerodrome Forecast (TAF): A concise statement of the expected meteorological conditions at an aerodrome for a specified period (not less than 6 hours or more than 30 hours). At present, TAFs are valid at International Airports in the ICAO AFI Region (Africa and Indian Ocean) for 24 or 30 hours. TAFs are usually issued four times per day, two to three hours prior to the beginning validity time, which coincides with the main synoptic hours (00, 06, 12, and 18 UTC).

Aerodrome Warning (AD WRNG): A message issued by the meteorological office to give concise information on meteorological conditions that could adversely affect aircraft on the ground, including parked aircraft, and the aerodrome facilities and services. AD WRNGs are issued in plain or abbreviated language. AD WRNGs should relate to the occurrence or expected occurrence of one or more of the following phenomena: tropical cyclone, thunderstorm, hail, snow, freezing precipitation, hoar frost or rime, sandstorm, dust storm, rising sand or dust, strong surface wind and gusts, squall, frost, volcanic ash, tsunami, volcanic ash deposition, toxic chemicals, other phenomena as agreed locally.

Flight documentation: Written or printed documents, including charts or forms, that contain meteorological information for a flight; is given to pilots

MET REPORT: A local routine meteorological report for aviation that may or may not contain a trend forecast. It is issued half-hourly or hourly and disseminated at the aerodrome of origin. It is intended for arriving and departing aircraft.

METAR: A routine meteorological report for aviation that may or may not contain a trend forecast. It's issued half-hourly or hourly, disseminated beyond the aerodrome of origin, and is mainly intended for flight planning.

OPMET (OPerational METeorology) : Meteorological messages used for aviation operations, such as METARs, TAFs, and SIGMETs

QNH: The pressure at mean sea level (MSL) calculated from a barometric pressure at ground level using the ICAO standard for the part between MSL and ground level.

SIGMET: A warning message issued by a Meteorological Watch Office (MWO) concerning the occurrence or expected occurrence of specified en-route weather phenomena that may affect the safety of aircraft operations.

Significant Weather Chart (SIGWX): A weather chart representing the most important meteorological phenomena especially relevant for air traffic transport. SIGWX charts are issued by the World Area Forecast Centers. However, at the present time, low- and medium-level SIGWX charts may be produced locally by the Meteorological Office.

SPECI: A special meteorological report for aviation that may or may not contain a trend forecast. It is issued whenever specified changes occur with respect of surface wind, visibility, runway visual range, present weather, clouds and/or air temperature. It is disseminated beyond the aerodrome of origin and is mainly intended for flight planning. Note that SPECIS are not issued for changes in temperature in Central and Western Africa.

SPECIAL: A local routine special meteorological report for aviation that may or may not contain a trend forecast. It is issued whenever specified changes occur with respect to surface wind, visibility, runway visual range, present weather, clouds and/or air temperature. It is disseminated at the aerodrome of origin and is intended for arriving and departing aircraft. Note that SPECIS are not issued for changes in temperature in Central and Western Africa.

TREND: A trend or trend forecast consists of a concise statement of the expected significant changes in the meteorological conditions at an aerodrome. It is appended to a local METAR, SPECI, MET REPORT, or SPECIAL. The period of validity is two hours from the time of the report and forms part of the landing forecast.

Wind Shear Warning (WS WRNG): A message issued by the meteorological office that gives concise information on the observed or expected existence of wind shear that could adversely affect aircraft on approach or take-off paths or during circling approaches between the runway level and 500 meters (1600 ft) above; and aircraft on the runway during the landing roll or take-off run.

Change Indicators

Change Indicators Used in TAFs, METARs, SPECIs, MET REPORTS, and SPECIALs:

AT: Used in TREND forecast with associated time group together with BECMG to indicate the specified time the change is forecast to occur.

BECMG: Change indicator used with associated time group in TAFs and TREND forecasts to describe changes where the meteorological conditions are expected to reach or pass through specified threshold values at a regular or irregular rate and at an unspecified time during the time period. In TAFs the time period should normally not exceed 2 hours but in any case should not exceed 4 hours. In TREND forecasts BECMG may be used with FM, TL, and AT as time indicators (see definitions of FM, TL, and AT).
FM: Change indicator used in TAFs with associated time group to indicate a significant change in most weather elements occurring at a precise time (in UTC) during the validity of the TAF and all the elements given before “FM” are to be included following “FM” (i.e. they are all superseded by those following the abbreviation).

In TREND forecasts, FM with associated time group together with BECMG or TEMPO to indicate the time the change is forecast to commence.

PROB: Change indicator used in TAFs with associated probability value to indicate the probability of occurrence of an alternative value of a forecast element(s). A probability of an alternative value or change of less than 30 per cent should not be considered sufficiently significant to be indicated. A probability of an alternative value or change of 50 per cent or more, for aviation purposes, should not be considered a probability but instead should be indicated, as necessary, by use of the change indicators “BECMG”, “TEMPO” or FM as appropriate.

TEMPO: Change indicator used with associated time group in TAFs and TREND forecasts to describe expected frequent or infrequent temporary fluctuations in the meteorological conditions which reach or pass specified threshold values and last for a period of less than one hour in each instance and, in the aggregate, cover less than one-half of the forecast period during which the fluctuations are expected to occur.

TL: Used in TREND forecast with associated time group together with BECMG or TEMPO to indicate the time the change is forecast to be completed.