5.3.2 Forecast Weather Reporting

In this paragraph the aerodrome forecast ‘TAF’ will be discussed. The ‘TAF’ describes the forecast prevailing conditions at an aerodrome and cover a period from 9 to 24 hours.

TAFs valid for 9 hours are issued every 3 hours, those valid for 12 to 24 hours every 6 hours.

Amendments are issued as and when necessary and are indicated by the code ‘AMD’ added to the identifier. It is assumed that a later issued TAF automatically amends and updates the ones issued previously.

A forecast period of a TAF may be divided into several self containing parts by the use of the abbreviation ‘FM’, followed by a time group. ‘TAFs’ are using the same code groups as in use in the METAR.

The information given in a TAF are:

There are four components in the report identifier:

Example: TAF EDDF 090430Z 090618 (= TAF of Frankfurt valid from 0600 till 1800 hrs).

In meteorological bulletins containing several station forecasts there will be only one general identifier heading the bulletin. Here the code ‘FC’ or ‘FT’ replaces TAF and the individual identifiers only contain station ident and validity time.

Same as in METAR.

As in the METAR the four figure group indicates the forecast minimum horizontal visibility. As with the METAR code the numbers are the expected values in meters, except that 9999 indicates a visibility of 10 km or more.

Only phenomena significant to aircraft operations are mentioned otherwise the group is omitted.

However, after a change group if the weather ceases to be significant, the abbreviation ‘NSW’ is used.

Cloud information is presented in the same format as in the METAR. The group usually consists of six characters, the first three indicating the expected cloud amount, using the abbreviations:

FEW (Few) Indicates 1 to 2 OKTASSCT (Scattered) Indicates 3 to 4 OKTASBKN (Broken) Indicates 5 to 7 OKTAS (= Ceiling)OVC (Overcast) Indicates 8 OKTAS.

The last three figures indicates the expected height of the base of the cloud, in units of 30 meters (100 ft).

Only the cloud type cumulonimbus (CB) is indicated.

Example:

– 4/8 stratus at 1000 ft is coded as SCT010.

– 7/8 cumulonimbus at 1000 ft as BKN010CB.

Criteria for the significance of changes are mentioned in 5.3.1 – Trend.

In the TAF there are four change indicators possible:

The code FM (from) followed by a time group is used to indicate the beginning of a self contained part of the forecast. All conditions given before this group are superseded by conditions after the group.

Example: TAF EDDF 060430Z 060618 27015KT 6000 NSCFM 12 27019KT 4000 BKN015.

The code ‘BECMG’ followed by a four digit time group indicates a regular or irregular change to the forecast weather conditions mentioned thereafter at an unspecified time within the time period mentioned in the group. This period will normally not exceed two hours, but in any case even more than four hours.

The change only affects the meteorological elements which are forecast to change significantly.

Unless a further set of change groups are used, the conditions given after BECMG are expected to prevail from the time to the end of the forecast period.

Example: TAF EDDF 060430Z 060618 27015KT 6000 NSCBECMG 1012 4000 BKN012.

(= After 1200Z the conditions are forecast to be: Wind 270 degrees at 15 knots, VIS 4000 metres, clouds broken at 1200 feet).

The code ‘TEMPO’ followed by a four digit time group indicates temporary fluctuations to the forecast conditions which may occur at any time during the mentioned time group. The conditions following this group are expected to last less than one hour in each instance and in aggregate less than half the period indicated by the time group.

Example: TAF EDDF 060430Z 060618 27015KT 6000 BKN015.TEMPO 1013 3000 RA.

The code ‘PROB’ followed by two digits ’30’ or ’40’ indicates a probability of resp. 30 or 40% occurrence of significant forecast elements. A PROB group is always followed by a time group or change group and a time group.

Example: TAF EHAM 200830Z 081212 20003KT 4000 SCT010BECMG 0305 1500 PROB 30 0507 0800 FG BKN004.

(= Visibility will fall to 1500 m between 0300 and 0500 UTC with a probability of 30% of fog with visibility 800 m between 0500 and 0700 UTC).

a. TAF EHAM 281500Z 281812 09004KT 2000 BR SCT003 OVC006 BECMG 0305 1200 BCFG PROB 40 TEMPO 0509 0400 FG FM 10 18008KT 5000 SCT015.

b. TAF LFPG 010000Z 010606 32020KT 9999 SCT018 SCT030CB BECMG 1012 32025G40KT BKN025 CB TEMPO 1216 2000 +SHRAGR BECMG 1618 35015KT.

a. Forecast issued on the 28th at 1500 UTC for Amsterdam valid from 1800 UTC till 1200 UTC: Wind 090 degrees at 4 kts, visibility 2000 metres in mist, clouds scattered at 300 ft, overcast at 600 ft. At 0500 UTC visibility will be down to 1200 metres in fog patches. A 40% probability that between 0500 and 0900 UTC, the visibility goes temporarily down to 400 metres in fog. From 1000 UTC wind 180 degrees with 8 kts, VIS 5000 metres in mist, clouds scattered at 1500 ft.

b. Forecast issued on the 1st at 0000 UTC for Paris-Ch. De Gaulle valid from 0600 UTC till 0600 UTC: Wind 320 degrees at 20 kts, visibility 10 km or more, clouds scattered at 1800 ft and scattered with CB’s at 3000 ft. Between 1000 and 1200 UTC increasing wind to 320 degrees with 25 kts gusting to 40 kts, clouds broken with CBs at 2500 ft. Between 1200 and 1600 UTC temporarily visibility 2000 metres in heavy showers with rain and hail. Between 1600 and 1800 UTC the wind will change to be from 350 degrees with 15 kts.