Page 141 - UK ATM ANS Regulations (Consolidated) 201121

Part MET - ANNEX V - Specific Requirements for the Providers of Meteorological
Services

more of the following values, or when the vertical visibility is forecasted to deteriorate and
pass through one or more of the following values: 100, 200, 500 or 1 000 ft (30, 60, 150 or
300 m); and
(j) any other criteria based on local aerodrome operating minima, as agreed between the
aerodrome meteorological office and the operators.
MET.TR.220(f)(1) GM1 Aerodrome forecasts
TAF USE OF CHANGE AND TIME INDICATORS
Guidance on the use of change and time indicators in TAF is given below:

MET.TR.220(g) AMC1 Aerodrome forecasts
USE OF PROBABILITY INDICATORS
(a) The number of change and probability groups should be kept to a minimum.
(b) The probability of occurrence of an alternative value of a forecast element or elements
should be placed after the element or elements forecast and be followed by the alternative
value of the element or elements.
MET.TR.220(g) GM1 Aerodrome forecasts
USE OF PROBABILITY INDICATORS
(a) A probability of an alternative value or change of less than 30 % should not be considered
sufficiently significant to be indicated.
(b) A probability of an alternative value or change of 50 % or more should not be considered a
probability but, instead, should be indicated, as necessary, by use of the change
indicators 'BECMG' or 'TEMPO' or by subdivision of the validity period using the
abbreviation 'FM'. The probability group should neither be used to qualify the change
indicator 'BECMG' nor the time indicator 'FM'.
MET.TR.225 Forecasts for landing
(a) TREND forecasts shall be issued in accordance with Appendix 1.
(b) The units and scales used in the TREND forecast shall be the same as those used in the
report to which it is appended.
(c) The TREND forecast shall indicate significant changes in respect of one or more of the
elements: surface wind, visibility, weather phenomena and clouds. Only those elements
for which a significant change is expected shall be included. However, in the case of
significant changes in respect of cloud, all cloud groups, including layers or masses not
expected to change, shall be indicated. In the case of a significant change in visibility, the
phenomenon causing the reduction of visibility shall also be indicated. When no change is
expected to occur, this shall be indicated by the term 'NOSIG'.
(1) Surface wind
The TREND forecast shall indicate changes in the surface wind which involve:
(i) a change in the mean wind direction of 60° or more, the mean speed before
and/or after the change being 10 kt (5 m/s) or more;
(ii) a change in mean wind speed of 10 kt (5 m/s) or more;
(iii) changes in the wind through values of operational significance.
(2) Visibility
(i) When the visibility is expected to improve and change to or pass through one
or more of the following values, or when the visibility is expected to
deteriorate and pass through one or more of the following values: 150, 350,
600, 800, 1 500 or 3000 m, the trend forecast shall indicate the change.
(ii) When significant numbers of flights are conducted in accordance with the
visual flight rules, the forecast shall additionally indicate changes to or
passing through 5 000 m.
(iii) In TREND forecasts appended to METAR, visibility shall refer to the forecast
prevailing visibility.
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