AMC1 MET.TR.220(f) Aerodrome
forecasts
TAF — USE OF CHANGE GROUPS
The criteria used for the inclusion of change groups in TAF or amendments to TAF should be based on the following:
(a) when the mean surface wind direction is
forecasted to change by 60° or more, the mean speed before and/or after the
change being 10 kt (5 m/s) or more;
(b) when the mean surface wind speed is
forecasted to change by 10 kt (5 m/s) or more;
(c) when the variation from the mean surface
wind speed (gusts) is forecasted to change by 10 kt (5 m/s) or more,
the mean speed before and/or after the change being 15 kt (7.5 m/s)
or more;
(d) when the surface wind is forecasted to
change through values of operational significance;
(e) when the visibility is forecasted to
improve and change to or pass through one or more of the following values, or
when the visibility is forecasted to deteriorate and pass through one or more
of the following values:
(1) 150, 350, 600, 800, 1 500 or
3 000 m; and
(2) 5 000 m in cases where
significant numbers of flights are operated in accordance with the visual
flight rules;
(f) when any of the following weather
phenomena, or combinations thereof, are forecasted to begin or end:
(1) low drifting dust, sand or snow;
(2) blowing dust, sand or snow;
(3) squall; and
(4) funnel cloud (tornado or waterspout);
(g) when the height of base of the lowest
layer or mass of cloud of BKN or OVC extent is forecasted to lift and change
to or pass through one or more of the following values, or when the height of
the lowest layer or mass of cloud of BKN or OVC extent is forecasted to lower
and pass through one or more of the following values:
(1) 100, 200, 500 or 1 000 ft (30,
60, 150 or 300 m); or
(2) 1 500 ft (450 m) in cases
where significant numbers of flights are operated in accordance with the
visual flight rules;
(h) when the amount of a layer or mass of
cloud below 1 500 ft (450 m) is forecasted to change:
(1) from NSC, FEW or SCT to BKN or OVC; or
(2) from BKN or OVC to NSC, FEW or SCT;
(i) when the vertical visibility is
forecasted to improve and change to or pass through one or 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.
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