METEOROLOGY FLIGHT PLANNING
FM (From) and BECMG (Becoming) are indicators of expected speed of change. FM is used
when the change is expected to be quick, and BECMG is used when the change is expected to
occur over a longer period of time. FM indicates a permanent, dramatic or relatively dramatic,
change to a weather pattern is forecast to occur in a short period. All elements of the forecast
conditions will be listed on that TAF line. BECMG indicates some forecast elements are going
to change permanently, or possibly all of the forecast elements will change. TEMPO
(Temporary) means just that: a temporary or non-permanent change to the overall weather
KNSE TAF 260909 28004KT 9000 HZ SCT020 SCT200 QNH2998INS
FM1200 26007KT 9000 HZ SCT025 SCT080 BKN250 QNH2996INS
BECMG 1416 9999 SCT025CB SCT250
BECMG 1718 23015G25KT 530004
TEMPO 1902 8000 TSSHRA SCT010 BKN025CB
FM0200 27010KT 9999 SCT030 BKN080 BKN250 QNH3001INS 20/09Z
Figure 1-28 TAF Change Groups
The heading "FM" followed immediately by a time (hours and minutes) indicates the forecast
weather is expected to change rapidly to the conditions on that line. In other words, the time
indicates the beginning of a significant and permanent change in the whole weather pattern, and
all previously forecast conditions are superseded by the conditions forecasted on this line.
Additionally, the "FM" line includes all elements of a normal forecast as discussed above.
Using Figure 1-28 as an example, the change group "FM1200" starts the change line, and
indicates a change is forecasted to occur at 1200Z. All elements on that line will be in effect
from 1200Z to the end of the original 24-hour period (0900Z in this example), unless changed
later in the forecast by another change group (as is the case here).
A line beginning with the heading "BECMG" indicates a change to forecast conditions is
expected to occur slowly within the period designated in the time group immediately following
the heading. In this time group of four digits, the first two indicate the beginning hour, and the
last two represent the ending hour during which the change will take place. The duration of this
change is normally about 2 hours, 4 at most.
The elements included in the BECMG line will supercede some of the previous TAF groups, but
it is possible all the groups may change. Any group omitted in the BECMG line will be the same
during the BECMG period as indicated in the main TAF line. These new conditions are
1-24 AVIATION ROUTINE WEATHER REPORTS AND TERMINAL AERODROME