BECMG Group

CHAPTER ONE

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

pattern.

KNSE TAF 260909 28004KT 9000 HZ SCT020 SCT200 QNH2998INS

FM1200 26007KT 9000 HZ SCT025 SCT080 BKN250 QNH2996INS

VCSHRA

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

FM Group

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all previously forecast conditions are superseded by the conditions forecasted on this line.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).

BECMG Group

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

FORECASTS