Definition
A section within a Terminal Aerodrome Forecast (TAF) that describes an expected change in weather conditions at an airport during the forecast period. Change groups are introduced by indicators such as FM (from), BECMG (becoming), TEMPO (temporary), or PROB (probability), each followed by the time the change is expected and the new conditions forecast.
Plain English
A part of a TAF that tells you when the weather at the airport is expected to change and what it will change to.
Context Anchor
Seen inside TAF reports, after the initial weather forecast for the airport.
Why Pilots Care
It tells a pilot exactly when weather will improve or worsen during the planned arrival or departure window, directly affecting go/no-go and fuel decisions.
Intuition Check
Do not read this as a new forecast issued later. A forecast change group is a time-marked section inside the same airport forecast that describes an expected change.
Example Sentence 1
The TAF for the destination included a BECMG change group showing the ceiling lowering to 800 feet by mid-afternoon.
Example Sentence 2
After the initial clear forecast, the next forecast change group indicated the ceiling would lower to broken at three thousand feet after 2200Z.