FM vs TEMPO vs BECMG: The Three TAF Change Groups
FM vs TEMPO vs BECMG is the comparison that decides whether you can read a TAF at all. The weather groups are the same ones a METAR uses. What makes a forecast hard is the timeline built out of these three words.
Get them wrong and you will read a brief shower as an afternoon of rain, or miss a change that cancels everything you planned around.
What FM vs TEMPO vs BECMG each do
FM means from. It starts a new steady state at a stated time, and it cancels
everything before it. FM051800 means from 1800 on the fifth, the forecast is
now this, and the previous conditions are finished.
The change is treated as rapid, normally under an hour. Everything after the group is the new prevailing weather until the next change group or the end of the forecast.
TEMPO means temporary. It describes fluctuations expected inside a window,
without replacing what the forecast already said. The conditions underneath are
still the baseline. TEMPO is an interruption.
BECMG means becoming. It describes a gradual change from one state to another
across a window. The change starts somewhere in that window and finishes by the
end of it.
The one that catches people
TEMPO is the group most often read wrongly, and it is worth being deliberate about.
TEMPO 1418/1422 2SM TSRA does not mean four hours of thunderstorms. It means
that between 1800 and 2200, brief periods of two miles visibility in
thunderstorms and rain are expected. Each burst should last under an hour. All
the bursts together should cover under half the window.
The rest of that window is whatever the prevailing forecast said. If the line above it said six miles and broken cloud, that is what you get between the bursts.
So TEMPO describes the worst it gets, not what it is like throughout. Plan fuel and alternates against the TEMPO conditions, because you cannot pick which hour you arrive in.
Reading BECMG without guessing
BECMG is vaguer than the other two by design, and the vagueness is the point.
BECMG 2022/2024 24015KT says the wind is becoming 240 at 15 knots somewhere
between 2200 and 2400. It does not say when. The forecaster knows the change is
coming and does not know the hour.
The practical rule is to assume the worse of the two states across the whole window. If the new conditions are worse, treat them as arriving at the start. If the new conditions are better, treat them as arriving at the end.
That way the window is covered whichever way it falls. After the window closes, the new state is the prevailing forecast and stays that way until something else changes it.
How they stack together
The three groups are not alternatives. A single TAF uses all of them, layered.
The body opens with a prevailing state. FM groups cut the forecast into
segments, each one a new baseline. Inside any of those segments, TEMPO and
PROB groups can sit on top, describing interruptions to that segment.
So the reading order matters. Find the FM segment covering your arrival time
first. That is the baseline. Then look for any TEMPO, BECMG or PROB group
whose window overlaps your time. Those modify it.
A common mistake is to read straight down the page and take the last line seen as current. The last line is only current if its window covers your time.
Which one signals most confidence
The choice of group tells you something about how sure the forecaster is.
FM is the confident one. A firm time, a firm new state. Somebody expects a
front or a sea breeze or a diurnal shift at a known hour.
BECMG is less certain about timing but confident about the outcome. The change
is coming, the hour is fuzzy.
TEMPO is confident that conditions will wobble and unwilling to say the wobble
becomes the norm. And PROB30 or PROB40 in front of it drops the confidence
again, to a chance that the wobble happens at all.
Read the groups as a confidence gradient and the forecast becomes easier to weigh.
Verdict on FM vs TEMPO vs BECMG
Sort them by what they do to the line above. FM cancels it. BECMG replaces
it slowly. TEMPO leaves it standing and interrupts it.
That single question, does this replace the baseline or sit on top of it, resolves almost every TAF reading problem. Work out your arrival time first, find the segment that covers it, then apply whatever is layered over that segment.
Frequently asked questions
What is the difference between FM, TEMPO and BECMG?
FM starts a new steady state and cancels everything before it. TEMPO describes brief fluctuations that interrupt the prevailing forecast without replacing it. BECMG describes a gradual change from one state to another across a window.
Does TEMPO replace the forecast around it?
No. TEMPO sits on top of the prevailing conditions. When the temporary period ends, the forecast underneath resumes. That is the main way it differs from FM, which cancels what came before it.
How long can a TEMPO period last?
Each fluctuation should last under an hour, and together they should cover less than half the stated window. If conditions would persist longer than that, the forecaster is supposed to use BECMG or FM instead.
What time does a BECMG change actually happen?
Somewhere inside the window given, and it is not specified more precisely. Plan on the new conditions from the start of the window if they are worse, and from the end of it if they are better.
Which change group cancels the previous conditions?
FM does, completely. BECMG also replaces the previous state once the transition finishes. TEMPO never does, because the conditions it describes are interruptions rather than a new baseline.