AMD vs COR: Amended or Corrected, and Which One Replaces What
AMD vs COR is a small distinction with a large consequence. One of these words means the weather is going to be different from what you were told. The other means somebody fixed a typing error.
Read them as interchangeable and you will either ignore a changed forecast or scramble over a fixed punctuation mark.
What AMD vs COR each mean
AMD is short for amended. It appears on a TAF when the forecaster decides the
current forecast no longer matches what is expected to happen. The meteorology
changed, so the forecast changed.
COR is short for corrected. It appears when the message that went out was
wrong as transmitted. A digit dropped, a group garbled, a station identifier
mistyped. The weather did not change. The record of it did.
That is the whole difference, and it decides how much attention the new message
deserves. AMD is news. COR is housekeeping.
Why a METAR can never be amended
This trips people who expect the two codes to be available everywhere.
A METAR is an observation. It records what was measured at a specific minute that has now passed. You cannot revise what the weather was doing at 2153Z, because it already did it. So there is no such thing as an amended METAR.
If the 2153Z observation went out with the wrong altimeter setting, the station
issues METAR COR and the corrected figure. The moment being described is the
same moment. Only the description was faulty.
A TAF is a forecast, which is a statement about the future. The future is allowed to change, so a TAF can be amended and frequently is.
When a forecaster issues an amendment
Amendment is not discretionary in the way it sounds. There are triggers.
The usual ones are a change in flight category, the arrival or departure of a thunderstorm, freezing precipitation starting or stopping, and a wind shift or speed change beyond set limits. If the current forecast would have a pilot planning for the wrong category, it gets amended.
A new TAF at an unscheduled time is a signal in itself. The routine issues come four times a day at fixed hours. Anything arriving between those hours arrived because somebody decided it had to.
That makes an off-cycle TAF AMD worth reading closely even before you decode
it. Somebody was concerned enough to rewrite the forecast early.
How an amendment replaces the old forecast
An amended TAF supersedes the previous one for the rest of the valid period. It does not patch it.
So you do not read the old forecast for the hours the new one skips over. The new message is the forecast now, complete in itself, and the old one stops existing the moment it arrives.
The valid period usually shortens as a result. An amendment issued at 1500 for a forecast that ran to 0600 the next day will cover 1500 to 0600, not a fresh 24 hours. The end time stays where it was.
The AMD NOT SKED remark
Some military and smaller civil fields carry AMD NOT SKED in the remarks, and
it is easy to read as reassurance. It is the opposite.
It means amendments are not scheduled at this station, normally because it is not staffed continuously. Nobody is watching the forecast to see whether it is still right.
Often it comes with hours attached, such as amendments available only between certain times. Outside those hours the forecast stands unattended, however wrong it becomes.
Treat a field carrying that remark as one where the forecast is a snapshot of somebody's opinion hours ago, rather than a live document.
Spotting which one you have
Both words sit near the front of the report. Look right after the type.
A TAF header reads TAF AMD KDFW 291720Z. A corrected observation reads
METAR COR KDFW 292153Z. The word comes before the station, so you meet it
early.
Some services put COR after the station instead. Either position means the
same thing. What matters is which of the two words is there.
You may also see RTD, meaning a delayed routine report. That is neither an
amendment nor a correction. It is the scheduled message arriving late.
If two versions of the same report reach you, take the later one. The timestamp settles it. There is no case where an older version wins.
Verdict on AMD vs COR
Sort them by what changed. AMD means the weather outlook moved, so re-plan.
COR means the message was faulty, so re-read.
Both replace what came before, so never work from the superseded version of
either. And if you see AMD NOT SKED, remember that the absence of an amendment
at that field is not the same as the forecast still being right.
Frequently asked questions
What is the difference between AMD and COR?
AMD means the forecast itself changed, so the weather outlook is different from the one issued before. COR means the previous message was transmitted wrongly and this version fixes it. Only AMD means the meteorology moved.
Can a METAR be amended?
No. A METAR is an observation of a moment that has passed, and you cannot amend the past. An observation that went out wrong is corrected with COR. Only a forecast such as a TAF can be amended.
Does an amended TAF replace the whole previous TAF?
Yes. An amended TAF supersedes the earlier one completely for the remainder of the valid period. You do not merge the two or read the old one for the parts the new one leaves out.
What does AMD NOT SKED mean?
Amendments are not scheduled for this station, usually because it is not staffed around the clock. It means nobody will be issuing corrections outside those hours, so treat the forecast as unwatched rather than as confirmed.
Where do AMD and COR appear in the report?
Near the front, right after the report type and before or just after the station identifier. A TAF header reads TAF AMD KDFW. A corrected observation reads METAR COR KDFW.