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dream747
12th Mar 2009, 11:17
Hi all,

I've got a couple of questions regarding decoding of this TAF.

EGPK 1221Z 22015KT 6000 RA OVC030 BECMG 1315 25010KT 3000 +DZ OVC004 FM 18 29025 9999 BKN018 PROB 40 1921 1500 TSGR SCT010CB=


I was asked what is the worst visibility forcast for 1800 UTC. The answer I was given is 10km. I thought FM refers to a quick (in less than 60 min) change in weather occurring at hh hours and mm minutes. If this is the case, wouldn't the change starts to take place anytime from 1800 and visibility at the very worst be at 3000m still?

Thanks!

cc2180
12th Mar 2009, 12:32
FM in a TAF supercedes the previous forecast with immediate effect. So assume that from 18, the new vis is over 10k.

You are thinking of FM in a METAR. :ok:

jamie230985
12th Mar 2009, 17:46
You are thinking of FM in a METAR

Why would FM ever appear in a METAR? a METAR is an indication of the current weather at an aerodrome at that precise moment so FM would never be included as that assumes something will occur.

The answer to your question is 3000m, FM indicates the weather WILL change from 1800 NOT at 1800!!

Keygrip
12th Mar 2009, 18:48
Hmmm, this TAF has me puzzled. It doesn't have a date and time code on it - just the time of issue...so I tend not to trust it.

However, it *does* say that the British weather will be crap - so seems fairly accurate. :}

(I'm teasing, btw!).

cc2180
12th Mar 2009, 20:02
Yes you use it in a METAR, but it is used differently.

FM, TL, AT are used to specify timings within a TEMPO of a METAR.

In a METAR it is used to indicate temporary changes from a certain time.
In a TAF it is used to indicate permanent change of FORECAST with immediate effect.

Dont spread your own confusion onto the lad Jamie23

The answer is as he was originally given, 9999.

jamie230985
12th Mar 2009, 21:43
Dont spread your own confusion onto the lad Jamie23

sorry mate but i don't agree & neither do any of the other instructors at the flight school, I seriously think you are misunderstanding what a TAF & METAR are.

I agree that in the trend section of a METAR the term from will indicate a change that may occur from a specific time however my understanding of the term FM in a TAF indicates a change from that time hence as the clock strikes 1800 the worst case scenario will be 3000m vis.

dream747
12th Mar 2009, 23:06
Hi all,

Thanks very much for the replies.

The notes I have says in a TAF, FM is used to show the beginning of a self-contained part in the forecast. All conditions before this group are superseded. So I took it as the weather will change to replace the previous weather complete from this 1800 so the worst case is 3000m visi at 1800. If the weather will change before 1800 (to give 9999) at 1800, I'm sure they would stated BCMG 1719 or something like this?

cc2180
13th Mar 2009, 12:33
/sigh


The notes I have says in a TAF, FM is used to show the beginning of a self-contained part in the forecast. All conditions before this group are superseded.


Yes! It is a forecast, therefore the change is immediate. You have written it yourself. It is there in your notes to see. In a metar it is how you describe, but this is a TAF.

Are all textbooks, syllabus, logic, pilots, question and answer wrong then?

Tick whatever box you want, but I have explained it exactly like it is. Believe the guy who claimed you never use FM in a metar if it pleases you. Your choice, your exam.


however my understanding of the term FM in a TAF indicates a change from that time hence as the clock strikes 1800 the worst case scenario will be 3000m vis.

And here you write it yourself Jamie, except you cant apply the logic.


You: my understanding of the term FM in a TAF indicates a change from that time

The TAF says: FM 18, 9999.

At 1759 the forecast is 3000, but from 18, the forecast becomes 9999.

You dont predict a prediction.

That is the terminology of a TAF defined by ICAO as opposed to a METAR. There is nothing to argue about.

sorry mate but i don't agree & neither do any of the other instructors at the flight school

So from 8pm to 9pm you went and asked all your instructors? Sure you did. I think they'd be embarrassed to see you claim their opinion supports yours. Which magical FTO are you at that believes this?

RTN11
13th Mar 2009, 13:17
This is the problem with TAF codes, they don't give enough definitive examples, so people argue over what it actually means.

Looking on the metoffice abbreviations section, FM is listed as:

FM... From (followed by time weather change is to begin)

so If I saw that, and was planning a flight, I would assume the vis at 1800 would be 9999.

jamie230985
13th Mar 2009, 22:36
So from 8pm to 9pm you went and asked all your instructors? Sure you did. I think they'd be embarrassed to see you claim their opinion supports yours. Which magical FTO are you at that believes this?seriously i'm not going to get petty but I start work at 8am in my FTO, I asked 4 instructors that have more experience than I do (two of which are PPL examinars) what their understanding of FM meant to check that what I believed was correct & all aggreed with my comments. I also got one of them to check in a met textbook for a definitive answer which was;

"The FM indication in a TAF denotes a change in the forecast conditions begining to occur after the time designator"

baring in mind that one of the four instructors I checked with has been teaching groundschool for over 20 years & another has a degree in Met as well as the fact that the PPL textbook states the same as what I thought to be true I think this is conclusive.

& for the record, if the TAF had stated AT 1800 then I would have agreed that the vis would have been 10km+ however FM & AT are two very different things.

Gargleblaster
13th Mar 2009, 22:51
Surprising how hard it is to find definitive answers on this, but the DK met office has a (seemingly authoritative, as from WMO) scanned copy of the METAR AND TAF codes here: http://www.dmi.dk/dmi/koder.pdf

Page 2: FM: "...all forecast conditions before this group are superseeded by conditions indicated after this group."

stue
13th Mar 2009, 22:53
OK, Just a quick question on the TAF? After the FM 18 what is the 25025? Is it just the fact there should be KT after it to indicate the wind? Also, I would have though the FM 18 would be written as FM1800?

I take it this is a TAF for an exam, not a real life one?

jamie230985
13th Mar 2009, 23:07
I think its safe to say this is a TAF written for an exam which is probably why there are a few of ur disagreeing over semantics.

The 25025 is the wind in degrees true, 250 deg 25 kts!!

stue
13th Mar 2009, 23:12
Thought so, just wanted to check I was reading it right and not missed something! :)

cc2180
14th Mar 2009, 23:56
Holy jeeze Jamie...

Inbetween evening posts you claimed that all your instructors had agreed with you. You are lying to create support for yourself, further confusing the original poster. It is unethical. Please name your FTO if you are so convinced. Nothing to be worried about surely?

Once again you quote a textbook answer, but somehow ignore the meaning of the words.

If you are going to be blindly stubborn in the face of all logic, definition, and explanation then that's your problem. I realise you want to save face, but if the question comes up, Dream747 tick 10k. That's all I worry about.

Shouldn't be answering peoples questions so confidently when you dont know yourself. :=


Maybe this will help clear things up:

BCMG is used in a TAF for the gradual change you are talking about.
FM is used in a TAF for an immediate change.

ICAO terminology. How can you argue with that?

For anyone else reading, please use ICAO instead of jamies mystery PPL examiner.

helimutt
15th Mar 2009, 07:41
Have to say I totally agree with cc2180 on this one. I'd tick the 10k answer if in an exam. FM 18:- 9999.

What would I know, it's nearly always less than 10k over the North Sea.:{

helimutt
15th Mar 2009, 07:43
Have to say I totally agree with cc2180 on this one. I'd tick the 10k answer if in an exam. FM 18:- 9999.

What would I know, it's nearly always less than 10k over the North Sea.:{

Not entirely sure you would have a good case, in the event of getting it wrong, that you could argue the TAF was incorrectly written and not what you would expect to find in everyday use, with the missing letters here and there. Correct terminology.????


If in doubt, mark 'C'