There is no reason to favor your filed alternate over any other alternative. Just because you put it in your flight plan doesn't mean you are in any way expected or required to use it.
Assess your situation, communicate with ATC to request any information you need and then make up your mind.
In my opinion, the alternate is -- just like almost everything else on the flight plan -- just for the loss of communication (7600) case.
While that is true, I'm afraid that for Part-NCO, which is 3 or so years off, they got this one by me...
NCO.OP.140 Destination alternate aerodromes — aeroplanes For IFR flights, the pilot-in-command shall specify at least one weather-permissible destination alternate aerodrome in the flight plan, unless:...
The gold plated alternate: This is for those really crap weather days and is the place you will go to if there is a significant and unforcasted degradation in the weather
I flew Cologne (EDDK) to Cambridge (EGSC) last Tuesday, going out to the aircraft at 1100, taking off from EDDK at 1145, 2 hours enroute. Usual alternate Southend (EGMC). I need RVR 800 m. Probably a good case study.
TAF EGSC 230805Z 2309/2318 01006KT 0400 FG OVC001 BECMG 2309/2312 2000 BR BKN004 PROB40 2312/2318 6000 NSW BKN005= METAR EGSC 230850Z 35005KT 310V020 0450 R23/1300U R05/1000N FG OVC001 12/11 Q1024= METAR EGSC 230920Z 35005KT 310V020 0450 R23/0900U R05/1000N FG OVC001 12/11 Q1024= METAR EGSC 230950Z 35005KT 320V040 0350 R23/0900N R05/0900N FG OVC001 12/11 Q1024= METAR EGSC 231020Z 34005KT 300V020 0350 R23/1000N R05/1000N FG OVC002 12/12 Q1024= METAR EGSC 231050Z 34005KT 270V050 0500 R23/1100N R05/1100U FG OVC002 12/12 Q1024= TAF EGMC 230804Z 2309/2318 02004KT 0400 FG BKN001 BECMG 2309/2312 3000 BR BKN005 PROB30 TEMPO 2312/2318 6000 NSW BKN012= METAR EGMC 230850Z 36005KT 320V050 0400 R24/0550 FG OVC001 13/12 Q1024= METAR EGMC 230920Z 36004KT 0400 R24/0600 FG OVC001 13/12 Q1024= METAR EGMC 230950Z 01004KT 320V060 0400 R24/0600 FG VV/// 13/12 Q1024= METAR EGMC 231020Z 36004KT 0400 R24/0600 FG VV/// 13/12 Q1024= METAR EGMC 231050Z VRB03KT 0450 R24/0900 FG VV/// 13/12 Q1024= TAF AMD EBOS 230904Z 2306/2412 06005KT 0450 FG BKN001 BECMG 2310/2312 1500 BR SCT001 BECMG 2312/2314 6000 NSW BECMG 2322/2324 3000 BR PROB40 TEMPO 2402/2407 0400 FG VV001 BECMG 2409/2411 8000 NSW= METAR EBOS 230850Z VRB01KT 0450 R08/0500N R26/0600VP2000U FG BKN001 11/09 Q1023 BECMG 0900 FG SCT001= METAR EBOS 230920Z 11002KT 0700 R08/0300V0450N R26/0500N FG SCT001 12/10 Q1023 BECMG 1600 BR= METAR EBOS 230950Z VRB02KT 0300 R08/0225V0325N R26/0350V0600N FG SCT001 13/11 Q1023 BECMG 0900 FG= METAR EBOS 231020Z VRB02KT 2500 BR FEW002 14/12 Q1023 BECMG 3500 BR= METAR EBOS 231050Z VRB02KT 5000 BR NSC 15/13 Q1023 NOSIG=
So Southend (EGMC) is legal for my time of arrival -- for those of you used to Part 91, under ICAO Annex 6 Part 2 and UK rules, I just need my destination or my alternate above AOM for legality. So I could, in theory, take-off with 20 mins alternate fuel, just enough to get to Southend. Sensible? You have to be f%$£ing joking.
The gold plated alternate is Ostend, where I know it's already 5000.
This is what the Met Office put out just after I was unable to access them.
TAF EGSC 231105Z 2312/2320 35006KT 0400 FG OVC001 TEMPO 2312/2318 OVC002 PROB40 2312/2318 1500 BR= TAF EGMC 231105Z 2312/2321 36004KT 0400 FG BKN001 BECMG 2312/2315 0800 BKN006 PROB30 TEMPO 2315/2321 0400 FG BKN001=
And just like that, the whole of the SE of England went from forecast above to forecast below minima. BPF, you are sooooo right.
FWIW, when I got to Cambridge it was positively glorious (Luton and Stansted were hovering).
METAR EGSC 231320Z 35006KT 320V020 1300 R23/P2000 R05/P2000 BR OVC002 13/13 Q1024= METAR EGMC 231320Z 33004KT 1600 R24/1200 BR BKN002 13/13 Q1023= METAR EGGW 231250Z 01006KT 360V130 0250 R08/0400 FG OVC001 12/12 Q1024= METAR EGGW 231320Z 01007KT 0350 R08/0900 FG OVC001 13/13 Q1024= METAR EGGW 231350Z 36007KT 0300 R08/0600 FG OVC001 13/13 Q1024= METAR EGSS 231150Z 35005KT 0300 R04/0450 FG OVC001 12/12 Q1024= METAR EGSS 231220Z 34007KT 0300 R04/0650 FG OVC001 12/12 Q1024= METAR EGSS 231250Z 34006KT 0550 R04/1100 FG OVC001 13/12 Q1024= METAR EGSS 231320Z 35005KT 0500 R04/0800 FG OVC001 13/13 Q1024= METAR EGSS 231350Z 36006KT 0500 R04/0900 FG OVC001 13/13 Q1024=
I'm afraid that for Part-NCO, which is 3 or so years off, they got this one by me...
One learns something every day...
I thought that for IFR in Europe, an alternate was always needed to be planned. The FAA "1-2-3" concession doesn't apply here, if there is a stricter requirement here.
Or should I hope that the IRE which I have to fly with sometime in the next 3 months isn't reading this? 
Unfortunately he almost certainly is!
I agree you were legal to go but with
TAF EGMC 230804Z 2309/2318 02004KT 0400 FG BKN001 BECMG 2309/2312 3000 BR BKN005
and a 1345 ETA one is definitely hoping that the "BECMG 2309/2312" will happen, and if it doesn't, the options are bleak.
With the kind of wx data you have posted I would have stayed indoors, or loaded up enough fuel to go somewhere nice and warm, plus 2hrs' reserve 
I find it easier to do this kind of boundary pushing with a £300 satphone from Ebay, for getting wide ranging tafs and metars.
The scenario given at post 6 is IMO a situation you do not want to ever be in.
Thinking about where you are going to go After you have failed twice to get in to your planned destination is too late. Therefore if you are down to 2 hours of fuel you had better have a plan since if you missed at the destination then the weather is obviously worse then forecast.
One of the challenges of flying SEP/light piston twins in real world IFR A to B trips, is any place an hour away is only going to be 120 to 160 miles away which means that it probably has the same crap weather as where you are in now so you better have a good idea of what is making the weather. If it is frontal weather then an airport that is in a direction perpendicular to the front either behind or in front of the front (depending on where it is and how fast it is moving) is probably your best bet.
This is why you should have a solid gold alternate and the fuel to make it. On days with widespread bad weather there may be no good options for a place to go that is going to be guaranteed good so, this would be enough for me to cancel the flight.
Driving along on a crap day and not getting updated weather is IMO not acceptable. There is always a way to get current weather. So in your post # 6 scenario the deteriorating destination weather and the new plan B should have been known before you started your first approach.
Finally over 20 plus years I have been flying IFR I bet the number of occasions I have missed an IFR approach and then immediately returned for a second one could be counted on one hand with fingers left over. Unless the weather is changing rapidly and for the better, after missing the first approach I would either go and hold until I get steady above limits weather or go to my alternate. The accident stats for approach accidents are frightening. For multiple consecutive ILS approaches you are 5 times more likely to crash on the second approach and 15 times more likely on the third or more consecutive attempt.......
One of the challenges of flying SEP/light piston twins in real world IFR A to B trips, is any place an hour away is only going to be 120 to 160 miles away which means that it probably has the same crap weather
Yes; I think the biggest thing in bookworm's weather situation is that those conditions are persistent (fog etc) and the chief characteristic of such conditions is that nobody is able to forecast with any accuracy when they come or when they dissipate.
A few years ago I set out to organise an "IFR weather" seminar, trying to drag in an ex UK Met Office forecaster who was then running some "aviation" seminars. Ultimately I was unsuccessful in pinning him down on any dates so it all fell apart, but in the process I did ask a few dozen pilots about what they were hoping to take away from such a seminar, and the most common thing was predicting fog.
My view on forecasting is that a normal pilot is never going to out-forecast the professionals who do it all day long, so time spent on a detailed understanding of meteorology (as on e.g. the JAA IR course) is best spent doing something useful. But not even the pros can forecast the timing of these persistent conditions.
I recall a few situations where I was going somewhere by airline and the pilots just sat on the ground until fog at the destination actually cleared. TAFs were simply ignored. They must know something
So one needs an alternate which is outside that whole affected region.
Finally over 20 plus years I have been flying IFR I bet the number of occasions I have missed an IFR approach and then immediately returned for a second one could be counted on one hand with fingers left over
I agree with that too (10 years' worth of IFR) but that's because - on an ILS - I have never seen the weather improve in the 5-10 mins it takes to go missed and come back for another go, not because I expected to crash on the missed approach
Sometimes one might have another go if one flew a nonprecision approach which was flown a bit too high up, so the MDH was not reached before one got to the MAP (which is not unusual because one is not supposed to go below the line) and then another go, taking one say 300ft lower, is perhaps worth a try...
I recall a few situations where I was going somewhere by airline and the pilots just sat on the ground until fog at the destination actually cleared. TAFs were simply ignored. They must know something.
I wondered this. Last Tuesday I was due to go to Frankfurt, from London City at 0900 on Lufthansa. During the course of the day I was re-scheduled due to cancelled flights four times at both City, and Heathrow, and only the 4th booking, did a plane arrive. So unlike us in GA, who sometimes just give it a go in the hope that it will actually be on the more optimistic side of a TAF, it seems the Airline pilots just wait until it is clear. I guess it's costly and embarassing to try and land somewhere, find all your alternates inaccesible too, and returning back to base.
I think that in commercial terms it would be completely stupid to depart unless a landing at the planned destination is practically assured, because the cost of transporting everybody in buses, and/or putting them in hotels, etc, is massive, whereas the cost of cancelling and having a load of people sleeping on the floor at the airport and cursing the airline is very low, and the airport makes a mint from the massively overpriced shopping
My understanding is that even if you have CAT3C, you still cannot land if the surface vis is below some figure, because you may not be able to taxi off the runway.
In GA terms, fog is a "killer" because large areas can be lost very quickly and most pilots do not have a means to get wx for numerous airports in one go (no ACARS and it's often not possible to ask ATC). You could easily lose the whole of the UK south east coast (i.e. Bournemouth, Southampton, obviously Shoreham, Lydd, Manston) within an hour or two. Southend tends to be hit less often. I recall one flight where I got into Lydd on a diversion minutes before that went below minima, and that was possible only because I was using my satellite phone to get wx for various places an hour or more before reaching the UK. Had I done the "classical IFR" thing and flown to the destination (Shoreham) first I would not have got in. But on that occasion the TAFs were nowhere near as bad as in Bookworm's scenario.
AeroPlus is an interesting new(?) iPhone/iPad app for forecasting cloud layer depths and fog; good tech support as well.
Besides info for METAR, TAF, ESTOFEX storm prediction, EUMETSTAT IR Cloud top temp. images, unstable atmospheric conditions etc. it gives this vertical cross-section for your dest/alt. airfield over 24 hours, with dew-point/temp split and freezing level:
Big Pistons Forever said:
"This is why you should have a solid gold alternate and the fuel to make it."
I FULLY agree.... and that is why I believe one should always attempt to flight-plan for "a solid gold alternate" rather than a purely "a legal one".
