I realise the weather is perfectly suitable and the fog should have cleared by 1000z in my example, I'm just asking specifically if you must legally declare two alternates in this instance.
Of course, it's only a forecast and you take it with a pinch of salt. Fog especially has a tendancy to stick around longer than the boys and girls in Exeter guess, and of course if this were a real scenario, I'd take a suitable amount of fuel. I'm just asking legally where you stand.
But why is the phrase "The prevailing weather conditions forecast in the initial part of the TAF should be fully applied" in a TAF if that's never the case. If there wasn't a BECMG/FM, the phrase would be irrelevent anyway, so why have they included it?
I don't meant to be obtuse and realise my question is entirely academic but I just don't understand the required criteria specifically laid out for flight plans.