Desmo
You seem to be mixing up 2 different circumstances.
The first is where:
your ETA or ETD is say in 6 hours
In that circumstance, your suggestion that we:
Look at the METAR or listen to the AWIS!
is nonsense, for precisely the reasons you identified. What the METAR or AWIS says now does not help in predicting what the weather will be in 6 hours. That's what TAFs and amended TAFs are for.
The second is where:
the time is 1145UTC.
The 1022 Taf says amongst other things
FM 11 prob 40 fog 500m temps 10 8 6 5
The 1130 metar and current AWIS say viz 8000m temp 9 dew point 9
You want to depart a runway with runway edge lighting spaced 75m:
1. What are your chances at 1300?
2. What are your chances at 1200?
If it's 1145 and you want to determine your 'chances' of departing at 1200, I'd suggest you look out the window. If the AWIS is accurate, I'd say you have a 100% chance of departing (or, more accurately, weather won't be an impediment to you departing) at 1200.
The METAR and AWIS, in the context of the TAF, indicate that the longer you wait, the increase in the likelihood in your being prevented, by fog, from departing. However, I don't see how that helps you, at 1145, determine the 'chances' of departure with any more or less accuracy than '40%'.
But I'm always happy to learn. It's 11.45. In your view, what are the 'chances' of departing at 1300 in your scenario?