Diversion decisions.
Every now and then I read posts in which people comment about how much fuel they think aircraft have, or alternates, or perhaps bemoaning the fact that they ended up diverting, and they subsequently found that the destination wasn't weather affected.
A recent example of how things can play out.
Flying from LA to MEL. On departure Melbourne had no weather requirements, and the maximum fuel possible was carried.
Just after departure, the weather forecast for Sydney was changed to include 60 minutes of holding due possible thunderstorms.
As the night progressed, Melbourne's forecast was changed so that it now included a 30% probability of broken cloud at 200'. This means that it now requires an alternate. Avalon's weather isn't as bad, but is still below the alternate requirement, so it can't be used.
So.....if Melbourne improves to put the cloud at 400' or greater, there's no requirement and you can continue. Or, if Avalon improves, you can continue to Melbourne and use AVV as the alternate.
If Melbourne does not improve, then you will need to divert from the cruise. But, Sydney has a 60 minute holding requirement due TS, and from experience it clogs up badly. So, that has the effect of pushing the diversion point back to about 200 miles out into the Tasman (from Moruya). In this instance, Sydney is a bad choice (because thunderstorms are mentioned). Next choice is Brisbane, which has no weather requirements, but the decision point is quite a long way back up the track (about 2.5 hours).
You can go past the Brisbane decision point as long as you have fuel for Sydney plus the holding requirement. But, actually going to Sydney is very undesirable, so you won't do so (go past the BNE decision point) unless the company met people think that the MEL forecast will improve. You can't go past the Sydney/Melbourne decision point unless the Melbourne forecast has actually been changed (not just met "think" it will improve).
In this instance, the holding requirement for Melbourne was removed about 3 hours out, so it all became moot. But, if it had still been in place 30 minutes later, Brisbane would have been on the agenda. Sydney's thunderstorms actually arrived, and the required 60 minutes of holding would have been inadequate. If the diversion to Brisbane had happened, on the ground you'd find that Melbourne was actually ok, and Sydney was terrible.