In order to have a "smoother" disappearing of trending posts from the Discovery panel, I suggest that the post position could be proportional to at least three factors: rating, upvotes, time span.
To be more precise, I imagine something like this:
rating * upvotes * (1 / (1 + hours))
So, the temporary floating of the post in the list would depend on these factors, where a stronger appreciation would help to resist obsolescence. Some very appreciated posts could resist for days - for a good reason - but sooner or later the age factor would prevail (exposure would still bring possible votes, but who has seen/voted yet, would no longer step in).
Of course, if other quality metrics are already taken into consideration and could improve the formula, the better.
I know formulas are confidential, for now, but since this is just a rough proposal I decided to leave this suggestion public.