either you have to be good at planning (and have players that don't veer too much from what you expect and/or tell them), or you have to be good on the fly.
one possible compromise between the two extremes -- make a list of clues that you want to pepper in at random times.
leave them fairly "in the background" but give them a mention as you describe areas. ie pretend one of your clues is "the weather starts to act up" and then when the PCs get to a town add in a random clue from your list. "you enter the woodland town, it starts to rain a little but the canopy of the trees provides some shelter as you move through to find the elven mayor that you seek ..." (just giving a slight nod to the clue hidden amongst more pressing info).
and then with time as you want the event to be closer then start giving more focus to the severity of the clue ("the rain is picking up and it's not even the rainy season, several townsfolk scrurry off to the shelter of their homes.")
one possible compromise between the two extremes -- make a list of clues that you want to pepper in at random times.
leave them fairly "in the background" but give them a mention as you describe areas. ie pretend one of your clues is "the weather starts to act up" and then when the PCs get to a town add in a random clue from your list. "you enter the woodland town, it starts to rain a little but the canopy of the trees provides some shelter as you move through to find the elven mayor that you seek ..." (just giving a slight nod to the clue hidden amongst more pressing info).
and then with time as you want the event to be closer then start giving more focus to the severity of the clue ("the rain is picking up and it's not even the rainy season, several townsfolk scrurry off to the shelter of their homes.")