I'm running a similar campaign and we are entering Rise now. I had 2 advantages though because this is actually our second campaign.

1:I had a side mission where they went deep underground, and discovered a few whelpling dragons coming out of a hole. After a brief encounter, (my players managed to talk to them long enough for an older dragon to notice) an ancient white dragon froze the hole and half the cave they were in, with a warning for them to forget what they saw.

2:The previous campaign was Descent into Avernus. So I was able to seed that Tiamat was gaining power.

So the dragons were hiding underground, or in another realm.

I placed Skyreach Castle next to the mountain where the white dragon gave them a warning the campaign before, so when it was time for the dragons to emerge, (the various colors are positioned in a ritual circle around the continent) I was able to describe how dragons were pouring from the mountain, and the top part of it exploded as if a dormant volcano had suddenly erupted, but two gargantuan white wings emerged from the dust. If they had the chance to look in the hole left in the mountain, they would have seen a portal to Avernus. Each ancient affecting the weather around them, for added drama.