Winter in Summer
Someone asked a question about the Frost Giants that I thought the answer was worth reposting so everyone could see it:
The Frost Giants are always around, but they're not necessarily right THERE all the time.
This is a war to them, and the war isn't with Midgard. Midgard and it's human inhabitants are incidental, so the the Jotun are mobilizing, searching for the best place to hold their positions all over the world so that when Loki and Thor finally face them, they'll have strongholds everywhere and be ready to fight on whichever front they're attacked from.
This means that at any time, your cities might be free of the Frost Giants as they move through to whatever point they're marching toward. Some might follow and fight, some might not. Because there are always MORE of them coming, and there could be another wave passing through the city, since it already appears to be on a path to wherever it is they're going.
But... your characters don't necessarily know that. ;)