Indication, Sign
A loitering mountain king in the winter is an indication or sign of something to come.
We don't want to focus too much on what the indication / sign actually is, though, because it will take away from the indication part (which we want to remember). So, focus on the fact that this is some kind of indication that's important, but you don't know what that indication means or is... because we'll use that with the reading.
ちょう
しるし
None
When you spot a loitering mountain king in the winter, you know it's an indication that soon Mrs. Chou (ちょう) will appear. It's like a Groundhog's Day kind of situation, but instead it means Mrs. Chou is about to come out, perhaps early for the season.
Imagine an announcer watching the loitering mountain king, "There he is... and it's still winter. We all know what that means. It's a sign that Mrs. Chou will be coming out of hibernation soon." Then, you can imagine her bursting forth from the mountain, running down her street.