The world map places are rainy. Could this be another plot, perhaps?
There's an increased chance of Tempest appearing in rainy areas.
Edit: Got that a bit wrong. Not just rainy areas. It's all over the map, and increases the chances of her appearing anywhere.
Happens for 3 hours on Monday's only. Tempest doesn't like Monday.
I've never happened to visit the map during the right time, and had no idea this even existed! Very cool :)