plus, Springfield Kentucky is not in the northern part of the state
There was another episode in which they listed several states which were east of spring field. One of those states was Oregon. Therefore, Springfield is probably in Alaska. This location would be ideal:
Every state mentioned in the episodes is to the east.
Every state is to the south (so tenessee is still south of the border)
There are lots of towns in Alaska that no one, including arguably the townsfolk, has ever heard of. This would explain how Shelbyville can be next to Springfield.
This would fit just about every possibility that I can recall except for the fact that they seem to have a normal day/night cycle