Regions is indeed the way to do this. They control weather, choosing from the available options based on where the player is and what the weights are.
The chances are weights, not percents, I believe. Meaning if they don't add up to 100, their still weighted that heavily against one another. If A has 25 weight, and B has 75 weight, I believe their chances are the same as if A had 100, and B had 400 weight.
Remember that most weather regions are contained under Tamriel as that's the world space most of the world exists in. I'm not sure how worldspace parenting affects regions and weather choice.
Oh, thanks for the info. Anyway I finally did some testing, ok, just 2 test runs, but indeed the mod does seem to work!
1st test without the mod and I got foggy weather. 2nd test with the mod and I actually got sunny weather. So yeah, I guess I was just being too paranoid. I'm not too familiar with the CK, even till now.