In case you're not familiar with this real weather event:
http://en.wikipedia.org/wiki/Dirty_thunderstorm
http://geology.com/articles/volcanic-lightning/

During Ash/Blight storms in the Red Mountain region, there could be thunder and lightning happening as well.
I think this would be a interesting idea for a Morrowind mod. I'm not sure how it would really look and feel in-game, but I think it's worth testing out to see if it fits.

I admit I'm a bit rusty with the CS lately, so I thought I'd just throw out the idea and see if someone runs with it.
Or leave some advice on how to best implement it.
Weather is one thing I'm not really sure we could modify like this.