Good news and bad news on the topic of making the storm into a map effect instead of a unit. I have some proof of concept. But I can't use the sandstorm unit which Deliverator has created, and I don't know enough about effects to create the right one.
In the BTS installation, there is a mod Gods Of Old, which has certain effects like plague and tsunami. The effects for these are in BTS/Assets/Art/Effects/Gods_Of_Old and BTS/Assets/Art/Effects/Sunami. There are no units around these effects, they are directly displayed on the map using CyEngine.triggerEffect(). I can do the same thing now. But neither of these effects does what I want. So far it is a proof of concept and most of the code still remains to be written.
The plague effect is too light and easy to miss, and the tsunami effect appears to last too long. When I move the plague, it does not leave a ghost behind. When I move the tsunami, it leaves a ghost for a long time, at least 10-15 seconds. The tsunami also has rain, which is obviously wrong, and lightning, which is pretty cool actually.
I don't understand the relationship between units and effects very well, and I don't know enough about nifs to tune either one.
Is it possible for some artist person to come up with something that doesn't leave a ghost but does look like a sandstorm? Lightning would be cool too.