I know of this thread over at the TWC, which principally discusses battle weather, but they briefly touch on campaign storms.
Midway down, DrJambo says:
Also, can I suggest using the toggle_fow RomeShell command - it will make monitoring for storms much easier, as it removes the fog of war, allowing you to monitor the whole map.I may be wrong, but I think that when modifying files that are included in the same folder as the campaign text files (e.g. disasters, terrain, etc), the effects may only take effect when a new campaign is started. I.e. changes will not affect saved games.
HTH.
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