You need a common event+variable.And the random raining wouldnt seem to help,as It would proabably start while you were inside a building,randomly.Not a great idea,but go for it if you REALLY need it,just used common events and variables.
Thats my 4 cents.
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Black
-EDIT-Also,it would have to know the names of your maps,so It could do it,and that seems a bit too hassleesh...hassleish.Really,just have it transition rain,and have it go off.Use the weather like normal.It just seems easier to do that then make a RGSS that would comply with the Night/Day scripts.