What you do, is that if you can, create two nuetral players that are unallied to eathother, but treat all other players as allies. one of them has one unit wich has locust and 99999999 hp and 999999 divine armor, no shadow, no unit file and the monsoon ability that will effect the enitre map. The second one has as many units as you want there to be, the same thing as the first but without the monsoon ability, and the wander ability (nuetral hostile). Then at map initialisation order it to cast monsoon. (set monsoon's duration to 0, that will make it never ending) and you should get the desired effect (monsson comes with rain.
Or WeU has an advanced trigger thatll make you a lightning storm.