I'm too lazy to check this in WE, but I think this will work. Edit your trigger so it looks like this
Event
Map initialization
Condition
(I left this one blank)
Action
Environment
Create at playable map the weather effect Ashenvale rain heavy
Environment
Enable Last Ceated Weather Effect
If this still doesn't work, change the event to Time Elapsed - Elapsed Game Time is 0.01 seconds (I don't think this is necessary, but as I said, too lazy to check)