you could have a huge white sprite that covers the lightning area(+)
set its visability invisible, also put it on a separate layer so that you can padlock it and it wont get in the way as you are designing. (it will have to be a high layer)
give it a booleon. so when you enter the raining area (or don't if its always raining),
as an event, have when the booleon is true, every x second (random(1,10)) make lightning sprite visible
then have another event when it is visible, that either fades (its a behaviour) or just a 0.2 wait before it goes invisible again.
also, sure Theres a 1 event way to do this.