If you want it to look like a real lightning strike, use this in your script: (Just change LIGHTNING to the name of your overlay)
&overlay LIGHTNING create
Shift your overlay using what IsabelleV said
&overlay LIGHTNING opacity 100% in 0.07 then overlay LIGHTNING opacity 0% in 0.07 then overlay LIGHTNING opacity 100% in 0.07 then overlay LIGHTNING opacity 0% in 1 then overlay LIGHTNING opacity 0.1% in 1 loop INFINITE times