Lightning affect?

In one of the night elf TFT missions (i think its the one where furion and Tyrande have to get to mieves base before the naga destroy it) there is thunder and lightning. When the lightning occurs, the screen flashes blue and makes it look like real lightning. How do i do this in WE?

I believe this is an alpha fade. You just fade blue fast, then fade back to transparent. I cant provide a complete guide cause Im not sure where my wc3c CD is...