just color the background. 0,0,0 when there's no thunder. and set color to 1,1,1 when there comes a thunder and slowly decrease it downto 0,0,0 again when the thunder is over

a simple looping animated texture with raindrops splashing and fading out should do the trick with the rainy thing. maybe a little particle system could be a fine supplement. you could eventually blend two of the animated raindrop textures with different scaling/positioning for variation