More than one witch has expressed the idea that they find it distinctly empowering to work magick during thunderstorms. According to Christos Beest, whenever he would use his art to channel the demons of the Goetia, storms would accompany his channelings. The deliberate causation of storms (at the hands of the archfiends) implies a magickal efficacy which is inherent in storms.
The rain serves an obvious purpose, as running water is a magickally powerful thing. Fire is known to be a magickally powerful thing as well, so perhaps lightning serves a similar function. Here follow two excerpts from one of the allegories in the Deofel Quintet, the first of which is only one sentence long.
“The storm pleased Melanie and she danced naked in her garden while the rain washed her body as she sucked the storm’s health in.
…The twilight of closing cloud and strong wind coloured the sky near the descending sun, and Melanie stood in the circle’s centre calling on the storms to break. Thunder cloud rushed toward her, killing the colour, as the wind graved strong and heavy around. There was no thunder, only a sudden and prolonged burst of rain, which Melanie laughing let soak through her thin dress to the warm flesh beneath. She became intoxicated by the power of wind and rain, and danced around the circle calling on the names of her gods… She felt her crystal, many miles distant, begin to respond and draw power from the Abyss beyond. The power came to her, slowly, through the gate in the fabric of space-time, a chaos of energies from the dimensions of darkness. Her consciousness was beginning to transcend to the acausal spaces where the Dark Gods waited and she sensed their longing to return, to fill again the spaces of her causal time. They were there, chattering in lipsed words she could not understand, roused from sleep by the power of her previous rites, ready to seep past the gate…” –Temple of Satan by Anton Long