Originally Posted by
Alhanelem
You cast the storm spell that creates the conditions ideal for the spell you want to cast on the target. Casting Firestorm for instance, will benefit the fire element spells you cast (especially the helixes), so yes, that is what you would cast on a monster with a weakness to fire.
This is the same for natural weather effects. When heat waves are in effect, fire spells are more effective. Storm spells simply create that weather effect any time you want, for the person receiving the effect.