One thing that strikes me as maybe being a bit too good is that you seem to imply that the caster can designate the targets that the spell will arc to. That gives you way more control over the spell than any other area spell. Take fireball for example, you decide the target center of the spell and provided it doesn't scatter you hit anything in the spells radius, whereas with your spell you could essentially throw it into a melee and have it hit only enemies. I would change it to "the spell arcs to the closest viable target regardless of whether that is a friend or a foe or a device or a vehicle..etc.". In this way the spell is more in line with the other indirect area spells.
Instant spells are basically fire and forget more or less, the kind of fine control that it would take to direct the arcs would probably require the spell to be sustained, which is of course also an alternative, making it similar to Flame Burst from shadow spells. In that case I would make it so that you could direct the spell to a new target every initiative phase with either a free or a simple action, of course that would count as your attack action for that phase and you would take the usual sustaining penalty to all other tests, not to the attack though.
