The initial Stormfire projectile explodes on contact, but its fragments don’t. They pass through enemies and explode only after hitting the ground and letting their expiration timer end. So it could take upwards of 1.5 seconds between casting Stormfire and its fragments exploding. For a spam skill, which should be highly responsive to the situation, this is simply unacceptable.
The fragments should explode on contact, just like the initial projectile.