Thinking about this some more, an alternative might be to effectively make a scene aspect on a zone (and it only really makes sense, to me, for a zone for this purpose) where the power of the spell is its targeting value (no Weapon rating).
So if, say, you wanted to clear a room, you might create a 4-strength firestorm that'd last for three rounds as a 9-shift spell, cast it, and then whoever's in that zone endures a 4-shift attack each of those three rounds.
This would mitigate the overpoweredness of the attack roll + weapon rating combo, but I think it only works as a zone attack, as that helps mitigate its overall power (since you need to spend 2 shifts for a zone effect), creates limits to its use (you'd be reluctant to abuse it if your allies are in the zone), and lets you do other things.
Thoughts?