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Because of the danger level, I'm assuming that you have shield worn but spear away to cast spells? Unlike a light shield, a heavy shield (like yours) requires the full use of the hand and arm and you can't switch a weapon to it as a brief FA to cast a spell. I do have down that you have magic weapon on spear.
Oh, and I understand story-stuff, but you still have to cast the spell ... it's not an SLA so it still shows as you spoke, invoked the winter forces, and made sure the symbol was visible.
Because you didn't define and it is now the Mother of All's turn, this is what I'll assume .... you need the protection.