You can cast this spell only at the beginning of your turn, before you take any other actions.

After casting arrow storm, you can use a full-round action to make one ranged attack with a bow with which you are proficient against every foe within a distance equal to the weapon's range increment.

You can attack a maximum number of individual targets equal to your character level.

If you choose not to spend a full-round action in this fashion after casting the spell, the spell has no effect.