I was curious if when you cast a 1st or higher level spell as a reaction on a hostile creature's turn when its movement provokes OA with War Caster if it would trigger the Storm Sorcerer's Tempestuous magic to Fly 10 ft.
Starting at 1st level, you can use a bonus action on your turn to cause whirling gusts of elemental air to briefly surround you, immediately before or after you cast a spell of 1st level of higher. Doing so allows you to fly up to 10 feet without provoking opportunity attacks.
—ScaG, pg. 137
War Caster
You have practiced casting spells in the midst of combat, learning techniques that grant you the following benefits:
- When a hostile creature's movement provokes an opportunity attack from you, you can use your reaction to cast a spell at the creature, rather than making an opportunity attack. The spell must have a casting time of 1 action and must target only that creature.
- …
—PH, pg. 170
Best Answer
No, you cannot.
Per the Tempestuous magic description:
It can only be used on your turn, and if you are using the War Caster's reaction on an enemy who has provoked an Opportunity Attack, it is not your turn.
Note that, if your enemy moves as a reaction during your turn, then you could use your reaction granted from War Caster to cast a spell, and assuming you haven't already used your bonus action, use Tempestuous magic either before or after the casting of the spell.