A 20-foot-radius sphere of whirling air springs into existence, centered on a point you choose within range. The sphere remains for the spell's duration. Each creature in the sphere when it appears or that ends its turn there must succeed on a Strength saving throw or take 2d6 bludgeoning damage. The sphere's space is difficult terrain.   Until the spell ends, you can use a bonus action on each of your turns to cause a bolt of lightning to leap from the center of the sphere toward one creature you choose within 60 feet of the center. Make a ranged spell attack. You have advantage on the attack roll if the target is in the sphere. On a hit, the target takes 4d6 lightning damage.   Creatures within 30 feet of the sphere have disadvantage on Wisdom (Perception) checks made to listen.