Barda's Tempest

You create a tempestuous lightning storm. Choose any point within 120 feet to create a storm of lightning and thunder with a radius of 20 feet. For the duration, motes of lightning and rolls of thunder leap from the ground in this area. Any creature within the storm takes 2d6 lightning damage and 2d4 thunder damage, and must succeed on a DEX saving throw or be knocked prone. Any creature that starts it's turn within the storm takes an additional 2d6 lightning damage. 

As a bonus action on your turn, you can move the storm 20 feet within range. 

When cast at higher levels, the number of dice for the lightning damage increases by 1d6 for each additional spell slot.