A storm rolls in, and lightning strikes the ground at four different points you see within range - each creature in a 30 ft. radius sphere centered on each point you choose must make a dexterity saving throw - it spreads around corners.  A creature takes 20d8 lightning damage & 10d6 fire damage on a failed save, or half as much damage on a successful one.  A creature in the area of more than one is only affected by one. 

Additionally, creatures in a 40 ft. radius - from the center point of each sphere - must also make a constitution saving throw.  They take 10d8 thunder damage on a failed save, or half as much on a success.  The spell damages objects in the 30 ft. area, and ignites flammable objects that aren't being worn or carried.