When Cast on Yourself:
Thunderstorm
Instant, 45 second cooldown
You call down a bolt of lightning, energizing you and damaging nearby enemies within 10 yards. Restores 8% mana to you and deals 1450 to 1656 Nature damage to all nearby enemies, knocking them back 20 yards.
When cast on an Enemy:
Thunderstorm
Instant, 45 second cooldown
30 yard range
You call down a bolt of lightning on your enemy, energizing you and damaging enemies within 10 yards. Restores 8% mana to you and deals 1450 to 1656 Nature damage to all nearby enemies.
So in other words, when you cast it on an enemy, it does the same exact thing, except with no knockback.