It should be buffed, really.
When compared with Rain of Fire/Fire Storm,
Thunderstorm costs 2
for 1 damage with 1 card, i.e. 6
for 3 damage with 3 cards
while Rain of Fire costs 7
for 3 damage
with 1 card,
By "Less is more" rule, Rain of Fire is preferentially chosen rather than thunderstorm, maybe in rainbow deck.
I am thinking of buffing this card in these way:
1. thunderstorm costs 1
and Lightning Storm costs nothing to cast, OR
2. thunderstorm removes all momentums of the enemy's creatures