Rolling Thunder

• WWizards of the Coast2004-10-04
  Official Ruling
  If X is 0, you can’t choose any targets.
• WWizards of the Coast2015-08-25
  Official Ruling
  You choose how many targets Rolling Thunder has and how the damage is divided as you cast the spell. Each target must receive at least 1 damage.
• WWizards of the Coast2015-08-25
  Official Ruling
  If some, but not all, of the targets become illegal by the time Rolling Thunder tries to resolve, the original division of damage still applies, but no damage is dealt to illegal targets. If all the targets are illegal, the spell won’t resolve.
• WWizards of the Coast2015-08-25
  Official Ruling
  If Rolling Thunder is copied, the copy will retain the original damage divsion. The spell or ability will likely allow you to choose new targets, but you can’t change the way damage is divided.

Rulings via Scryfall