I have a character in the throes of a Storm of Vengeance spell and two Whirlwinds, he's a 19th-level spellcaster, and will inevitably use magic to solve his problem.

How do I calculate the Concentration DC?
He's not being dealt damage, and the Concentration Skill says that 'Weather caused by a spell, such as storm of vengeance.', uses that spell's DC.
So automatically a base 10...

Do I just add the Whirlwinds to one base Save DC? Or does he Save against three DCs?