That answer doesn't actually address the question asked. The question is: when do you pick targets for the spell? The PHB does not actually say. I can see the logic of choosing targets as each blast resolves (and even moving between them*), since that is how extra attack works, but it would be nice if they had addressed it.not according to JC.
*ETA: This is not true. the rule specifies weapon attacks in order to break up movement between attack rolls.