Tantrums and Meltdowns are two different things. A tantrums typically derives from NOT liking, wanting, agreeing with, getting the solicited response or answer. Tantrums in general give a hint of a reason and what response is sought. A meltdown stems from being overwhelmed and can be a reaction to something evident or completely unseen. A tantrum will most often stop when your child receives the elicited response. A meltdown often occurs when there are no words to put to the emotions and sensorial input bodies are receiving most often unsolicitedly.
Take a look at the short clip below for a sampling of scaled teaching materials that can be used to teach your child words to go with those sensorial inputs.