So I was wondering if the weather was changed would mega evoling abomasnow/tyranitar retrigger the weather?
If so, It would happen after switches, so switching in drizzle poiltoed, would set rain but then mega abomasnow would change it back to snow?
although prankster triggers after mega evo's, It's useful considering no pranksters have access to hail/sand.
/
that said tyranitar can now DDance without worrying that the weathers going strip away its SD hull

TO my knowledge it will change the weather back when TTar/Snow Mega-evos during another weather.

I'm pretty sure that Charizard-Y activates Drought upon mega-evolution, so it follows that M-Tyranitar and M-Abomasnow would do the same for their own abilities. I don't think it adds to your count, though, if their respective weather is already active.