The following information comes straight from Bulbapedia:
"When used as a
Z-Move, Weather Ball will be turned into
Breakneck Blitz, but then Breakneck Blitz will turn into the Z-Move of the type corresponding to the type Weather Ball would be in that weather. This does not occur if Weather Ball is turned into a Z-Move via another move like
Sleep Talk."
Yet, for some reason, when I use Z-Weather Ball, it doesn't change from Breakneck Blitz into the Z-Move I want it to change into. For example, I have a Pokebank Ubers rain team with Weather Ball Raikou holding a Normalium Z. When I turn Weather Ball into Z-Weather Ball while in the rain (without Cloud Nine or Air Lock in effect), it doesn't transfrom into Hydro Vortex--it stays Breakneck Blitz. Furthermore, it doesn't deal as much damage as regular Weather Ball.