So, I was messing around with a trick room double's team, and I discovered something very interesting about Abomasnow and his Mega self. Even if you already had the ability Snow Warning, if the weather has changed (or subsided) since you sent him out, Mega Evolving will count as "entering the battle" or as freshly gaining the ability, making it hail again. This, combined with his natural slowness, makes him ideal for messing up opposing weather teams. He's slower than most weather inducers so in most cases your weather will be the prevailing one if both are sent out at the start, and if they try to set up again by either switching out or using the relevant move, just save your Mega Evolution to get a free reset.
Example from the Battle Maison - 5DKW-WWWW-WWW4-R4EC