Snow Warning will summon a hailstorm in battle. Its effects are the same as Hail but, instead of lasting for five turns, it lasts for the whole battle until replaced. -Bulbepedia
Snow Warning summons a hail storm.
It's like the move Hail, but instead of 5 turns, it's until another weather move/ability replaces it.
Abomasnow and Snover are the only Pokemon with Snow Warning
Hope This Helped :D