Snow Warning summons a hailstorm in battle as soon as a Pokémon with Snow Warning enters the battle. Its effects are the same as Hail, but instead of lasting for five turns, it lasts for the whole battle until replaced. If a Pokémon with Snow Warning is sent out in the same turn as another Pokémon whose Ability affects the weather, the slower Pokémon's Ability will override the faster Ability.
In Generation VI, the hail will last only for five turns instead. This can be extended to eight turns if the Pokémon is holding an Icy Rock.
In Super Mystery Dungeon, Snow Warning changes the weather to hail upon entering a floor. Hail lasts a certain number of turns (in multiples of 66) before clearing. After the weather has cleared, Snow Warning will again activate after waiting a certain number of turns (in multiples of 66).