Snow Storm is a Patience deck specific effect card. Since Beta 28.0, it kills all monsters on the board that are paralyzed, including your own. This makes it work as the paralysis equivelant to its KR counterpart: Perseverance's Contamination (aside from the new spells added).
This is very useful if you want to get rid of powerful cards of the opponent, and useless cards of your own. This works especially well with Cold Winter, due to its ability to paralyze up to 3 enemy monsters at once.