Snow won't affect the bullet's flight in any measurable way.
It will, however, have an affect on the shooter and the shooter's view of the target, and the temperature may change the behavior of the firearm and ammunition.
A few years ago, we had a 2-page discussion about the affects of heavy rain on a bullet's flight
. In the end, it was agreed that the odds of a bullet hitting a drop of water, even in rain as heavy as 2" per hour, were low enough that it didn't matter. In the case of the bullet actually hitting a rain drop, the influence on the bullet would be insignificant -- to the point that it is essentially imperceptible at the target. Snow, of course, would have even less of an influence.