Icons
The icon for the pieces bouncing around is for hail, and I don't think that's intended to be used for forecasts, since hail is usually a highly acute condition of thunderstorms. Rather, it's probably used as the larger "current conditions" icon when a nearby thunderstorm is expected to produce hail. In iOS 6, I have seen this icon, though, in forecasts that included thunderstorms showing the possibility of producing hail.
The icon with just the snowflakes is flurries or otherwise light snow that is not expected to impede travel much. The other snow icon with the white on the bottom is actually supposed to represent snow accumulated on the ground rather than a cloud, and is used to indicate a blizzard or other heavy snowfall conditions that are widely expected to significantly impede travel for significant amounts of time.