A clean bore will effect velocity for a few shots until it gets dirty again. You may see this as vertical stringing on the target. Knocking some crud off the crown, or more likely wearing the crown with the cleaning rod will also move your point of impact.
Most likely you tweaked your zero by moving something as you were cleaning.
Snow doesn't matter, but temperature affects the powder burn rate and air density, and thus changes in velocity cause a vertical shift again. But you won't see much at 100 yards.