I think you (dionythicus) are right...Avalanche I believe is amel, anery and diffused. And Whiteout is amel charcoal and diffused.
To the original question, snows are not het anery, they are homozygous recessive anery. For most cornsnake traits, a snake that is het for something will not be visibly expressing that trait. So a snake that was het for anery and amel would look like a normal cornsnake but carrying the genes that make it possible to produce an anery and amel.
Each trait has 2 copies of a gene, either recessive or dominant. One copy from the mother, one from the father. Snake needs both copies to be recessive in order to be visibly expressed. Therefore if you want anything, both parents must have at least 1 recessive copy of the trait. The snow has 2 recessive copies of amel, and 2 recessive of anery. Therefore, to produce snows you will need to breed it to a male that has either 1 or 2 recessive copies for both amel and anery (therefore, either it is anery and amel, or het anery and amel, or whatever). It can have as many other traits as you want it to have, as long as it has recessive copy of anery and amel. So Breeding it to an avalanche would work, as you would get offspring that are snow het for diffused. A snow stripe would work, a hypo snow would work, a caramel snow would work, a lavender het amel and anery would work (though you would get some normal offspring as well). As long as it has at least 1 recessive copy of both amel and anery you should get some snows.
Because of the half anery offspring you got from the snow x motley diffused, the motley diffused is also het anery.