hopefully you are not planning on putting chains on summer tires...
to be honest, 21" 30 series tires for winter is probably the worst thing you can do, i dont recommend it at all.
if you are going to some where heavily snowed in i recommend steel 19" wheels with studded tires and chains.
if you hit anything under the snow with 21" wheels, i guarantee that you will bend the rim and blow the tire.