I have spent several nights in a bivvy with a sleeping bag during snow storms and been rather comfortable. It is important to have a top of the line bivvy bag, preferably with waterproof, breathable fabric on top and something waterproof on the bottom, and of course a decently warm bag. Usually the pack and spare clothing provides insulation on the bottom. One of my favorites was large enough to hold two people; the shared heat renders the setup quite effective and efficient.