Is an anorak warm enough for cold winter days?

Yes, often warmer than you might expect. The sealed front panel eliminates the draft pathway that exists even with a fully zipped full-zip jacket, so a shell anorak can feel noticeably warmer than a shell full-zip. For very cold days, add a mid-layer underneath. Just be cautious about over-insulating — since you cannot unzip the front to cool down quickly, it is better to start slightly cool and add a layer than to overheat in an anorak.