Because many have a fear of driving in such conditions, they worry about being stuck at home with no groceries until the snow and ice have melted. As a result, grocery stores are flooded with shoppers when snow is in the forecast. The most commonly sought after items are milk and bread.
What is perplexing about this phenomenon is that the snow and ice rarely remain on the roads for more than a day or two. So as a general rule, it's safe to assume that you will be able to get back out for groceries in just a couple of days and a pre-snow grocery run probably isn't necessary.
Of course, some people are after snow day fare like hot chocolate and marshmallows, rather than milk and bread. In any case, be prepared to battle the crowds at the grocery store any time snow is in the forecast.