Just goes to show how precise the demand/supply ratio is. In the event of a natural disaster grocery stores will be empty within 2-3 days. Gas stations within 1-2 days.
I lived in Banff when the highway washed out a few years ago, and the two grocery stores in town were barren of produce immediately. Word around town was that the restaurants cleaned them out (and being a tourist town, there are a lot of restaurants). It was nothing but canned and frozen food for a couple days before Safeway was finally allowed to get a truck through.