Good comment...so say there's enough liquids in an average persons's home to sustain them for ____ number of days + some will loot from stores so they could last _____number of days. Others would attempt to collect from local streams and last for ___number of days but eventually die from dyssentary. Minimal well digging will occur as it's community activity and difficult to do in Winter. It will happen. Little rainfall in Winter, but some snowmelt in limited areas at higher elevations and probably lower population density most places, but places like Denver might get a lot. Then take an average to see survivability over a month's duration for dehydration.

Some government officials would use law enforcement and probably firemen to seize supplies (commandeer them) for administrative and support staff, so looting of many caches of supplies might not be possible. I would expect this as a high probability outcome with the merchants giving them a bill of sale for it, and a promissory note. Very plausible, for you have to provide water for deputies, policemen, firemen, otherwise complete breakdown of society.

A lot of bushcraft folks know other ways of extracting drinkable water from maples(first) and birches(later) but this is contingent upon weather such that sap rises, otherwise it won't work, and this is very much later. You need frosty nights and warm days. So this again won't work for the beginning of Winter at all.