This map shows the historic date by which there’s a 50% chance of at least 0.1” of snow on the ground, based on each location’s snowfall history from 1981-2010, based on the latest U.S. Climate Normals from NOAA’s National Centers for Environmental Information.
A quick look at the map reveals the obvious: the main factors that govern the average day of your first snow are the same main factors that govern your other major climate characteristics: latitude and altitude. In general, the farther north you are, and the farther up you are, the earlier the threat of first snow. In fact, the highest elevation stations along the spines of the Rockies have a year-round threat of snow. On the other end of the spectrum, the Deep South, Gulf Coast, Desert Southwest and Hawaii have many stations that get snow so infrequently, there’s no date listed (empty circles). In these places, there just aren’t enough events to make robust statistics. – NOAA