Over the past weekend, southern Wisconsin experienced its first cold snap of the season with widespread morning lows in the upper 20s on Friday and Saturday mornings.
Often, cold snaps in autumn are short-lived, as this recent example was, affecting usually one or two nights at most.
There are a variety of reasons for this brevity. First, cold snaps at this time of year require a substantial southerly excursion of cold air from high latitude Canada to get us cold. Though the Arctic night creeps ever farther south each day after the autumnal equinox, its southerly progress is slow.
Consequently, cold air production is limited to the very highest latitudes well into November, which means that any cold air that makes it as far south as Madison is not well connected to a broader reservoir of cold air that would allow the cold to remain.
Second, there is little snow on the ground even in central Canada at this time of year and so the cold air that does migrate southward is modified (warmed) to a greater degree on that migration at this time of year than in the late fall and winter when snowcover is widespread.
Third, the lack of snow on the ground in Madison itself limits the longevity of cold snaps. This is even true in the wintertime. Overnight cooling over a snowfield can reinvigorate the chill of a cold snap and render it longer lasting as a result. So, if you were complaining about the cold this weekend, be glad we are still in October and much more likely to experience a quick rebound.
