Coldest Days of Winter
By Climate Central
As the national map above shows, the coldest time of year, based on the 1981-2010 climate “normals,” can come as early as mid-December in parts of the West and Southwest, in mid-January across much of the Southeast, and anywhere from late January to mid-February — and even late February in some spots — in much of the Upper Midwest and Northeast.
Why the difference? One major reason, according to NOAA, is snow cover: places that tend to have the most, including the Northeast and Upper Midwest, but also some high-altitude regions in the West, reflect the most sunlight back into space — sunlight that would otherwise tend to warm the ground, at least somewhat. Winter snow cover is actually increasing in North America, but the trend isn't significant. In comparison, spring snow cover is declining significantly. What both those trends mean for when the coldest day of the year hits in a given location isn't entirely clear, though.
These are the averages, but conditions in any given year can stray from the average in either direction. So while Wisconsin’s coldest days tend to come a couple of weeks from now, this week could end up being the coldest period this time around (or maybe the worst is yet to come).