It's December, and it's also considered the start of the meteorological winter, even though the season doesn't officially start on the calendar for another three weeks. 

Meteorological winter is a three month period that runs from Dec 1st to the end of February. It is the coldest three month period of the year in the northern hemisphere. Astronomical winter is based on when the sun reaches the most southern point on the globe, the Tropic of Capricorn. If you are located right on the Tropic of Capricorn at 12:00 noon on the first day of astronomical winter, the sun will be directly overhead. Also, on the first day of astronomical winter, the sun is at its lowest point in our sky at 12:00 noon and, of course, it is the shortest day of the year. Obviously, it is the first day of the summer season in the southern hemisphere.

If you look at the December data for Milwaukee, you see how temperatures really start to fall.  In the beginning of the month, the normal high and low are 38 and 25 respectively.  By the end of the month (and year), the "normals" are 31 and 17.  We can also expect more than ten inches of snow to fall.

And in case you're wondering, at least according to the calendar, winter officially begins on December 21 at 10:48 pm CST.