Temperatures in the upper midwest have been anywhere from 8 to 10 degrees below normal. That may not sound so dramatic but considering normal high temperatures in this area are in the mid 20's, average highs this month have been in the teens. A closer look at home shows that Philadelphia, points north and west have been anywhere from four to six degrees below normal with a little moderation towards the coast. Temperatures are not expected to reach above freezing again until Saturday but with it comes the chance of some snow. It's not until late January through February when the major cities along the coast see their snowfall.
Winds blow counter clockwise around a low-pressure system.
So as low pressure moves up the coast it draws in the warmer ocean air. Because of the close proximity of the cities to the ocean, this warmer air quickly changes snow over to rain. However, by late January and February the ocean temperature has cooled down significantly. Now as low pressure travels up the coastline those east winds draw in air that is too cold to change snow over to rain, thus giving the likelihood of an all snow event. The current ocean water temperature is 30 degrees, and that's plenty cold enough.