WeatherWhys for January 9,2015 from accuweather.com

The Dead of Winter is the one-month period when normal temperatures reach their lowest levels. For most of North America, this occurs from the second week of January until the second week of February. The reason this period does not line up with the first day of winter when the amount of incoming energy from the sun is lowest is due to the seasonal lag. The land and oceans continue to lose more heat than is gained this time of year and will do so until mid-February.