Another factor is the North Atlantic Oscillation (NAO). The NAO is based on pressure differences between the Icelandic Low (a permanent low pressure system stationed over Iceland) and the Azores High (a permanent high pressure stationed over the Azores). When the pressure differences are large, the NAO is considered positive. When the pressure differences are lower, the NAO is considered negative. Below is an image describing it.
The NAO has major variability and is something special to watch in the winter. Below is an image displaying the effects the NAO has on the US in the winter months.
When the NAO is positive, the Eastern US is warmer. However, when the NAO is negative, the East US experiences colder and snowier conditions as the jet stream dips southward, releasing colder air into the region. Below is a general overview of the different NAO conditions and subsequent effects thanks to Ian Bell's website:
Positive NAO Index
Negative NAO Index
More and stronger winter storms crossing the Atlantic Ocean on a more northerly track
Fewer and weaker winter storms crossing on a more west-east pathway
Warm and wet winters in Europe
Moist air into the Mediterranean and cold air to northern Europe
Cold and dry winters in northern Canada and Greenland
Milder winter temperatures in Greenland
US east coast experiences mild and wet winter conditions
US east coast experiences more cold air outbreaks and hence snowy weather conditions
Did it seem cold last year? If you thought it did, it was. Last year had a negative NAO for much of the winter months. This year, we are getting some indication that the NAO may also be negative this winter, but the specifics remain to be seen.