Colder winters along the North American Atlantic northeast
As air moves over the surface of the earth, the properties of the surface (such as temperature) can alter the air. Since at those particular latitudes, the predominant wind direction is from the west and partially the north, it is important to note what is north and west of both locations.

West of the northeast Atlantic is land, and northwest is Canada. This means that the air is drier, and prone to have characteristics of mimicking continental- polar (cP) air. Continental polar air tends to be dry (since it is over land) and cold (since it is coming from an area that receives less sunlight).

West of the northwest pacific is the Pacific Ocean, and northwest is still the Pacific Ocean. This means that the air is more moist and prone to have characteristics mimicking the air over the ocean, called Maritime polar (mP). Since the water tends to warm slower than the land and also slower than the land, the air over the water is usually warmer during the winter and cooler over the summer than the air over the land. 

To summarize:


At those latitudes, the wind usually blows from the north and west.
During the winter, air over the ocean is warmer than air over land.
North and West of the Northwest coast is the Pacific ocean, while land is west of the Atlantic coast.
Therefore, the winter in the northwest coast is usually warmer than the winter along the northeast coast.