While most cold fronts track in a general west-to-east direction across the United States, sometimes they can move in the opposite direction and follow an east-to-west or northeast-to-southwest path.
This is called a "backdoor cold front."
Common in the Northeast during the spring months of March, April and May, these backdoor cold fronts will cause the winds to shift out of an easterly or northeasterly direction after they move westward or southwestward through an area. That pulls in much cooler air off the Atlantic Ocean, and temperatures can drop significantly in a very short amount of time.
A perfect example was seen in eastern New England on the morning of May 31 when the temperature in Boston dropped 19 degrees in only 10 minutes, falling from 82 to 63, as a backdoor cold front sliced from northeast to southwest across the region.
The layer of cooler, marine air ushered in behind a backdoor cold front is typically shallow, extending only a few thousand feet above the Earth's surface, so the front usually struggles to advance over mountain ranges such as the Appalachians.