This analysis provides evidence for the existence of a regionally coherent January Thaw across the northern United States. These events are the most prominent in the East, where seasonal temperatures are naturally more variable. Further, a regionally cohesive January Freeze, often experienced just prior to the Thaw, occurs in the western United States. The trend of both warm and cool singularities is generally consistent inter-regionally, moving eastward across the country and appearing to follow typical mid-latitude circulation.
Thaw and Freeze events are related to variable synoptic conditions across the United States rather than the advection of a single air mass type. Perhaps as expected, in some areas of the country Thaws are associated with more frequent warm air mass types, like MT in the Midwest. In other regions like the Great Plains, Thaws occur while the character of cold air masses change, exhibiting fewer of the very coldest days. These situations are sometimes accompanied with overall less frequent cold air mass types. Freeze singularities are often associated with more frequent DP- air masses, though in general are related to more frequent cold air masses and less frequent warm air masses.