I'll let Keith C. Heidorn, PhD, (THE WEATHER DOCTOR) provide the technical details.
A January Thaw is expected to fall within the third week of January, usually following a strong cold snap, but need not occur every year. In fact, during the very cold winters of the late 1970s, there were no January Thaws in the New England region during 1978 and 1979. (And they could have used a thaw!) There have also been recent mild winters when the Thaw was absent or hardly noticeable...Technically,the January Thaw is what is known as a weather singularity, which is a weather phenomenon likely to occur with reasonable regularity around a specific approximate calendar date. That date is typically January 23-25...and those of us living in these parts know that we have not observed any type of thaw over the past several days nor do we have any type of thaw in store over the next few days.
In prime January Thaw country, the ideal weather pattern characteristic of the Thaw period unfolds in this manner. It begins after a cold air mass from northern or
western regions has slid over the region (A), eventually moving out over the Atlantic Ocean. As that air mass leaves, the Bermuda High strengthens (B,C,D) and becomes positioned over the southern Atlantic Coast or southeastern US
states while a broad low pressure trough moves slowly across northern Ontario and Quebec (B-E). The juxtaposition of the isobar patterns of these two map features (E) places the northeastern US and southeastern Canada border region into a south-southwesterly flow of warm air from the Gulf of Mexico. This air advects over the northern snow and ice fields and begins a thawing.
Often during this time, the upper air wind patterns are in a period of readjustment, and thus surface weather systems stall or creep slowly across the eastern continent's mid-latitude belt. After several days of warmth, the regional weather again comes under the influence of a strengthened polar high (F,G), and cold weather returns.
If I had known there would be no January Thaw this year, I would have made a greater effort to get on the bike trainer. Yeah, right!!