Lanzante J., and R. Harnack, 1982: The January thaw at New Brunswick,
N. J. Mon. Wea. Rev., 110, 792-799.

Abstract:

An investigation of the January than phenomenon, a period of unseasonable
warmth, was conducted using daily maximum temperatures recorded at
New Brunswick, New Jersey, from 1858-1981. Student’s t-tests,
comparing long-term means of daily maximum temperature to values
from a fitted seasonal trend curve, indicate temperatures higher on
22-23 January and lower on the 29th than seasonally expected.

It was found that the January thaw does not have a fixed time of occurrence
but occurs most frequently from the 19th to the 28th. During this time
the interannual variability of daily maximum temperature is significantly
higher than during the remainder of the month.

Evidence of a tendency for a secondary thaw maximum to occur, centered on the
26th, is evident in several different analyses. Examination of daily
temperature curves for 10-, 20- and 40-year periods reveals a shift in the
mean thaw date from 22-23 January to the 26th. This change has evolved over
the last 30-40 years. It was concluded that the January thaw is more
pronounced when the mean circulation is characterized by a contracted
polar vortex over North America and abnormally strong midlatitude westerlies.