Why is it so warm? What is the “January Thaw”?

After a bitter freeze (like the one experienced during the first week of 2017), it may seem weird to then experience balmy conditions in mid-to-late January.

But this pattern does happen quite frequently, and it has a name -- the "January Thaw."

High pressure sets up over Bermuda, pulling warmer air over the eastern third of the U.S. during the month of January.

High pressure sets up over Bermuda, pulling warmer air over the eastern third of the U.S. during the month of January.

Most notably, the South and Northeast experience a period of well-above average temperatures while the Great Plains, Midwest, and Great Lakes regions become entrenched in bitterly cold, wintry weather.

This is due to a high pressure system that sets up near Bermuda -- known as the "Bermuda High" -- that drives warm, humid air into the eastern third of the country. The Bermuda High blocks the polar jet stream from diving south, effectively diverting it towards northeastern Canada. As a result, the South and East Coast enjoy temperatures that can climb 15 to 20 degrees above average.

In meteorology, the "January Thaw" is considered a statistical singularity, which the American Meteorological Society defines as "a meteorological condition that tends to occur on or near a specific date more frequently than chance would indicate."

Surface map for January 25, 1950. The high temperature reached 77 degrees in Huntsville, which is the warmest day in the warmest January on record for the city. (Source: NOAA)

Surface map for January 25, 1950. The high temperature reached 77 degrees in Huntsville, which is the warmest day in the warmest January on record for the city. (Source: NOAA)

 

For example, WGN chief meteorologist Tom Skilling says that Chicago experiences its thaw around January 20-22, "when the long-term average temperature breaks the freezing mark."

Closer to home, analysis of the top 10 warmest Januaries on record corroborate the theory of the "January Thaw":

1.) 1950: Max temperature of 77 degrees on Jan. 25.

2.) 1937: Max temperature of 77 degrees on Jan. 22.

3.) 1933: Max temperature of 78 degrees on Jan. 22.

4.) 1952: Max temperature of 79 degrees on Jan. 1; it should be noted that this particular year, temperatures ranged from the mid-40s to upper 60s during the days spanning from Jan. 20 - Jan. 28.

5.) 1974: Max temperature of 70 degrees on Jan. 22.

6.) 1949: Max temperature of 77 degrees on Jan. 10.

7.) 1916: Max temperature of 75 degrees on Jan. 11.

8.) 1923: Max temperature of 69 degrees on Jan. 31.

9.) 2006: Max temperature of 74 degrees on Jan. 2; it should be noted that for this particular year, temperatures spanned from the 50s to 60s during the days spanning from Jan. 20 - Jan. 28.

10.) 1953: Max temperature of 69 degrees on Jan. 15.