Recent January DCA Temperatures Appear Inconsistent with 1981-2010 Normals

[RodneyS]

On 1/21/2022 at 9:06 PM, RodneyS said:

Recent DCA temperatures offer more support for the "January 13th thaw-January 21st freeze" hypothesis, with the former day in 2022 averaging 41.5 degrees and the latter day averaging 23.0 degrees.  The new NCDC 1991-2020 DCA normals now reflect a bottoming out of yearly temperatures in the January 18th-22nd time frame (at 37.0 degrees), with January 13th 0.2 degrees higher.  The previous 1981-2020 normals showed a bottoming out right around January 13th (at 35.7 degrees), with January 21st 0.2 degrees higher.  While this update is a step in the right direction by NCDC, it does not come close to reflecting the reality that during 1991-2020 at DCA January 13th temperatures averaged 9.0 degrees higher than January 21st (41.6 vs. 32.6).  

 

What convinces me that this differential is not merely a weird aberration, but rather an unexplained phenomenon, is that I first noticed it nine years ago, and it has continued.  Specifically, during the last nine years (2014-2022), January 13th temperatures at DCA have averaged 6.7 degrees higher than January 21st (40.4 vs. 33.7).   While this differential is not quite as high as during 1991-2020, my first calculation of the differential was for the period 1984-2013, when it averaged 8.2 degrees (40.3 vs. 32.1). So, the differential actually increased when that 30 year period was moved forward by seven years to 1991-2020, and while it has been somewhat lower during the most recent nine years, it is still for those nine years way beyond what would be expected if 1984-2013 had been an aberration.

 

What is particularly interesting about all of this is that for the first 112 years of official daily temperature recordkeeping in DC (1872-1983), there tended to be a January 13th freeze and a January 21st thaw.  Specifically, during that time frame, January 13th averaged 3.5 degrees cooler than January 21st.  So, while for the most recent 39 years in DC, January 13th has averaged 7.0 degrees warmer than for the first 112 years (40.3 vs 33.3), January 21st has averaged 4.3 degrees cooler (32.5 vs 36.8).  Why the big shift? Inquiring minds want to know.

The beat goes on.  This year January 13th averaged 47.5 at DCA, while January 21st averaged 39.0.  So, the comparison of the average temperature for the two days between the first 112 years of official daily temperature recordkeeping in DC and the last 40 years is as follows:

Period          Jan 13           Jan 21     Jan 21 minus Jan 13

1872-1983    33.34           36.77      +3.43

1984-2023   40.49           32.65      -7.84

[RodneyS]

When I initiated this thread 11 years ago today, I wrote, regarding officially-reported DC temperatures for two days in January:  "January 13th has gone from being the coldest January day at 33.3 degrees on average during 1872-1983 to the warmest January day at 40.3 degrees on average during 1984-2013 -- an increase of 7.0 degrees. January 21st has been the mirror image of January 13th -- going from the second warmest January day at 36.8 degrees on average during 1872-1983 to the coldest day of the year at 32.1 degrees on average during 1984-2013 -- a decrease of 4.7 degrees."

 

So what has happened during the last 11 years?  Essentially, a repeat of 1984-2013, but with slightly warmer temperatures for the two days.  January 13th this year at DCA averaged 44.5 degrees, to bring the 2014-2024 average for that day to 41.5 -- 1.2 degrees warmer than 1984-2013. January 21st this year at DCA averaged 27.5 degrees, to bring the 2014-2024 average for that day to 33.6 -- 1.5 degrees warmer than 1984-2013.

 

Thus, instead of January 13th and January 21st temperatures at DCA each regressing to the mean January 13-21 temperature over the past 11 years, which would be likely if the prior 30 years had been a statistical aberration, January 13th has continued to be abnormally warm, while January 21st has continued to be abnormally cold. That suggests that this "thaw-freeze" situation in DC is caused by a repeating weather phenomenon, likely relating to warming temperatures worldwide.  But what is the specific mechanism that is responsible for this phenomenon?

[Rhino16]

@RodneyS

So... I downloaded some ERA5 Reanalysis data, 80 years worth from 1945-2024, found the average temperatures and then found the change in average temperature between the 2 40 year periods. I then plotted it and these are the results for both days. There's definitely something happening around our part of the country and north on the 13th... what exactly, I don't know.

 https://imgur.com/a/yuUYC5l

[RodneyS]

On 2/28/2024 at 3:14 PM, Rhino16 said:

@RodneyS

So... I downloaded some ERA5 Reanalysis data, 80 years worth from 1945-2024, found the average temperatures and then found the change in average temperature between the 2 40 year periods. I then plotted it and these are the results for both days. There's definitely something happening around our part of the country and north on the 13th... what exactly, I don't know.

 https://imgur.com/a/yuUYC5l

Excellent.  I'm delighted that you are extending my analysis to include additional areas.