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.   

[RodneyS]

On 1/22/2024 at 10:50 AM, RodneyS said:

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?

This year on January 13th DCA recorded a minimum of 28 and a maximum of 51 degrees, and on January 21st DCA recorded a minimum of 16 and a maximum of 24 degrees.  Those temperatures continue the strong trend toward a lower average temperature on January 21st than on January 13th, and bring both the January 13th and 21st DCA average temperatures for the 12 years that I have now been tracking them (2014-2025) very close to the prior DCA 30-year averages (1984-2013) for those two calendar days.  That, in turn, means that the differences at DCA in the January 13th and January 21st average temperatures for 2014-2025 and 1984-2013 are about the same during each period. Specifically, the average temperature at DCA during 2014-2025 for January 21st was 8.8 degrees lower than the average temperature for January 13th, whereas during 1984-2013 that lower difference was 8.2 degrees.  That provides even stronger evidence that what I found in January 2013 was not merely an aberration from the first 112 years of daily temperature recordkeeping in DC (1872-1983) -- in which the average temperature was 3.5 degrees higher on January 21st than on January 13th -- but rather a fundamentally new, if currently unexplained, reality:

Period         Jan 13     Jan 21     Jan 21 minus Jan 13

1872-1983  33.3        36.8        +3.5

1984-2013  40.3        32.1         -8.2

2014-2025   41.3         32.5         -8.8

Note that@Rhino16and I have done some additional research that indicates that a similar January 13th to 21st temperature shift has occured in other Eastern locations in recent years -- see my post of February 20, 2020 and his post of February 28, 2024 above.  

Does anyone here have a hypothesis as to what is causing this shift or know of someone who is researching this issue?

[pazzo83]

really good stuff - I wonder if this is related to what appears to be a more delayed onset to winter-like temps that we've seen over the past decade or so (although not every year).  In other words, the core of winter is a couple weeks later than it used to be.

[MN Transplant]

This continues to be a wild thread.  Keep up the interesting data!

[TSG]

Fascinating for sure and fun to follow along all these years. I was still in college when this first got posted and it really caught my attention as it was maybe the strangest weather related phenomenon I had ever come across/read about at that age. I hadn't even made an account on here yet, I was just a perennial lurker in the winter months. 

Keep it up Rodney!

[NVAwx]

Is there a website I can easily figure out how many hours we have been above freezing and below freezing since January 1?

 

For instance:

How many hours above freezing vs. below freezing at DCA and IAD

Number of hours below freezing in a row

 

Etc. 

[Rhino16]

2 hours ago, NVAwx said:

 

Is there a website I can easily figure out how many hours we have been above freezing and below freezing since January 1?

 

For instance:

How many hours above freezing vs. below freezing at DCA and IAD

Number of hours below freezing in a row

 

Etc. 

i’m not sure, but I’m sure it can be done with python and some form of either obs or reanalysis data.

If you can’t find anything, reach out in a PM and maybe I can fetch that for you if I can figure that code out and assuming I have the time…

[MN Transplant]

12 hours ago, NVAwx said:

 

Is there a website I can easily figure out how many hours we have been above freezing and below freezing since January 1?

 

For instance:

How many hours above freezing vs. below freezing at DCA and IAD

Number of hours below freezing in a row

 

Etc. 

You could do it brute force via:  https://www.wunderground.com/history/weekly/us/va/arlington/KDCA

 

[LongRanger]

If this temp pattern is anything but random, I would speculate it is related to the winter solstice. Minimum northern sunshine happens mid-late December. Perhaps the small increase in heating that occur in January is enough to instigate artic cold draining to the lower latutudes at a certain time, with the result DC sees unusually cold temps Jan 20 or so.

[RodneyS]

20 hours ago, LongRanger said:

If this temp pattern is anything but random, I would speculate it is related to the winter solstice. Minimum northern sunshine happens mid-late December. Perhaps the small increase in heating that occur in January is enough to instigate artic cold draining to the lower latutudes at a certain time, with the result DC sees unusually cold temps Jan 20 or so.

The mystery is why this January 13th/21st thaw-freeze scenario began happening about 41 years ago in DC.  Between 1872-1983, it was just the opposite, with the temperature rising on average by about 3.5 degrees between these two dates. Suddenly, around 1984, the temperature began falling on average by more than 8 degrees between these two dates, and it's still happening.

Because the recent period has coincided with warmer temperatures worldwide, I would guess this phenomenon is somehow related to those warmer temperatures.  However, why is it that January 21st in DC has defied the warming trend, by now being more than four degrees colder than it used to be? What is the forcing mechanism that brings about this counter-intuitive result?

[IronTy]

Could it have something to do with Arctic sea ice extent?  I have no idea if this is the case but I would assume with the warming base state the sea ice is a bit slower to reach Max extent coming out of warmer falls?

[RodneyS]

12 minutes ago, IronTy said:

Could it have something to do with Arctic sea ice extent?  I have no idea if this is the case but I would assume with the warming base state the sea ice is a bit slower to reach Max extent coming out of warmer falls?

Hmm, that may be something to look at -- I think the more creative ideas we have here, the better off we are. 

[IronTy]

27 minutes ago, RodneyS said:

Hmm, that may be something to look at -- I think the more creative ideas we have here, the better off we are. 

Assuming this phenomenon is real and not some sort of statistical aberration, the engineer in me says that the first place to look should be large, low speed system changes over time.  Solar output, ocean currents, ice extent, plate tectonics (in the form of subsea volcanism), orbital changes,  CO2 levels, pressure pattern changes, etc.    

[pazzo83]

1 hour ago, IronTy said:

Assuming this phenomenon is real and not some sort of statistical aberration, the engineer in me says that the first place to look should be large, low speed system changes over time.  Solar output, ocean currents, ice extent, plate tectonics (in the form of subsea volcanism), orbital changes,  CO2 levels, pressure pattern changes, etc.    

my best guess is just the delayed onset of winter conditions that we see throughout the northern hemisphere

[IronTy]

29 minutes ago, pazzo83 said:

my best guess is just the delayed onset of winter conditions that we see throughout the northern hemisphere

Agreed. Though the logical, and more important next question is which physical phenomena underlying that is the main driver?  That's why I went first to sea ice extent.  If global warming is causing a delay in sea ice advance by several weeks on average then is that albedo change delaying the most intense arctic outbreaks in the region by the same amount on average?  But I don't even know if Arctic sea ice shows a similar pattern as I haven't looked into it.  It was just the first thing that came to my mind.  

[TSG]

@RodneyS

Was curious if this view would reveal anything within the DC data. Only thing I've picked up so far is that this phenomenon seems to be visible on the 12th and 22nd as well, and that Jan 4th has been all over the place the last 40 years lol. Have you thought about including those two dates in your analysis?

 

 

[Rhino16]

It’s kind of weird coming back to this after a bit more meteorology knowledge has been gained. The 13th looks like it has warmed where you would expect to see Cold Air Damming occur. Problem is it kinda extends too far west for me to think it is involved. Maybe it would be useful to investigate CAD setups in this time frame and see if any differences appear.
 

See the ERA5 Reanalysis images from awhile back below:

[RodneyS]

49 minutes ago, TSG said:

@RodneyS

Was curious if this view would reveal anything. Only thing I've picked up so far is that this phenomenon seems to be visible on the 12th and 22nd as well, and that Jan 4th has been all over the place the last 40 years lol. Have you thought about including those two dates in your analysis?

 

 

Thanks, Tom. I have looked at other January dates, but I thought that the 13th and 21st were the best to illustrate this phenomenon.  I remember in the 1980s reading about the "January thaw" that tended to occur in the East around January 21st, which is now the coldest day of the year in DC!  However, my analysis is quite simplistic, and so expanding upon it may shed additional light as to what is going on. 

 It's great that some of you younger guys are now taking this seriously.  The way science usually progresses is for the older generation, which is in a position of authority, to dismiss any new idea that challenges the conventional wisdom as being unworthy of investigation.  But if something is really there, eventually the younger generation will take over that position of authority and make the new idea part of mainstream science. 

[TSG]

This one is kind of a recreation of Roger's from a couple years back but with the more recent data corrected for warming to make comparison easier. I took a flat -2.2F off the 84-24 values. Below is the same data but presented as the delta between the two data sets.

[TSG]

2 hours ago, Rhino16 said:

It’s kind of weird coming back to this after a bit more meteorology knowledge has been gained. The 13th looks like it has warmed where you would expect to see Cold Air Damming occur. Problem is it kinda extends too far west for me to think it is involved. Maybe it would be useful to investigate CAD setups in this time frame and see if any differences appear.
 

See the ERA5 Reanalysis images from awhile back below:

Could you do a 12/13 composite and a 21/22 composite? That may show an even stronger signal based on what I'm seeing. 

[Rhino16]

11 minutes ago, TSG said:

Could you do a 12/13 composite and a 21/22 composite? That may show an even stronger signal based on what I'm seeing. 

I think I still have the 14gb data file, but I might want to add this years data too and request new data.

[TSG]

Gave this some more thought in the context of how our winters have been changing overall and I think I might have something. I'm focusing on the idea that our seasons seem to have shifted "to the right". i.e. Winter conditions more common in Mar/Apr, Summer conditions in Sept/Oct, etc. 

I don't have any kind of explanation for why, but it looks like whatever consistent atmospheric features cause these anomalies may be (a) starting later in time, (b) extending longer in duration, and (c) amplifying. The late January thaw that used to be a feature of many pre-1980s winters may be pushed out into the first week of February on average at this point. I haven't grabbed February data yet to see if this theory would actually hold water, but my memory has more than a few "early feb thaws/heatwaves" vaguely floating around. There were a couple years of cherry blossom disappointment when I lived in DC 2015-2020 from temps crashing late-Feb/early-March after said warmup and killing off most of the blooms.

 

 

I could also be seeing things that aren't there 

[RodneyS]

2 hours ago, TSG said:

Gave this some more thought in the context of how our winters have been changing overall and I think I might have something. I'm focusing on the idea that our seasons seem to have shifted "to the right". i.e. Winter conditions more common in Mar/Apr, Summer conditions in Sept/Oct, etc. 

I don't have any kind of explanation for why, but it looks like whatever consistent atmospheric features cause these anomalies may be (a) starting later in time, (b) extending longer in duration, and (c) amplifying. The late January thaw that used to be a feature of many pre-1980s winters may be pushed out into the first week of February on average at this point. I haven't grabbed February data yet to see if this theory would actually hold water, but my memory has more than a few "early feb thaws/heatwaves" vaguely floating around. There were a couple years of cherry blossom disappointment when I lived in DC 2015-2020 from temps crashing late-Feb/early-March after said warmup and killing off most of the blooms.

 

 

I could also be seeing things that aren't there 

Thanks for this analysis, Tom. Regarding February in DC in recent years, your memory is spot-on.  If you compare DC temperature averages during 1872-1983 and 1984-2024, the average monthly increase from the earlier period to the latter period is 2.7 degrees.  But as I think you suspected, February has experienced the largest increase of any month, at 3.6 degrees.  December is second largest at 3.5 degrees.  So, in keeping with the idea that the winter is being pushed back a couple of months, January must be third largest, right?  Wrong!  January has the second smallest increase, at only 2.2 degrees.  Why?  Because while a number of January days have had large daily increases, including the largest of the year on January 13th (7.4 degrees), a number of other January days have experienced temperature decreases, including the largest of the year on January 21st and 22nd (-4.6 degrees on each day). So anyone attempting to calculate recent daily January Temperature Normals in DC is in for a head-spinning experience.  

Regarding "Winter conditions more common in Mar/Apr, Summer conditions in Sept/Oct" -- the numbers do not support that in DC.  Between 1872-1983 and 1984-2024, March has increased 3.1 degrees (3rd largest), and April has increased 3.0 degrees (tied for 4th largest), while September has increased only 2.4 degrees (4th smallest), and October has increased only 2.5 degrees (5th smallest). 

So, the picture is quite muddled, but January is in a class all its own.  

FYI, here are each of the average monthly temperature increases in DC from 1872-1983 to 1984-2024 in ascending order:

May 1.7, January 2.2, June and September 2.4, October 2.5, November 2.7, July 2.9, April and August 3.0, March 3.1, December 3.5, February 3.6. 

 

[wxdude64]

On 1/28/2025 at 5:29 PM, TSG said:

Gave this some more thought in the context of how our winters have been changing overall and I think I might have something. I'm focusing on the idea that our seasons seem to have shifted "to the right". i.e. Winter conditions more common in Mar/Apr, Summer conditions in Sept/Oct, etc. 

I don't have any kind of explanation for why, but it looks like whatever consistent atmospheric features cause these anomalies may be (a) starting later in time, (b) extending longer in duration, and (c) amplifying. The late January thaw that used to be a feature of many pre-1980s winters may be pushed out into the first week of February on average at this point. I haven't grabbed February data yet to see if this theory would actually hold water, but my memory has more than a few "early feb thaws/heatwaves" vaguely floating around. There were a couple years of cherry blossom disappointment when I lived in DC 2015-2020 from temps crashing late-Feb/early-March after said warmup and killing off most of the blooms.

 

 

I could also be seeing things that aren't there 

I've never looked at data for this, but have felt for several years now that weather has 'pushed' about 2 weeks later into the year. Interesting seeing actual data.