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The Great December Dump

Rain Cold

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#1
Since the start of December is now within the proverbial 10 day window, I'm going to go ahead and hoist the thread. The next 10 days or so should average seasonal to mild, bringing us into early December, where things begin to become less certain. How will the month play out? Well, there are some things to be optimistic about and there are some things to be less optimistic about. Here is the TLDR version. For the cliff notes, please skip down to the summary section at the bottom:

TLDR Version:



Since the start of December is now within the proverbial 10 day window, I'm going to go ahead and hoist the thread. The next 10 days or so should average seasonal to mild, bringing us into early December, where things begin to become less certain. How will the month play out? Well, there are some things to be optimistic about and there are some things to be less optimistic about.

The case for a below normal December:

1) There has been a tendency for a -NAO and blocking to show up in a favorable region, supporting a cooler eastern US. This is a good sign as we head into winter.
2) There has been an active STJ, along with several coastals. This is also a good sign, arguing for frequent clouds and precipitation chances.
3) There is a case to be made for the emergence of a stout -EPO regime, favoring cold air transport into the US, probably centered over the Plains. The orientation and position of the EPO ridge will make the difference for whether or not the SE can get cold or stay seasonal.
4) Warming of the stratosphere could lead to enhanced blocking, featuring a turn to cold later in the month.
5) So far, warm patterns seem to get pushed back or are transient, with no static SE ridge.
6) The MJO doesn't appear to want to hang out for weeks and weeks in the warm phases.
7) I like the Aleutian low that keeps wanting to appear.

The case against a below normal December:

1) Recent climatology. It has been forever since we've seen a below normal December. Given a warmer overall background state, there may be other forces at work that we don't fully understand, leading to warmer conditions to start winter.
2) Warm-neutral/weak +ENSO conditions tend to favor an AN December.
3) The NAO looks to break down. This may turn out to be ok, if we get a strong -EPO, positioned correctly. However, incorrectly positioned, it could hand-shake with a -PNA, creating a semblance of a SE ridge.
4) Some seasonal modeling, though I'm not sure how much predictive value they really carry, show a warm December.
5) Ridging north of HI has been showing up, which argues for riding in the east, all else equal.

I'm sure there are other items in both camps. Feel free to add them if you think of them.

Anyway, here are some maps and charts to start us off:

6z 240 hr GEFS:

gfs-ens_z500_mslp_namer_41.png

6z 384 hr GEFS:

gfs-ens_z500_mslp_namer_65.png

0z 240 hr GEPS:

gem-ens_z500_mslp_namer_41.png

0z 240 hr EPS:

ecmwf-ens_z500_mslp_namer_11.png

CFS December:

cfs-mon_01_z500a_namer_1.png

CPC Climate Indexes:

CPC1.jpg

EMWESYWFYCWF MJO:

ECMF_phase_MANOM_51m_small.gif

EPO:

EPO.jpg



Summary:

My feeling is that we average near or slightly below normal for the month and will see at least one winter storm.

Merry Christmas and Happy Holidays!

Welcome to winter!
 
Last edited:

Ollie Williams

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#4
Since the start of December is now within the proverbial 10 day window, I'm going to go ahead and hoist the thread. The next 10 days or so should average seasonal to mild, bringing us into early December, where things begin to become less certain. How will the month play out? Well, there are some things to be optimistic about and there are some things to be less optimistic about. Here is the TLDR version. For the cliff notes, please skip down to the summary section at the bottom:

TLDR Version:



Since the start of December is now within the proverbial 10 day window, I'm going to go ahead and hoist the thread. The next 10 days or so should average seasonal to mild, bringing us into early December, where things begin to become less certain. How will the month play out? Well, there are some things to be optimistic about and there are some things to be less optimistic about.

The case for a below normal December:

1) There has been a tendency for a -NAO and blocking to show up in a favorable region, supporting a cooler eastern US. This is a good sign as we head into winter.
2) There has been an active STJ, along with several coastals. This is also a good sign, arguing for frequent clouds and precipitation chances.
3) There is a case to be made for the emergence of a stout -EPO regime, favoring cold air transport into the US, probably centered over the Plains. The orientation and position of the EPO ridge will make the difference for whether or not the SE can get cold or stay seasonal.
4) Warming of the stratosphere could lead to enhanced blocking, featuring a turn to cold later in the month.
5) So far, warm patterns seem to get pushed back or are transient, with no static SE ridge.
6) The MJO doesn't appear to want to hang out for weeks and weeks in the warm phases.
7) I like the Aleutian low that keeps wanting to appear.

The case against a below normal December:

1) Recent climatology. It has been forever since we've seen a below normal December. Given a warmer overall background state, there may be other forces at work that we don't fully understand, leading to warmer conditions to start winter.
2) Warm-neutral/weak +ENSO conditions tend to favor an AN December.
3) The NAO looks to break down. This may turn out to be ok, if we get a strong -EPO, positioned correctly. However, incorrectly positioned, it could hand-shake with a -PNA, creating a semblance of a SE ridge.
4) Some seasonal modeling, though I'm not sure how much predictive value they really carry, show a warm December.
5) Ridging north of HI has been showing up, which argues for riding in the east, all else equal.

I'm sure there are other items in both camps. Feel free to add them if you think of them.

Anyway, here are some maps and charts to start us off:

6z 240 hr GEFS:

View attachment 26312

6z 384 hr GEFS:

View attachment 26313

0z 240 hr GEPS:

View attachment 26314

0z 240 hr EPS:

View attachment 26315

CFS December:

View attachment 26316

CPC Climate Indexes:

View attachment 26317

EMWESYWFYCWF MJO:

View attachment 26318

EPO:

View attachment 26319



Summary:

My feeling is that we average near or slightly below normal for the month and will see at least one winter storm.

Merry Christmas and Happy Holidays!
Welcome to winter!
The potential SSWE is a bit controversial, if we can get a strong event, we can see a strong +AO in the stratosphere and a -AO in the troposphere bringing a flow of cooler air into the us, dampening any potential warmup. If its too weak, the stratosphere could fix itself and a -AO in the stratosphere but a +AO in the troposphere flowing in mild air. I believe that was what happened last winter. We shall see, what happens. The weather will do what it wants regardless.
 
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#5
Past 6 AN winters and the DEC pattern.

NPrTg7BD5i.png


Good news is neither the GEFS/EPS show the big eastern ridge...yet. The GOA/Aleutian ridge is burning my retinas on the EPS though.

ecmwf-ensemble-avg-nhemi-z500_anom_5day-5676800.png gfs-ensemble-all-avg-nhemi-z500_anom_5day-5784800.png
 
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#8
I love the thread title! Question is will it be a dump of the Arctic or a dump of poop? I like the GEFS with the -EPO. That's how you get cold. But the NAO vanishing is par for the course. But it's hard to get both at the same time. I would like to see a -NAO at some point though to slow the flow a little where the timing isnt as critical.
 
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#10
Even though it's the last few frames of the 06z GFS, it does look like some wintry weather is starting to show up for parts of Tennessee/ northern Arkansas and northward. Could this continue to show and trend southward? Time will tell!
 

Rain Cold

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#11
I love the thread title! Question is will it be a dump of the Arctic or a dump of poop? I like the GEFS with the -EPO. That's how you get cold. But the NAO vanishing is par for the course. But it's hard to get both at the same time. I would like to see a -NAO at some point though to slow the flow a little where the timing isnt as critical.
Thanks! I left it up for interpretation. Could be an arctic dump or a big snow dump or a garbage dump or a massive dump of posters or.... JK :)
 

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#20
Folks,
Even though the GEFS has been cold in the 11-15 for several days, always beware its strong cold bias mainly due to too cold a pattern! Per Maxar, the last 30 days have been averaging a whopping 6 too cold just in the 6-10 at ATL and that even incorporates the very cold early Nov when the model was not cold enough!! So, it would have been even warmer without that very cold period. Even the EPS has been 3 too cold at ATL the last 30 days.

Nationally the GEFS has only been 3 too cold, but that's because cold biases are less in the Midwest and near 0 with even a slight warm bias in some western areas. The EPS has been 1 too cold nationally. I had thought the EPS had averaged slightly too warm but that hasn't been the case.

As an example of how bad the GEFS cold bias has been on a national basis, I looked at the nationally weighted heating degree days (US HDD) of the 12Z GEFS from one week ago today and compared its week 2 to today's 12Z GEFS week 1. The results are horrible. I mean fail/flunky bad. Every day of 11/23-30 was predicted by the GEFS of a week ago to be solidly colder than normal. The current GEFS has all of those days near normal.


US HDD
Date.....12Z 11/15 GEFS.....12Z 11/22 GEFS

11/23.......….......21......…...............……..19
11/24...…….……22.......................……....19
11/25.........……..23.............…......……….18
11/26......…….....22.........….……...…...….16
11/27......…….….22................…...…...…..17
11/28......………..23......…....…...………….19
11/29......…….….23.............…...…...........20
11/30...…...……..24......….......…..............19

11/23-30 avg...22.5.................….......18.5

The 22.5 US HDD forecasted a week ago is 22.5, which is a whopping 4.5 higher (colder) than the normal of only 18. The current run has only 18.5, which is 4 lower (warmer) than the run from a week ago and is only 0.5 higher (colder) than the normal of 18. It really stinks, folks. To make matters worse, even today's GEFS prediction of near normal for 11/23-30 will come in quite a bit too cold as a warmer than normal US is expected. So, the too cold miss of 4 HDD on the week ago run will verify even further off once 11/23-30 actuals come in. SMH. Fake extreme GFS/GEFS cold never ends!
 
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Rain Cold

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#24
But don't ignore the horrible GEFS cold bias either. I recommend you take the GEFS and warm it up considerably if you're going to use it for a forecast.
Dude! I think we can pretty much all agree on this by now. It has a cold/cold pattern bias every year. Has for years.
 

GaWx

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#25
Dude! I think we can pretty much all agree on this by now. It has a cold/cold pattern bias every year. Has for years.
And dude, this needs to be emphasized as long as the maps are shown without the "cold pattern/bias" disclaimer. Not everyone is aware of this or wants to accept this and there are also new readers.
 

Rain Cold

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#26
And dude, this needs to be emphasized as long as the maps are shown without the "cold pattern/bias" disclaimer. Not everyone is aware of this or wants to accept this and there are also new readers.
I think the people who are going to get the concept get it by now. That's kind of like having to remind people not to believe a 10 day map every single time a 10 day map is posted. Or like saying the models flip flop every time the models flip flop. Some people aren't going to care about anything but whether or not there's blue cold or white snow being shown over their house.
 
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#27
I think the people who are going to get the concept get it by now. That's kind of like having to remind people not to believe a 10 day map every single time a 10 day map is posted. Or like saying the models flip flop every time the models flip flop. Some people aren't going to care about anything but whether or not there's blue cold or white snow being shown over their house.
I always consider the "no snow for the SE" bias on all models. If it shows cold and snow, it's corrupted and should be discounted. Always. ;)

I'm not ready to say the GEFS has got the wrong pattern. It may be overdoing the degree of cold, but the pattern could be correct IMO. EPS versus GEFS both have their strengths and weaknesses I think.
 
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#28
But don't ignore the horrible GEFS cold bias either. I recommend you take the GEFS and warm it up considerably if you're going to use it for a forecast.
I use a blend of both ensembles, it's never a good idea to take one model at face value. I was just verifying if that massive Bermuda high was there on the GEFS. Since that is 300+ hours out, that Bermuda high does not have a chance at verifying at this time, considering that the GEFS does not have it. If other ensembles don't show that Bermuda high, the chance of that Bermuda high is slim. Of course, things can always change.
 

Rain Cold

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#29
I always consider the "no snow for the SE" bias on all models. If it shows cold and snow, it's corrupted and should be discounted. Always. ;)

I'm not ready to say the GEFS has got the wrong pattern. It may be overdoing the degree of cold, but the pattern could be correct IMO. EPS versus GEFS both have their strengths and weaknesses I think.
I always view anything 10 days or over with skepticism. What I try to use to give me a little more confidence is agreement between model/ensemble suites on the general pattern, as well as pattern recognition. IMO, like Larry has been saying, the GFS tends to push lower heights too far south in the east in the LR. That leads to colder temps than what will eventually occur. It's been that way for a long time. BUT, it doesn't mean it doesn't have the general pattern right.

The take-away is that, even if there is a cold bias, it more or less means that it will average warmer than it's showing. But when it comes to specific daily temps or specific storm chances, there is no appreciable skill way out there. But if the general pattern is favorable, it tells you that there is at least a period of interest coming up....if that general look holds as we move in and especially if there is agreement between model suites and especially if there are features that are present that suggest a stability to the pattern.
 
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#30
Part of the problem is that any model when over analyzed past day 10, especially day 15, is going to show some large inconsistencies...it seems the GEFS biased cold or pattern biased. EPS has large errors day 10+ too.

But, to start December both the GEFS and EPS are showing a -EPO with an eroding -NAO. Question is how far east/west does the actual ridge setup in the pacific. Is it out over the GOA like the 0z EPS or is it splitting the Yukon like the GEFS.

gfs-ensemble-all-avg-nhemi-z500_anom_5day-5460800.png
 
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