December 2025 Short/Medium Range Forecast Thread

[John1122]

Been reading on the MJO today as I wait for the Tennessee game to start. The MJO phase only affects North American temperatures apparently in relation to the proposed in the paper, MVP (multivariate PNA). For instance, in the article I am reading, phase 5 is only warm if the MVP is also negative. 

This is from that paper. 

"The MVP is the first combined EOF of 20–100-day OLR, 850-hPa streamfunction, and 200-hPa streamfunction over the North Pacific and North America. The North American temperature patterns that follow each phase of the MJO change with the sign of the MVP. For example, real-time multivariate MJO (RMM) phase 5 usually leads to warm anomalies over eastern North America. This relationship was only found when the MVP was negative, and it was not associated with El Niño or La Niña. RMM phase 8, on the other hand, usually leads to cold anomalies. Those anomalies only occur if the MVP is positive, which happens somewhat more frequently during La Niña years. Composite analyses based on combinations of the MJO and the MVP show that variability in the Pacific jet and its associated wave breaking play a key role in determining whether and how the MJO affects North American temperatures."

The article can be found here. 

https://journals.ametsoc.org/configurable/content/journals$002fmwre$002f141$002f11$002fmwr-d-13-00118.1.xml?t:ac=journals%24002fmwre%24002f141%24002f11%24002fmwr-d-13-00118.1.xml

[John1122]

I remember a few years ago we passed through phase 8 and it didn't get cold. I believe it was an el Niño, so the mentioned/proposed MVP likely wasn't favorable. 

I do believe ph 7 is generally normally BN here in winter. The article I linked above noted that phase 7 seemed to be unaffected by the state of the proposed MVP.

[Carvers Gap]

That is insanely good information, John.   

[John1122]

I do like that we may get to phase 8, and low amp (which is usually the coldest version of 8) and that La Nina is much more likely to produce a +MVP, which means a deep Eastern trough. 

[Holston_River_Rambler]

Pretty nice run of the Euro AIFS at 6z. 

[Daniel Boone]

Trainwreck. SER flexes on Weeklies. Looks like only part of the US that's going to benefit from the weakened PV will be mainly the Rockies and Plains. Euro Weeklies go from well above first 2 weeks to a basic Average look weeks 3 and 4 as SER is finally squashed.  Of course, nothing is set in Stone as we've witnessed these change drastically recently. If they're not factoring the MJO much there may be a drastic change if MJO 8 flexes. Fwiw, the CFSv2 Weeklies are even worse. Wall to wall well above Normal.

[Carvers Gap]

The good thing about the Weeklies.....it is still shoulder season and they apparently have missed days 1-10 of December badly - just speaking of the hemispheric long wave pattern.   In other words, do we trust a model which likely is going to miss badly?  The Weeklies will almost undoubtedly miss embedded cold.

Models are swinging back and forth wildly right now.  That tells me cold air is incoming into NA.  The Mountain West looks likely simply because that has been the pattern since 2015 or so.  I really doubt models are done with their wild swings.  The GFS trying to "cook up" a Gulf hurricane tells us this could be wild.

But really, I think the first week of December has been pretty well documented as being warm.   This was a textbook kick the can or Lucy football situation.  The second week of December should be a transition w/ the time around Christmas "maybe" being very cold.  

I also think SSWs upends thing.  Within days of it being mentioned, the LR pattern went warm-ish.  We have seen that occur several times.  That said, I do think we have some chances upcoming.  Either way, early December is rarely good snow climatology.  I do think there is a bit more pressure to have cold weather in December during Nina years as they are front loaded.

 If Cosgrove is right, the last half of winter should be good.  He certainly has stuck to his ideas and appears to be ready to score a coup of sorts for December.

Again, after models missed the Baja low (multiple lows in succession), I have been waiting for the dust to settle so to speak. If the end result of this is getting a cold or seasonal air mass around the holidays...I will gladly take it.

[Carvers Gap]

One last note, the Euro Weeklies are very dependent upon the first two weeks of the 0z deterministic.  I knew today's Weeklies would be warm.  It was simply an extrapolation of the deterministic which is struggling greatly with what is likely Southwest feedback..  It would not surprise me to see them snap back cold if the MJO continues to rotate.  I have definitely seen models not recognize the MJO impact until day 10-12, and we are not to that point yet.  I would suggest modeling will trend much colder as it becomes clear to modeling that we are entering a phase 7-8 rotation, and I wouldn't be surprised to see it rotate right back to 6-7-8.

[Carvers Gap]

FWIW, the 12z GEFS looked good in terms of cold.  I noticed that one of the mets in the MA noted that the GEFS is running circles around other modeling right now in the extended range.  I don't have that data, and am to lazy to look...so just look in their December thread for reference.

[Carvers Gap]

Really there are two BIG things models are trying to navigate.  It is going to take some time for them to "get right."

1.  SSW - to what extant (if any) does this impact NA or the surface at all???

2.  The cutoffs in the SW.  It sure looks like feedback after system one.

I have been fooled by modeled cool forecasts for December before.  Conversely, I have seen LR extended modeling show cold, reverse course to warm, and then revert back to the original cold look.  So, I am gonna just "chill," enjoy the Florida beat down aftermath, and eat some great Thanksgiving food this week.  We certainly could see a warm temps in phases 8-1-2.  It has happened before, and could happened again.  But I have also seen deterministic modeling flip very cold once they recognize the MJO phases - almost all at once.  

[Carvers Gap]

For clarification of my above model verification comments.  @Terpeastposted this.

For future reference, based on MJO verification over the last 30 days, GEFS is really the one to watch when MJO is medium to high amplitude. And it’s not even competitive.

[Carvers Gap]

The last in a flurry of posts...I do like the GFS at this time of year.  By mid-late December, I like the Euro better for medium and long range accuracy.  The 18z GFS isn't a bad pattern.  Maybe keep an eye on the Dec 6 timeframe.  For the past several runs, the GFS really wants to lift something out of the Gulf.   I have definitely noted over the years that the GFS will often spot a cold outbreak more quickly on the deterministic d10-15.

[Carvers Gap]

Bonus post....sorry.  This is why the LR is a mess in terms of, "What will it do?"  On this map, we have many conflicting signals. We have  a second tropical-ish system south of the Yucatan, a low east of Hawaii(good), a PNA ridge(good), a cold front crashing eastward (good), a -NAO (not good), a block and a half over Alaska and the Bering Sea( I can't even remember the Bering Sea rules), a -AO(not good), a GOA low(normally not great), a western Atlantic Ridge (WAR), and cold over the interior of Canada(good).  

I could make a pretty good case that if the "movie is played forward" that this is has the potential to empty a lot of cold into NA and eventually the EC by the second half of December.  Again, if any model is going to sniff out a cold shot at this time of year(December), my money is on the GFS.

By my count the 18z GFS has (4) cool or cold fronts which make it to the west side of the Apps.

And that is it for a bit...I have been watching things today, and wanted to sit on a few model cycles before posting.  Plus, it has been a busy weekend.

 

[Carvers Gap]

Model whiplash....12z CFSv2 and 18z CFSv2.

[Daniel Boone]

1 hour ago, Carvers Gap said:

Model whiplash....12z CFSv2 and 18z CFSv2.

There about as lost as I've ever seen them. 

[jaxjagman]

Still look like its getting into Africa  before Christmas

[Carvers Gap]

The difference between 0z and 6z on the GFS is stark regarding the long range pattern.  I think it likely we deal with warm temps through the first week of December, but that is not set in stone as there is a cold front possibly coming through Dec 6th(up in the air so to speak).

[Carvers Gap]

If I am cherry picking models...I sure like the 6z AIFS paired w/ the 6z GFS.

[Carvers Gap]

I do believe the GFS is now starting to sense the MJO.  That is a MONSTER EPO ridge on that model to begin December.  I highly doubt the downstream consequences of that are worked out yet.  

[Carvers Gap]

I think the end result of all of this will be some wild swings between cold and warm.  The frequency of the troughs and ridges is high.  The troughs are deep and the ridges are high.  That is a formula for wild swings from model run to model run.  I do think troughs tucking under the EPO ridge is probably an error.  It is not without precedent, but it usually isn't common this early in the season and especially with an MJO that would want to kick a trough out of the West.  The runs which make the most sense are the ones where the troughs get booted if/when they form(from the West).  I think the progressive nature of the GFS is probably muting that feedback.  

[Carvers Gap]

The 12z GFS now has a true Arctic outbreak at the end of its run which is exactly what we would expect to see with the MJO flirting with phase 8.  And I have said this many times, the GFS deterministic would likely be the first to spot something like that.  I don't know if it is right, but it fired the first shots yesterday at 18z.

[Carvers Gap]

And it is very important to be careful w/ 500 maps with cold air on the  prowl.  I was watching temp maps that were frigid at the surface, but the 500 map looked benign.  The surface map(first image) shows a strong cold outbreak into the Tenn Valley after 300 hours.  See that string of high pressures.  However, the 500 map looks kind of meh.  I think there is a 50/50 chance that we see a strong cold outbreak in/around December 6th.

 

[Carvers Gap]

And this is a setup that says, "Look out!"  This is sitting at d10.  As @Holston_River_Ramblershared yesterday, the AIFS caught wind of this possibility first - I failed to note that early...sorry Holston!  The GFS is now picking up on the possibility.  That is a mechanism for fiercely cold air to be discharged into the Lower 48, especially the Northern Plains, Ohio River Valley, and maybe the Tenn River Valley.  There is the possibility that this is being under done.  Either way, this is a very cold look at the surface.  

[Carvers Gap]

And this would be phase 8.  Now, I must warn everyone(newbies especially) that model talk past d10 is not without peril.  But it sure seems like the GFS (and about half of the recent AIFS runs) are sniffing out a potential cold shot around December 6th.

[Carvers Gap]

The 12z Euro AIF is rolling.  You can send me the Christmas card later (for saying the first week of December being warm was a foregone conclusion).  LOL.  That right there is not a warm look for the eastern 2/3 of North America.

[Carvers Gap]

The 12z CMC almost has a winter storm by 198.  Big changes on the deterministic models at 12z today - noted that the Euro has yet to run.  The Euro AIFS, GFS, and CMC are showing a cold shot around d10-12 which wasn't there on yesterday's runs.  It is a bruiser.  I doubt the ensembles will switch that quickly, but it is a trend worth watching.  This cold front had been on LR ext modeling for weeks, and then disappeared.  edit...the CMC is actually close to something good 3x.

[Carvers Gap]

The 12z CMC has a below zero air mass sitting in the Plains and Ohio River Valley as a couple of impulses originate in the Gulf and head northeastward.

[Carvers Gap]

And the 12z CMC has similar look to both the AIFS and GFS.  EPO anyone?

[Carvers Gap]

This is the 5-day map from WxBell of the above pattern depicted by the 12z CMC....that is a BIG change.

What we are seeing at 12z might explain the model mayhem.  

Is it the MJO finally exerting influence on modeling?  I have sometimes noticed that the MJO doesn't really exert influence on modeling until about d10-12.  I have noticed this in reverse when the pattern looked cold d10-15, but the MJO looked like it might stay cold while in the warm phases - nope, almost always flips as reality approaches.  

Is the SSW starting to have an early impact on the troposphere?  Maybe, but it is a bit early for that influence.  

Either way, the EPO is showing up on modeling big time at 12z.  Let's see if the Euro gets on board.  I kind of doubt that it does as it IMHO is dealing with feedback issues in the SW.  But let's see if it gets moved off of its spot just a little.

[Carvers Gap]

The 12z Euro looks remarkably similar to the 12z CMC.  I am kind of surprised by that.  Winter storm potential exists on it as well.