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.