December 2025 Short/Medium Range Forecast Thread

[John1122]

The Euro AI has a very potent cold front in early December that really battles the SE ridge. It slowly overtakes the western forum first and it takes 36 hours or so to cross the state. Get some rain to snow in NW Tennessee that slowly spreads eastward and lightens as it goes east. Often times the cold comes a little quicker than models manage it in those situations, but sometimes it just sticks in place.

[John1122]

The cold is wild in the midwest. 40 degrees below normal with -10s. Most of our forum area is in the 10 to 20 degrees bn range from the 3rd to the end of the run on the 6th.

[John1122]

The Euro might be trying to go at a winter storm for the western areas at 288 this run. It's warmer than the AI but cooler across our forum area with less extremes on either side of the front.

[John1122]

The Euro ends up cutting the mid-state with western areas miserable in the 30s with rain, warm in the east ahead of the cutter. SE ridge wins on it vs the AI.

[jaxjagman]

Id agree with what Jeff said up above .We seem to be seeing the the SSW underway with the sharp rise in temps  into the stratosphere but even so ,the cold seems to be getting pushed back seemingly each day.The JMA  right now shows a strong MJO signal in the WP into the first of Dec,while this isnt bad it still should have reflection to  more or less a SER,just transient cold shots.Might not be a bad thing if you want a cold Christmas or even possibly beyond that for a couple weeks anyways

[jaxjagman]

GEFS looks more Euro now with any SSW,you can see it here

[jaxjagman]

In case you dont have this you can look at Planetary and Equatorial Waves using the Euro Modes,ill shut up now

[Carvers Gap]

The flip has been quick!  The trend has not been our friend, but it is only November and we have just begun to fight...or something like that.

The pivot away from BN heights over Tenn Valley has voluminous support both in width and depth in the model world.  From ensembles to Weeklies....the trend is towards a standing wave SER over the SE.  It can be beaten down, but it immediately returns.  

Outlier models such at the Euro AIFS and GEPS persist.  I don't discount those.  If cold is coming, the Canadian model will see it first.  What we really want to see is the GFS deterministic to start bringing cold eastward OR the Euro control weeklies.  That Weeklies Euro control is a nifty tool.  With the exception of a few days earlier this week, it simply refused to be cold for December.

The repeated troughs into the Southwest are an ominous look if you like snow and cold.  I keep asking myself this question, "Have I been burned more by Baja lows which actually formed...OR have I been burned more by modeling which erroneously (due to feedback) put that feature in the Southwest?"  I guess we will find out soon!  Modeling often performs so poorly over that region.

I find it pretty hard to believe that the MJO in phase 8 is going to end up being warm.  So, I do think there will be some moderation(from AN heights) back towards seasonal and maybe even BN heights at some point for a 7-10 day period during December.  The Euro Weeklies today are cold at the surface, but I now think that is a bad, bad bias(and a new one at that) and an error.  Its 500 heights are now pretty much AN for most of December.  That would imply cold will push some, but the SER is the base pattern.

Again, this is shoulder season, and trends will likely still change - we can hope, right?!  However, for me anyway, my hopes that models are delaying the cold diminish with each passing run.  The reality is that it might not come to pass for much of December.  But it only takes one memorable snowstorm to make folks forget a warm pattern!  Besides, we usually don't get a lot of tracking in the valleys until January or Feb.  

We aren't even gonna talk about the retrograding BN heights(over the HB) into the West to start January!  If LR ext modeling was wrong about December cold, it could certainly be wrong about a January pattern at range!  I wish I had better news.  I will give it a few more days, and then I will almost certainly have to adjust my winter ideas.  

Have a good weekend, everyone!  At least we are getting some much needed rain.

[Carvers Gap]

If the Baja/Southwest cutoff is simply feedback, we should start to see the slp slowly creep eastward with each run, especially the first (in a series of 3!).  If that is indeed feedback, you can bet the rest of the (previous runs) are probably not correct.  Something to watch on this rainy, fall night.

[Carvers Gap]

Haha.  I know I have lived this discussion before!  This is why I love this site...we can look back at other winters. 

Also, google our forum for Baja cutoffs.  Lots of great discussion for those.

 

[Carvers Gap]

Just watching the 18z GFS run, it is already very different over the Southwest.  The second shortwave does not dig over the Southwest.  That yields an entirely different outcome.  That subtle change illustrates how important that sequence is....

[Carvers Gap]

Look at the different in the eastern half of the US.  The first low(now in the Plains) is shallow and digs less.  The second vortex spins off the Cali coast(westward).  Not sure how much of that is believable, but this illustrates what happens when perpetual lows don't form in the SW.

[Carvers Gap]

The second vortex (which follows the Four Corners low) is the problem.  During the 18z run it literally cuts directly through the ridge in the GOA.  At 12z, it cuts over the top and dives into Baja which deepens the western trough.  You can see the variations on the panels of the ensembles.  About half have a trough in the East and half have a ridge - thanks to that little vortex.  The EPO ridge is so tall that it is rotating energy back under it which is causing westward flow of Cali.  If that vortex doesn't sit over Cali, I do wonder if Santa Anna winds might develop as a result.

edit:  When the first Four Corners low doesn't dig as much, it doesn't really leave much room for the second to follow.  All of this is something to watch on future runs.  Until this gets worked out, everything from Weeklies to deterministic runs are going to be impact IMHO.  Why?  That series of cutoffs takes almost a week to resolve.  In other words, there are 5-7 days of cutoffs if not more (on previous runs).  If we are looking at just 24-48 hours of a single, cutoff low...that allows for the trough to form in the East.

It is a great example of the butterfly effect.  The difference between record highs at 12z and the PV heading south at 18z....look nor further than the SW sequence.

[Carvers Gap]

Night and day differences when the SW doesn't have multiple lows cutting off...I don't know which is right.  But the 18z GFS gives me some hope that the multitude of cutoffs in the SW may well be a feedback error.

[Carvers Gap]

Option 1(and there will sure be options 3,4, and 5!) is that the trailer vortex spins out to sea.  That is the 18z version.  Option 2 is what happened at 12z.  The trailing vortex followed and dug out a huge trough which allows a third vortex to drop into.  We need that first vortex (even if it goes to the FC...to dig less and kick earlier).  I think the first vortex digging into the SW is legit.  I am not sure I buy the second and third vortices doing the same - that smacks of feedback.  If the second and third vortex simply "don't go to Baja" then the trough likely digs into the EC.  

Take a minute and looks at the little vortices which modeling is juggling.  There are four rotating around the EPO ridge - not counting the Four Corners low.  This set-up is an absolute thorn to work out.  The downstream effects are huge.  It will be interesting to see how this works itself out.

OK, enough stream of though posting.  It is Friday night, and I am chilling.  Will check back later.

 

[Carvers Gap]

Hour 276....we have some things to get worked out.  

The difference in the Euro AIFS and GFS ta 18z over Nashville is 58 degrees.  The Real Feel on the AIFS is 3.  The GFS is 61.   The AIFS did not dig into the SW w/ the SLP FWIW, but it did the second.  That still allowed a massive cold front to crash eastward on Dec 3 on the AIFS.  The AIFS even dropped 1-3" of snow north of I-40 from Nashville eastward.

[jaxjagman]

East Asia gives you some insight of the pattern ahead,yes its never gonna be right because we have different teleconnections,sorta speaking.But honesltly 1,E,you see a ridge building into Mongolia into maybe even Russia,nothing but a trough in a few days as we head towards T-GIVING

[jaxjagman]

If you ask me this pattern we are fixing to get looks more severe in early Dec,You got heights rising before t-giving  into the Sea of Okhotsk,with a trough in East Asia and rising heights into Mongolia

[Carvers Gap]

Overnight, it looks like the GFS started to get rid of the feedback over the SW.  The 6z run pretty much eliminates it after the 6th.  There will be a cutoff out there, but the GFS(and its ensemble) kick out everything after it.   Is this because the model has a progressive bias?  Maybe.  But it would also be the first to fix whatever is going on(if indeed there is a feedback error which seems to be growing more likely).

[Carvers Gap]

I think the multiple cutoffs to Baja and/or Southern Cal....was an error.  The GFS has a bias to be progressive, but because of that it will work this out more quickly.  The 12z gfs and ensemble matched the 6z run.  The CMC is still coming around.  In fact, the GFS is flirting with snow around Dec7-8.  My guess is that cuts along the Ohio River Valley.  But things don't hang up...that cold front will come roaring through at the end of the first week of December.

[Carvers Gap]

The 12z Euro has correct as well.  Storms are kicking out of the Southwest as one would expect.  Now that modeling is getting that fixed, we should see some downstream corrections.  I would expect for something to track pretty soon.

[Carvers Gap]

The 12z Euro deterministic also kicks out the energy out of the Southwest.  The EPS is a little slower to do so, but I am going to guess it does the same.  Both the 12z GEFS and EPS have a formidable cold look now.  The SER flexes are short lived.   Just file this one away for later used.  I would say 75% chance that what we saw yesterday was feedback across all modeling.  It caused havoc downstream from that.  I would suggest modeling in the d8-14 is still in the process of correcting.

[Carvers Gap]

Noticeable shift colder today on most models with the exception of the CMC deterministic.  The Euro Weeklies have returned to their cold look and serviceable 500 pattern for December.  By the second week of December, they begin to push the cold for the rest of that month - equilibrium restored.  The Weeklies, contrary to some comments on other forums, had a major change about two days ago.  That first week of December has pretty much been warm on those Weeklies for some time.  Nobody is upset about that.  The thing that was frustrating to see was the Weeklies completely lost the rest of the month of December, especially after nearly 4-5 weeks of cold being shown.  The Euro Weeklies have been anything but steady this past week - welcome to shoulder season.  I really think the problem was feed back over the Southwest.  Now that is being resolved with each passing run, modeling(deterministic and ensembles) is beginning to correlate again with the MJO and LR ext modeling.

[Carvers Gap]

Comparing week 2 of December.  This is the change which is seen on the Euro Weeklies.  This was derived from the 0z run.  The 12z derivation would have certainly been even better.  Notice the SER is gone.  Notice the EPO ridge?  It isn't in the Aleutians which teleconnects to AN heights over the SE.  That is a 1,000mi change in the Eastern Pac ridge.  There are AN heights over Greenland.  Those changes IMHO are a direct result of things resolving w/ the erroneous(?) Baja low.  The SER doesn't get pumped long term.  

addendum:  Yesterday, the blues and yellows were exactly where u didn't want them.  Today, they are generally where you want them

[Kentucky]

I'm looking for anything right now, and the 12z Euro AI delivered a deep trough.

[Carvers Gap]

These are the changes for week 3 of December.  Ridge is out of the Aleutians.  There is no ridge bellying into the SE.  This would allow for northern stream systems and sliders.  The control is MUCH colder than this.  

 

[Carvers Gap]

1 minute ago, Kentucky said:

I'm looking for anything right now, and then 12z Euro AI delivered a deep trough.

I think that trough is baked in the cake.  It should be COLD.  Nice animation.  How did you get the GIF for WxBell to work?  

[Kentucky]

8 minutes ago, Carvers Gap said:

I think that trough is baked in the cake.  It should be COLD.  Nice animation.  How did you get the GIF for WxBell to work?  

Hopefully the start of a trend, copied the URL off Chris Bailey's latest blog post. Then just  clicked on other media and add image URL here https://kyweathercenter.com/?p=73876

[Carvers Gap]

Here is the difference in the control for all of December...Notice what is gone.  This also correlates with the MJO looks we have seen.  The SER wasn't pumped by multiple Baja lows.