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!  Take a look at the very FIRST thread on the forum.  Read until you get to John's map on Dec 15th.  This is why I love this site...we can look back at other winters.  And look at that map - mega dittos!!! I have no idea what the SSTs or ENSO were at the time, but wow....similar.  My intent was to start at the first thread, and find where we have had this conversation before during December.  I am sure there are others, but LOL...it is great.  

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.