December 2025 Short/Medium Range Forecast Thread

[Carvers Gap]

@Holston_River_Rambler, you jump right in here with your nifty short range graphics.  I don't want to be taking your job!  Here is the RGEM right before the map above.  It dissipates as it heads south.  As we so overnight, sometimes it hold together.  So, let's watch the trend on this.  The 3k NAM is generally just a higher elevation event.  With such a strong air mass coming in, the jet streak with this might help.  Also, a lot of models won't see lower level moisture getting squeezed out w/ below zero air masses.  This might be 15:1 or 20:1 type of stuff.

[Carvers Gap]

I am going to try a new way of posting.  For the 12z suite, I am going to put all note here instead of 5,000 posts in a stream of thought fashion.  I will simply edit and amend this post as the 12z suite progresses.  Feel free to comment!

12z CMC...is having nothing to do with a prolonged warm-up prior to 240.  It has the Dec 20th cold front.  

12z GFS...Doesn't have the 20th cold front, but has one on the 23rd.  The 12z GFS...gasp...erases the furnace by 276.  

JB mentioned that last night that all it will take to dump more cold into the East would be for a piece of that trough to split and head eastward.  That is the 12z GFS.

haha.  The 12z GFS has snow showers along the Virginia apps on Christmas Eve.  What a turn of events.

Feedback.  Have I talked about that?  Well, the deterministic models (GFS and CMC) have less of it over the NW.  That, in turn, reduces the raging chinook which develops with west-east flow.  It really isn't a problem to have a ridge roll through in front of a NW PAC slp.  It is a problem when that slp spins, reforms, spins, reforms, spins....  With that feedback loop lessened, the trough gets kicks eastward and an Alaska ridge pops.

The MJO plots on CPC are much more favorable than the previous days.  That may be why we are seeing more cold fronts on medium and long range modeling.  Will we see a complete reversal of the heat wave back to a cold shot?  Probably not, but just a normal ridge or two rolling through is not a huge problem.

If I was waiting on cold and saw the GFS/CMC begin to eat into it the way they just did the warm spell...I might call that a can kick and question whether it actually comes to pass in large quantities.  

To me, this looks like a back-and-forth pattern w/ a great cold source which comes eastward with any kind of amplification whatsoever.  

Good trends.  I will have a separate post on the Euro if it is similar.

Feedback on the GFS 300+ is pretty apparent off the West Coast.  If that is feedback, the trough roars into the East. ---> almost the exact some thing which led to the head fake(warm up) for earl December.  It pumps the ridge in the East.  Without out, that amplification is less or absent.

When we see a storm exit Washington state, drift southwest with no steering, and it amps up...that doesn't look realistic to me per the GFS.  If that gets worked out, a colder east is possible and even likely.

[*Flash*]

A bit more bleak for west/middle TN these days. Hoping some hope can pop onto the scene during this ROM torchfest.

[Carvers Gap]

5 minutes ago, *Flash* said:

A bit more bleak for west/middle TN these days. Hoping some hope can pop onto the scene during this ROM torchfest.

I think this winter is a second half winter for you all.  Middle and West have far outdone NE TN during the 2020s which is wild...even Knoxville has.  Generally, the cold finds you all at some point during recent La Nina climo.  I think u all will be good.

[Carvers Gap]

The 12z Euro continues the 12z suite trend of muting or erasing entire sections of the warm-up.  More posts to follow here...I will just edit this post.

Run-2-run changes for maybe a not so random day...ensembles don't reflect this yet, but the trend is pretty hard to ignore.  I wouldn't quite call it a flip as plenty of warm persists, but the standing wave - poof.

 

[Carvers Gap]

Essentially the pattern is this at 12z.  A slp slides into the NW(just ignore the runs where it stalls, reforms, slides to the southwest in perpetuity, etc).  When that strong lp slides across the Rockies, the counter-clockwise rotation forces a strong chinook which races eastward.  As the slp slides into the Plains, it drags extremely cold air behind it.  That cold then slides eastward.  Wash, rinse, repeat.  If the SLP stalls over the NW(likely feedback), then it pumps a crazy ridge in the east.  

Ensembles are washing this pattern out.  Now, I fully admit the SER could out-duel the incoming cold.  But the corrections at 12z seem to infer a very back-and-forth pattern which would feature NW flow snow, severe, record warmth, and possible record cold.  Wild West.

And really, it is not a SER which were are fighting...it is a chinook on steroids.

I think when the ridge in the East is anomalous, you will find a feedback issue off the West Coast.  It isn't quite the Baja low feedback issue, but it is in the same part of the world w/ the same result.

[jaxjagman]

12 minutes ago, *Flash* said:

A bit more bleak for west/middle TN these days. Hoping some hope can pop onto the scene during this ROM torchfest.

Like always in a NINA it seems,we see some flakes.If it snows it will be in March where it screws up severe

[Carvers Gap]

3 minutes ago, jaxjagman said:

Like always in a NINA it seems,we see some flakes.If it snows it will be in March where it screws up severe

Maybe I am wrong, I thought you all have had several snowstorms this decade.  We have had very few in NE TN.  I thought I rememberedr tracking several with you all.  Were those storms west of Nashville?  Not trying to be smart...I thought Nashville had some decent snows.  I could be misremembering.

[jaxjagman]

1 minute ago, Carvers Gap said:

Maybe I am wrong, I thought you all have had several snowstorms this decade.  We have had very few in NE TN.  I thought I rememberedr tracking several with you all.  Were those storms west of Nashville?  Not trying to be smart...I thought Nashville had some decent snows.  I could be misremembering.

NINOS,not NINAS.I'm not saying it wont snow but chances are better for you guys than us,it should flip next year

[Carvers Gap]

Same feedback error IMHO as ~ three weeks ago.  Two different models - still trying to figure out why there is so much feedback over the West during the past month.  Here is the Euro at 264.   That mid-continent ridge doesn't get pumped without that area of BN heights which has tracked from Calgary into the Pacific (yes, you read that correctly).  Here are the two timeframes....

 

[Carvers Gap]

Two different models, and BIG differences for our back yards as a result.  The 12z GFS has a cold front on Christmas Eve.  The 12z Euro...not so much.  The difference?  The Euro has a much deeper area of BN heights in the eastern Pac (which originated on the NA continent...I guess that could happen but I am no holding my breath).  Conversely, the GFS slides an area of BN heights over the GL(versus dumping it all into the Pac), and a strong cold front results over our region.  Look at the difference in the ridge over NA.  

Below is a nearer term difference.  This time the 12z Euro has a cold front over the East on Dec 20th.  The AIFS, CMC, and ICON all have this.  The GFS is the outlier.  The ridge and trough locations are exactly the opposite next Friday.  The GFS somehow loses energy heading East.  The results are polar opposites.

All of this is to say that small differences over the eastern Pacific are having huge downstream impacts.  In some cases, we are seeing 50-60 degree swings in modeling.  And I feel like I have typed that before...maybe later November?

I do think we see some Chinook winds.  That seems almost inescapable as SLPs transit the Rockies.   But cold air "should" cave behind the low as it passes.  Our advantage this years is that big green blob in northern Canada which recent climo had as AN heights - ie.  we have a cold air source.

I also think ensembles are washing things out due to the means.  IF(stress) we are seeing feedback which is amping ridges in the east too much, then the average temp of all members is going to be very skewed.  Things get washed out.  

IMO, a very back and forth, amplified pattern "could" be coming up as an alternative to the torch.  If we can swing a trough through on the 20th and on Christmas Eve, that is tolerable?

[jaxjagman]

Just saying, here in Mid Tn,during NINO'S we can have destructive ice storms compared to NINA.Its really rare in a NINO winter to not see ZR or IP fall underneath SN and not just all SN.We've had in the past I.E in the 50's and 90's some destructive ice storms in MID TN.,during a NINO,for what ever reason  NINA seems to bring late season snow here that falls at night/early morning and melts during the day,this is always late into winter early spring.But i'm not saying it cant snow,its winter,just  the odds are it might not happen

[Golf757075]

1 hour ago, jaxjagman said:

Just saying, here in Mid Tn,during NINO'S we can have destructive ice storms compared to NINA.Its really rare in a NINO winter to not see ZR or IP fall underneath SN and not just all SN.We've had in the past I.E in the 50's and 90's some destructive ice storms in MID TN.,during a NINO,for what ever reason  NINA seems to bring late season snow here that falls at night/early morning and melts during the day,this is always late into winter early spring.But i'm not saying it cant snow,its winter,just  the odds are it might not happen

I agree. Once we get to January, time really goes by fast and our window of opportunity drops significantly. I hope we get what's needed to have an overlap of cold and precip

[Matthew70]

Winter really doesn’t officially begin until the 21st.  You have all of January & February, that can produce good snows.  February is actually one of the top months for snow in this area.  Not sure why so many are already canceling winter when it hasn’t even begun. 

[jaxjagman]

But if the blocking isnt more into the AK but more Western AK and into Siberia like the GEFS shows, assuming this is even right right,its not a cold look,you probably be seeing boundaries stalling out in NA somewhere and not your typical 1-40 and if the 850 V-winds kick up its probably more severe than winter

[Carvers Gap]

20 minutes ago, Matthew70 said:

Winter really doesn’t officially begin until the 21st.  You have all of January & February, that can produce good snows.  February is actually one of the top months for snow in this area.  Not sure why so many are already canceling winter when it hasn’t even begun. 

Yeah, it's just December 11th.  Today's Euro Weeklies control flips the pattern back during the second week of January.  IF we can pull that off, we have cold during the coldest weeks of the year.  The warm solutions(and they are WARM) in ensembles are skewing the mean.  For example, there might 14/25 members which are BN with temps, but the other 11 are raging warm.  The mean would be well AN, but the median might be cold.  I have always been taught that reality subtracts (as we get closer) the less likely members, and the more likely members are left.  Do I think we see some warm-ups this winter?  Absolutely, and models are honking at some chinook warmups Between Dec 20th and Jan 8th.  That is a pretty normal time for a thaw.  Nina winters really like to pull the trough back West for about 2/3 of winter, but the 1/3 we get can often be good.  I don't see a thing which surprises me at this point.  That said, I am not sweating AN temps when we have very cold temps on the way Sunday and Monday.  I truly need about 7-10 days to warm-up.  The cold in NE TN has been pretty relentless.

[Carvers Gap]

MRX graphic from social media.  Feel free to post your local.  

[Carvers Gap]

Afternoon disco from MRX...

922
FXUS64 KMRX 121734
AFDMRX

Area Forecast Discussion
National Weather Service Morristown TN
1234 PM EST Fri Dec 12 2025

...New DISCUSSION, AVIATION...

.KEY MESSAGES...
Updated at 1232 PM EST Fri Dec 12 2025

- Light snow accumulations possible mainly in the mountains Saturday
  night into Sunday.

- Bitter cold will surge into the area for Sunday into Monday.

- Warming trend begins Tuesday.


&&

.DISCUSSION...
Issued at 1232 PM EST Fri Dec 12 2025

Tonight into Saturday we will be under the influence of surface
high pressure, and high temperatures will be near to slightly
above normal for this time of year on Saturday.

The brief warmup will quickly be forgotten as a strong cold front
surges through our area Saturday night ushering in an arctic air
mass for Sunday into Monday. The front will have little moisture to
work with, but we will likely see some showers quickly changing over
to snow showers and flurries Saturday night as the front moves
through, with snow showers and flurries lingering into Sunday
especially over the normally favored higher elevation areas as the
northwest flow and cold advection continues.  Right now, it appears
any snow accumulations will be most likely over the higher
elevations of the E TN mountains and SW VA as is typical in these
scenarios. Current ensemble data suggests a very low chance (around
10-30%) of exceeding one inch of snow even in these favored areas,
but think this is underdone at this point and as more hi-res
guidance is incorporated these probabilities will likely increase.
Even so, any snow accumulations are expected to be light.

The bigger story for most folks will be the cold. High temperatures
Sunday daytime temperatures will generally be near or below freezing
even in valley areas, and the wind will make it feel even colder.
Lows Sunday night will be in the single digits and teens, and while
it currently appears winds will be on the decline during Sunday
night which will suppress what could be even worse wind chills,
still wind chill values in the single digits will be common in the
valleys with below zero values for the higher mountains at times
Sunday night into Monday morning. It is still unclear how much if
any of the area will dip into cold weather advisory territory Sunday
night into early Monday, but it looks close enough to warrant
continued inclusion in the HWO for now.

The center of surface high pressure will eventually shift to our
east by Tuesday allowing for a gradual warmup to begin, and
temperatures will likely be above normal by Thursday. Both Tuesday
and Wednesday will be dry, but moisture will begin to increase later
in the week. Models are still in poor agreement on exactly when the
next chance for precipitation will arrive, but current ensemble data
supports having chances for rain back in the forecast by Thursday.

 

[Carvers Gap]

This is probably a little bit over done, but wow.  The 18z GFS puts down 2-3" on Sunday for TRI and the foothills.  I would not be surprised to see more areas which see at least some snow in the air - maybe not even seen on radar.

[Carvers Gap]

18z suite commentary...I will just update this post.  

18z GFS:  At 162, it looks like it is about to unload some cold air into the Lower 48.  The trough in the PNW is less, and the BN heights over northern Canada look to be rotating southward.

At 183, this looks like it is gonna send it all???