December 2025 Short/Medium Range Forecast Thread

[housemtnTN]

Medium Flurry, dime size flakes in Mascot, TN about 10 mins ago.

[Carvers Gap]

The 18z RGEM has company…the 12z NAM.  Both are at range so the usual caveats apply.  The 18z GFS wasn’t too far off.

[Carvers Gap]

Haha.  If u want to see a wild swing, and it has been going on for a bit…look at 360 on the GFS at 18z and then flip back and look at 12z (temp anomalies).  We are talking 40-60 degree swings!  Good times.

[Carvers Gap]

The 18z GFS shows one of two extremes which have been modeled in the past 36 hours.  That is a crazy cold run.  Real feels have ranged from the 70s to below zero with these swings - for the same hour!!!

[midwoodian]

I created a Winter 25/26 general obs thread in case anyone sees some snow this AM 
[mention=10790]Knoxtron[/mention] and [mention=499]John1122[/mention] I'm staring at y'all with jealousy. 

Heavy flurries in Signal Mountain this afternoon


.

[Carvers Gap]

The last four runs of the GFS for Dec 12-17th.  

 

[Carvers Gap]

The 18z AIFS has 2-3 very strong cold fronts w/ the anafront ending the run.

[Carvers Gap]

[Carvers Gap]

The 0z 3kNAM and 0z RGEM show frozen precip over NE TN and SW VA over the weekend.  Just something to keep an eye on....

[John1122]

The UKIE, GGEM, and ICON show a winter event of varying degrees in the area. The Canadian would be a major winter storm with significant ice along the 40 corridor from the mid-state to the mountains. Some snow on top of the ice across areas north of 40. UKIE looks like a long duration sleet event. The ICON is a 1-2 inch snow north of 40.

[John1122]

Euro is just a little too warm for most of Tennessee heading into the weekend, with very light precip.  After, it gets cold, but barely any qpf, looks like warmth and moisture return together mid-month but that may be a head fake too. 

[Holston_River_Rambler]

Overnight ensembles are interested wrt wintry weather in the period around Dec 11-12. As many changes as we've seen with this upcoming system overnight Thursday into Friday AM, will be interesting to see how the 11-12th evolves over the next 5 days or so. 

[Carvers Gap]

The good thing that I see with regards to the mid-month ridge rolling through is that it is temporary on all ensembles this morning.  It is always in the realm of possibility that the ridge is a pattern change, but for now it looks transient at best(3-4 days at the longest).  That ridge, as long as it rolls through, could very well set the stage for the coldest air of the season(so far) to fill that trough behind it.  There are pretty strong mechanisms in place to deliver cold air d10-15.

[Carvers Gap]

No wonder models are all over the place....

 

[Carvers Gap]

JB mentioned that all of the predicted stalls of the MJO have not occurred yet.  He said modeling is having to adjust to the idea that the MJO is just continuing on around.  Now, FTR I am not opposed to the Euro idea of a stalled MJO in 8-1!   I really don't know honestly.  I do think the MJO is going to reload though and rotate around once again.  I found the CFSv2 seasonal this AM to be reasonable for DJF.  This fits with a weak Nina.  I have noticed multiple models w/ the NAO beginning to fire.

To me this implies that cold drops into the Plains, very cold air at times.  Then, the cold pushes south eastward.  Basically this goes  in sequence....cold -> thaw -> very cold.

[Carvers Gap]

And that CFSv2 at 6z sure looks a lot like the GEFS ext last night for December.

[Carvers Gap]

We are close to having to start a thread for this weekend.  The 12z RGEM has ice over much of the eastern valley.  The 12z GFS is less formidable, but has snow for SW VA and NE TN.   The 12k(edit) NAM has snow over the Plateau and SW VA/E KY.  Very close call, and getting dicier with each run.

[Carvers Gap]

Let's try this one more time...the 12z GFS has a second wave of frozen precip Monday for areas along the northern Tenn border, KY, and SW VA.  The 0z CMC had this as well - waiting for its 12z run.

[Carvers Gap]

Very close to a Nor'Easter w/ the 8-10th system.  The ICON just about pulled it off.  The 12z GFS pops a coastal over eastern NC.  

[Holston_River_Rambler]

NAM looks to have come in a bit colder at 12z with a quicker precip onset for Thursday night. I'm honestly having a hard time believing what the RGEM is throwing out for MBY, but as Carver's will tell us, one disregards the RGEM for winter events at one's peril. 

I do like the RGEM, but:

 

 

If there's a warm nose coming up I-75, I usually get it too. 

[Daniel Boone]

Just now, Carvers Gap said:

The 12z CMC brings a second wave of frozen precip on Sunday.  The GFS is oh so close with that second wave.

Could be a rather long duration Event, combining the 2. Not saying anything like what I'm bringing up is possible this time but, for whatever reason made me think of one of Knoxvilles big, long duration Snowstorm Dec. 3-6 I believe way back in the late 1800's I read about. I think 1873. Would have to look it up. It was nearly 2 Feet ! I've often wondered if it was a product of overrunning or System or multiple Systems.

[Holston_River_Rambler]

2 minutes ago, Carvers Gap said:

Very close to a Nor'Easter w/ the 8-10th system.  The ICON just about pulled it off.  The 12z GFS pops a coastal over eastern NC.  

Based on what has happened with the Thursday night system, I think the 8-10th one is going to produce some snow from someone along and north of I-40, location in TN, NC, or VA to be determined

 

Trend on the GFS for Thursday night:

[Carvers Gap]

@Holston_River_Rambler, I do think the RGEM looks a bit too cold.  Time of day is key I "think."  For ice in the eastern valley, the cold air has to get trapped.  Usually for that to happen, we need a clear sky to start the evening and temps drop.  Then cloud cover slides over between 2-4AM and traps the cold air in the valley.  I just am not sure.  I do think SW VA looks prime for good snow as is SE KY and prob the Plateau if the precip can make it back that far.  NE TN is a crap shoot.  What interests me is the slp popping to the SE.

[Carvers Gap]

1 minute ago, Daniel Boone said:

Could be a rather long duration Event, combining the 2. Not saying anything like what I'm bringing up is possible this time but, for whatever reason made me think of one of Knoxvilles big, long duration Snowstorm Dec. 3-6 I believe way back in the late 1800's I read about. I think 1873. Would have to look it up. It was nearly 2 Feet ! I've often wondered if it was a product of overrunning or System or multiple Systems.

I have hated to even say it out loud, but that scenario has been on modeling for days...just a bit to our SE.  Now, things are trending a bit northwest.   It is a narrow boundary for that to occur and a bit early in the season for the valleys, but I could see places above say 1800' scoring with this.