Carvers Gap

MRX morning discussion.... Key Messages: 1. Extreme cold that has not been seen in this area for decades will build into the area on Thursday night and continue through Saturday. 2. Expected impacts from this extreme event will be gusty winds that may cause power outages, potentially hazardous road conditions from snow/ice, and dangerous wind chills. The arrival of an extremely cold air mass continues to be the big story for this period. From 00Z Fri to 12Z Fri, most of the area will see around a 30-40 degree drop in temperatures. The cold front will move through quickly, from around midnight to 4 AM, and dry air aloft builds in a few hours after surface fropa. So there will only be a small window when there will be potential for snow accumulation, which appears to be from 06Z to 12Z. For the most part, the SREF and NBM members have mostly settled on snow amounts from about a quarter inch to one inch for the Valley, with up to 2 inches in the higher elevations and SW VA, and the forecast amounts will fall in this range. Outside of some light snow showers or flurries adding a tenth or so in the mountains and SW VA, accumulating snowfall will mostly end by 12Z. As we have been emphasizing the past few days, despite the light snow amounts, travel in the area may be impacted by the rapid drop in temperatures causing any moisture on roads to quickly freeze. Icy road conditions may persist through Friday and potentially through Saturday as well, since max temperatures will be in the teens and 20s. With the frontal passage will come a sharp increase in winds, with sustained winds expected to be in the 15-30 mph range with gusts in the 40s, and near 50 mph in the mountains. A concern with these winds will be the potential for downed trees and power lines leading to power outages when temperatures will be getting extremely cold. Wind chill values on Friday will be below zero for the entire area through Friday and Friday night, and -15 to -25 in the mountains, Cumberland Plateau, and SW VA. Therefore, a Wind Chill Watch will be issued for the entire area from 09Z Fri through 18Z Sat. The TN Valley may end up in a Wind Chill Advisory, but at this point, will have a blanket Watch and let later shifts divide it by topography for an Advisory/Warning. Bottom line, this will be an impactful, extreme cold event that has not been seen in this area for decades. The rest of the forecast period will be dry with a gradual warming trend.

The global ensembles overnight are singing the same song....The Hudson Bay block sags a bit too far south to help at all. At 0z and 6z, I don't see any remarkable cool downs. Again, normal temps will get it done, and it may be a struggle to get those. All of that said, we are ten days from January....LONG way to go.

The 18z GEFS seems to have made a significant move to the 12z CMC at 500mb for the second week of January.

Yeah, it's rare air. I know everyone likes snow, but to get to witness this type of extreme in December is something we don't see every season. I looked at Cut Banks' wind chills and they are showing -50F.

I looked at Sheridan, WY, and they have wind chills head to -40. But -40 on the thermometer w/ wind chills is gonna get crazy.

Glad you mentioned that. Yeah, multiple runs on the GFS have had it. That would be something to behold. I read where several ski slopes are shutting down due to the extreme temps forecast in Montana. Those cats don't close their slopes much due to cold, especially during the holiday season. So, when they are taking precautions due to the cold....legit cold stuff.

18z GFS does not budge.

My yard has been dormant since mid November other than just picking up leftover leaves. After record cold or near record cold temps, I doubt my yard is gonna have much life it in after the extreme cold upcoming.

Interesting to see the mention of ZR in NE TN. The CMC nailed that if right. Otherwise, this is for posterity. MRX afternoon disco: Wendesday night into Thursday morning there is a possibility for a brief window of freezing rain in the far northeastern TN counties and SW VA as isentropic ascent leads to precip spreading back into the area from the south and southeast. Not expecting any impacts as precip amounts will be light, and increasing southeasterly flow will also yield some downslope warming. Regarding those winds, some models show 40-50kt flow around the 850mb level, which could yield some high winds in the mountains and foothills. However there`s enough uncertainty here about whether we even reach advisory criteria, that we will hold off on any wind headlines for Wed night into Thu morning. On Thursday a 500 mb low dives through the upper plains and midwest, into the Ohio Valley just north of our area, driving a strong cold front eastward into the region Thu night. More rain will spread into the forecast area ahead of this Thu evening. The front is expected to cross our area quickly during the overnight hours, between 06Z and 09Z. Temperatures will drop abruptly - and I mean very abruptly - immediately following this frontal passage, causing rain to transition to snow. This will occur between midnight and 3-4 am Friday morning, with temperatures falling through the day. Friday high temperatures will likely occur at or just after midnight, and temps will drop into the teens and single digits by the time you are waking up on Friday. Models suggesting anomalous 850 temps ranging from -25C to -27C. These dangerously cold temperatures will persist through the weekend and are not expected to climb above freezing again until Monday afternoon. In regards to snow amounts: only light amounts are anticipated, trace to 1" in most places, higher in mountains. Despite the minimal snow accumulations expected, the concern continues to be any precipitation or pre-existing rain on the roads freezing to the roadways which would be supported by the incredibly low temps. The frigid air fills in behind the front and will squeeze out any last precip, so isolated to scattered flurries and snow showers remain for Friday. Storm total snow is only a few hundredths of an inch across the valley, with higher totals around 0.75-1.4" in SW VA and in the mountains Late Thursday night and into Friday a Wind Advisory may be necessary as forecast wind gusts are expected to be 20-30+kts in the valley, and 25-35+ kts in the higher elevations. Piggy backing off of the that, another main concern with this system is the subsequent dangerously low wind chills due to the bitter cold and elevated wind speeds. Forecast wind chills on Friday are as low as -10 and even -20 in some higher elevations, which may also warrant a Wind Chill Advisory, and possibly a Wind Chill Warning beginning Friday morning. Will leave that to future shifts. To summarize: our main message continues to be the threat of dangerously cold temperatures and wind chills, as well as the potential for travel impacts following the cold frontal passage and arrival of frigid air Thursday overnight.

Winds 40+mph, wind chills -10F to -20F, light snow. That is winter at its best for December. It's beginning to feel a lot like Christmas....or Fairbanks, Alaska.

Since we have a thread for the last seven days of the month, I have started a January thread as we are getting close enough to actually see it on ten day forecasts tomorrow.

Looks like the first ten days or so will be warm, but that is not a lock. After three years of La Nina, we are probably closing-in on our last few months of this ENSO phase. La Nina's often yield warm Januarys, but something makes me think we buck the trend this month for at least part of the month. I have January as AN in my seasonal forecast, but a portion of the last 20 days of the month could be quite cold. I still expect some wild swings this winter w/ a base warm pattern and very cold interludes.

Looking down the road....The system on the 27th looks to close out the current cold cycle. The cold has been spent and the Pacific is not going to yield a PNA/EPO pattern without a fight (as models once hinted they would). In fact a stubborn GOA low is trying to set up shop. To me it looks like the pattern will be anomalously warm from December 28th to roughly January 10th. There are some signs that is a new pattern - cold in West and SE ridge hooked into HB block. That said, we have seen that error during late fall and early winter on more than one occasion. So, I am not ready to go there quite yet. The 12z GEPS is only slightly budging when it comes to keeping a trough in the East. But it has budged enough for me to think the Canadian ensemble is in error. The culprit? You know when we start talking about the MJO warmth is on the way. The MJO stalls like it is hitting a brick wall between phases 7 and 8. Phase 7 during December will work, but not January. I am nearly certain the cold equatorial/dateline Pacific SSTs are stalling the MJO waves from reaching prime longitude. There are some hints in LR modeling that the cold will return quite harshly after the 10th, but add a few days at least to account for can kicking.

The 12z Euro just about got to 12z CMC levels at 500 where it turned neutral and then negative. Something to watch for sure. I think the system on the 23rd being weaker has allowed room for the development of a system on its heals.....the gradient is still in place versus being sharpened up.

The Euro at 168 is much more potent on the 500vort map. That has trended much stronger though the surface doesn't reflect that yet.

Yes. Which is why the second system on the 27th makes sense.

Which to me means we are tracking an ana front as John noted and not a Plains/Midwest blizzard. I think the Siberian Express is a better term. There is some rotation over the Lakes, but this is a cold front blasting through. That changes things some. Now, the question is when/where the cold is...in relation to a front and not a strong SLP charging into Illinois from TX which would be cold chasing rain most likely. I like the GFS solution.

Very much agree.

So the 23rd system is now acting like a clipper? (Referencing the comment above). I actually agree with that if so.

The system on the 23rd (in the Midwest and eastern Plains) is a shell of its former modeling self. I am not sure I classify that as a cutter as no energy really cuts. That is a northern stream system which means the problem we fight is that it goes to shallow as at the even nears. This systems has gone from Miller A to hybrid to blizzard cutter to as John notes...almost a clipper (if I am reading that correctly as we are talking two storms now in this thread). As for the second system...not sure. The GFS and CMC handle in a completely different manner...one is a clipper and one is a Miller A. For the sake of clarity, I probably will move future comments about the "maybe" system on the 27th back to the pattern discussion thread. If it becomes anything, might worth a thread of its own.

One thing I am noticing is that the actual cutter is barely a cutter. I could be wrong(wouldn't be the first or last times), but it looks like a northern system w/ a cold front. The snow amounts in the Mid-west and eastern Plains have diminished remarkably. Not sure that changes the overall solution, but that is looking like an error(too strong) by all global models.

While modeling has struggled w slo placement, they have been incredibly accurate from d10+ re: temps.

It is important to remember that wx models are tools much in the same way carpenter’s use tools such as hammers or squares or levels. Whether they are used accurately is up to us. On to the ex...The 6z Gfs isn’t budging and is a true anafront where - the precip is behind the front. While I don’t trust that model, the Euri struggled with this setup a couple of years ago if I remember correctly. I don’t trust it either. think part of the issue is tracking storms from ten days out. There are 40 different runs(one just one operational model) but ultimately one verified reality. I think a blend of modeling is not a terrible idea at this point.

The CMC has been verifying higher than the GFS and Euro at d10+. Several times it has had a correlation coefficient of 0,8 or more.

The CMC has a major winter storm in the 27th. Modeling is all over the place with that system. Something to watch,