Carvers Gap

The 12z GFS is exhibit A in what happens when the trough doesn’t feedback over the NW.

The GFS has had a sensitive habit of being able to identify cold before other models. It is absolutely lit at 12z.

The 12z GFS again almost wipes out the weeks long AN temps regime. For portions of E TN, we are normal to below for the last ten days of the run!

Dabbling a bit w/ models this morning, the 6z GFS comes pretty close to erasing the standing ridge at the surface. One cold front after another. The Euro and CMC are less enthused, but as we've seen, that can change. The interesting thing this AM is a the TPV paying a visit to New England, and we get a backdoor cold front from it. That has been on several model runs. Been a while since we have seen an air mass strong enough to seep down the western slopes of the Apps(from the East).

I mean this is wild. That is a 5 day map. PNA ridge. EPO ridge. Alaska block. Low east of Hawaii. That is a snowstorm setup. Yes, it is a deterministic run at 300+ AND ensembles have yet to flip....but that is quite the opposite of what has been shown for the past few days. When I see an ensemble finally flip(and they will take some time if they are going to do it), then we know feedback(infinite loops) were at fault here. Let's see if the trend(which began a couple of days ago) ends with a decent pattern. No promises, but fun and interesting to track. Are models finally starting to feel the MJO? Maybe.

And a nice little inland runner snowstorm where that chinook influenced SER has been forecast to pop. I may be in the minority, but I really don't see the warm-up as a phase 6 SER. It is more like a chinook getting pumped by feedback over the NW. Get those lows moving more progressively, and the chinook dissipates as they pass to the east.

We have below zero wind chills in the middle of the warm-up. If that is the warm-up, I would sure hate to see what a cold snap would look like in January w/ that cold air mass parked in Canada.

And as we see the feedback over the NW resolved....modeling begins to look like an MJO on the left side of the plot. Those constant spinning vortices are replaced by more reasonable solutions. That allows multiple cold fronts to march eastward. I don't think it will be wall-to-wall cold like the first 15 days of December, but we could end with a back and forth pattern which is more amplified.

RIP to the southern standing wave on the 18z GFS. We hardly every knew ye.

Here is an article about atmospheric river events. During the 2022-2-2023 La Nina, California experienced nine atmospheric river events. Mammoth got absolutely buried with snow. I thought for sure ARs were Nino driven, but new research suggests it actually has less connection than I thought. An atmospheric river event can actually overwhelm either ENSO signal regardless. https://scripps.ucsd.edu/news/atmospheric-rivers-explain-atypical-el-nino-and-la-nina-years

Middle and Western forum areas definitely do better with a good PAC. The Atlantic helps us with inland runners. We can also do well with a good PAC in NE TN…but many good winters IMBY have good NAO setups, even if briefly. In some ways, TRI scores when DC does.

Definitely good trends here. If the Pac is gonna be taking its time getting its act together, we can try the Atlantic. With the QBO negative and the SSW, a strong NAO is possible and even likely. Keep in mind that -NAOs are often not forecast well in advance. They can upend LR modeling in a New York minute.

Area Forecast Discussion National Weather Service Morristown TN 154 PM EST Sat Dec 13 2025 ...New DISCUSSION, AVIATION... .KEY MESSAGES... Updated at 1236 PM EST Sat Dec 13 2025 - Light snow accumulations expected across the high terrain of East Tennessee and southwest Virginia overnight into Sunday morning. - Bitter cold will surge into the area for Sunday through Monday morning. Please see the Cold Weather Advisories for additional details. - Warming trend begins Tuesday with above normal temperatures by mid to late week. Conditions will be dry during this time. && .DISCUSSION... Issued at 1236 PM EST Sat Dec 13 2025 A broad upper level trough remains in place over much of the central and eastern CONUS. Shortwave energy translating through the synoptic feature tonight into Sunday will sharpen the troughing as a surface front progresses through the region. Some light ra/sn will occur during the overnight hours as this energy translates through the central and southern Appalachians, however, the greatest probabilities for light snow accumulation will be limited to higher terrain of the East Tennessee mountains and southwest Virginia. Overall, moisture in the DGZ is lacking and better northwest flow does not initiate until the bulk of the moisture is departing the region. Will cover this with an SPS for now, ending it at midnight to allow for the next forecast desk to make any additional considerations. The main concerns will be well below normal temperatures advecting into the region as winds veer NNWly and become gusty behind frontal passage. Temperatures around 20 degrees below normal are largely expected across the region Sunday into Sunday night. A Cold Weather Advisory has been issued for the Cumberland Plateau, southwest Virginia, and our East Tennessee mountain zones starting around sunrise tomorrow morning. The gusty winds and well below normal temperatures will result in wind chill values around zero, to as low as 10 below zero in the highest terrain of the mountains. By late Sunday afternoon, the remainder of the valley locations are introduced to the Advisory as cold air continues to filter into the region. Overnight lows in the low teens to single digits will be widespread. Many valley locations will see wind chills in the single digits to near zero. Cold Weather Advisory products will come to an end shortly after daybreak Monday, when a gradual warming trend is expected into the mid-week as the anomalous troughing ejects northeast and H5 heights slowly recover. For Wednesday night through Friday, the main storm track will be across the northern third of the nation. The fast but more zonal flow will quickly move a series of short-waves with associated frontal boundaries moving across the region. Ensemble cluster analysis shows differences in timing of systems but overall increasing chances of precipitation are expected towards the latter half of the week.

The CFS forecast has been a lot different than any other MJO forecast. Is it an outlier, but also correct?

I really hate having to dig through MJO data - yuck. Anyway, I read the MJO report from the 8th by CPC. So, I think I know how to read the following map. The orange is suppressed convection. Now, it is important to remember there is almost always convection at this time of year over the MC, and I think that is a mistake some(not in this forum!) are making. You can have convection, but it be suppressed IMHO. That simply means less than the norm. I don't "think" phase 8 and phase 5 have a one-size-fits all scale for convection. I would guess the MC has more convection, ie it could still have convection but less than the norm. The enhanced(more than normal) convection is blue...and that is phase 8. It isn't moving. In fact, you can see some convection firing in east Africa which is 1-2. What I want to know on the CHI plot is which color is enhanced and which is suppressed. I need to get that straightened out so that I can compare the two. @jaxjagman