Carvers Gap

Not sure there will be a ton of snowpack. It will depend on that first wave. If the first wave can deliver, many of us will see some snow before the ice. But for now(and I mean this exact moment at 11:44AM on Jan21), ice is the story IMHO. When you first watch the GEM, it looks remarkably similar to the GFS. The second wave is where they differ. As Jeff noted, reality is probable somewhere in the middle between the GEM and GFS.

12z GEM and GFS maps.

The 12z GEM is also a two wave event. It is a tick flatter with the first wave, and then similar w/ the second w/ the cutter. True over running event as well.

This could easily be just ebb and low the model...but the onset for the GEM has more snow north of I-40. The GEM is for sure south by a tick until it cuts after hour 87.

Strong rates falling into below freezing temps is not without precedent.

The 12z GEM is an ice storm for the eastern 2/3 of the forum. I have barely looked at the synoptics as I was watching the GFS.

So, snow w/ the first wave(prior to this slide). Then, it is a classic overrunning event w/ waves just riding the west-east gradient. The last of the energy kicks out and that low in the Panhandle transfers its energy to the low at Hatteras. E TN ends w/ heavy snow. 12-14" of snow for areas about 50 miles north of the Tenn state line - statewide. Has to be crazy amounts of ice south of it. I am only describing the model....I make ZERO claim as to its accuracy and not sure I even agree w/ it. However, that is a plausible scenario if the low doesn't consolidate and the high is strong. I continue to struggle to believe that it is going to rain w/ a 1050 hp leading this off...but I know it can happened. IF that is the likely evolution of the system...just about zero chance this is even remotely nailed down.

Haha. An inland runner out of all of that.

And now we wait for what appears to be an inland runner at 108. Absolutely no way any model would have this worked out quiet yet...if that is even real. The 12z ICON is very similar to the GFS FWIW.

Haha. So much fun. At 102, there is the ICON's Panhandle slp w/ frozen precip over the forum area...ice to sleet to snow the further one heads to NW TN.

Whew, dog....would I hate to have to be making a forecast for this. At 99, there is a slp sitting right over Hatteras w/ precip strung back to New Mexico.

The 12z GFS is a true over-running event. First wave rolls in flat w/ energy being held back over Texas. Next wave should be inbound in a sec...likely ice.

12z GFS is snow for the northern 2/3 of the forum for the first wave. @tnweathernut, that sure looks like it is going to string out into two waves...or more.

The 12z GFS is rolling....

The 12z ICON was significantly weaker (compared to 0z) and w/ less precip.

It wouldn't be the first time w/ an overrunning event - a true over-running event. I think you noted that often precip will get out front w overrunning, there is a pause, and the rest comes out. WPC said it best when it alluded to the illusion of consensus.... Looks to me like a thump of snow, sleet transitions, ice(a lot of it), and then rain.

The 12z ICON has a low in the Panhandle at 96 w/ a faint reflection west of the Apps. Looks icy...very icy for most of the forum area. It does try to form a Miller A, and that must be watched w/ a slp in the Panhandle, but it just isn't organized enough. Not a textbook cut, but some energy transfer is evident. Main low stays along the Panhandle into souther Georgia. The reflection into the Tennessee Valley causes thermals to get wrecked...and it produces ice.

The 12z ICON has ice into southern MS.

Take a look at the 12z NAM vs GEM after 72. NAM is amped. RGEM is much flatter. Again, short range models at range...buyer be warned.

12z RGEM at range. It will often give us clues for the upcoming GEM run. For posterity and the eventual forensics investigation into this...If I hadn't seen 0z last night, I would say is that is going to be a severe ice storm if that low kicks out late or a snowstorm if it goes with that 1050 high. Usually, 1050 highs are money for frozen in this forum. However, there is definite precedent for it to rain behind big highs like that, but still is crazy to watch.

Yeah, this winter Bam has flip-flopped more than a mackerel that just got pulled into the boat. But a broken clock is right twice a day....I think he went warm for the second half of January, and then walked it back the next day. I don't understand the fascination w/ their stuff. There are so many other great mets and met resources which don't push clicks. That is all from me on that subject. The GEM-para nailed this trend. It has been good this winter. The WPC post above is really well written. It explains some of the issues they are dealing with, and some of that we actually discussed here - I hadn't read that until about ten mins ago. Just been doing this hobby long enough to know the pitfalls w/ stuff like this. I think ultimately the mishandling of the Baja low was yet again a problem. What I will be interested in...does a cutter now set the table for the next system? If that verifies, it will change the next 5-7 days worth of tracks for vortices traversing the country.

This is Wall's X post, but cleaner and easier to read. This is the most recent WPC discussion about interaction between the northern and southern branches, the NBM being "off", and potential adjustments north(with the snow line) in their forecasts. Model guidance generally remains in good agreement with the aforementioned large scale pattern evolution, however differences in the details persist. The most notable question deals with how much interaction we see between the southern stream and northern stream energy over the Plains, and the overall depth/strength of the resultant trough. More interaction and a stronger trough would likely result in stronger downstream ridging and thus a farther north storm track. This is indeed what just about all the 00z models are trending towards. Generally a clean sweep, with the deterministic physics based models, ensemble means, and AI guidance all shifting north with the storm track. This trend really has little to no impact on the fact that a widespread and significant winter storm is going to occur...but it does play a big factor in exactly where the rain/ice/snow lines set up. So how confident are we in this trend, and will it continue or revert back to earlier runs, is the big question. Given the nearly unanimous trend in this direction, and at least a couple model runs in a row showing such a trend, it does seem like this shift has some merit. However, overall run to run consistency has shown pretty large swings with the exact handling of both the southern and northern stream energy. Suspect that the exact details of these features is far from settled upon...especially the northern stream energy which is currently strung out across much of western Canada into the arctic. Thus tend to think that the unanimous model trend could be a slight mirage and suggests more confidence in a solution than there actually is at this point. Either way, it should be stressed that in just about every outcome we get a widespread and major winter storm with many areas getting significant impacts regardless of these exact details. These details are, however, important for exactly where the maximum snow and ice totals occur. The current WPC QPF and temperatures were derived before much of this 00z guidance was available, and thus is a bit south of the new consensus. As described above, no guarantee we dont see a shift back south in later models...but assuming some persistence in the 06z/12z models then the WPC update today would likely shift north to at least some extent. Behind this system the general troughing in the east and ridging in the west will persist. There is quite a bit of run to run variability with the shortwave details which does result in quite a large spread in the max/min temperature forecast across the Plains. The NBM was in line with the model means, so we did not make any changes. However, it should be noted that we will likely have some larger than normal errors...just hard to stay whether colder or warmer wins out along this ridge/trough interface.

More likely than not...the GFS is probably mishandling the timing of the Baja low. However, none of the other models have been stellar with that area either. With one big camp calling for a lot of energy to go west of the Apps....tough for me to go against that camp. Probably have an increasingly diminishing windrow through Weds for this to adjust some. I really doubt it goes back to big snow, but hope I am wrong. The GFS standing alone(maybe some RFS and AIGFS support) rarely works out for me. I do wonder if the Euro is too amped as you noted above w/ some of its recent trends with other systems.

Yes. It has done that with many systems this winter. I don't know if there is a window where data is missing(systems going through blind spots like the Arctic Circle or eastern Pac) or if that is just a bias. OTH, the GFS has had big time problems w/ Baja energy this winter. At one point, it tried to tap a Pacific tropical system and funnel that moisture into E TN. But with trends across several models...sure looks like a warmer system(compared to yesterday's runs) is likely.

I should add that I would be shocked w/ a reversal. Just too many models showing the trend towards a warmer solution. However, with the Euro changing hp strength in the Plains...that could greatly affect where the phased system tracks. If the hp dives behind the system, it cuts. If it stays over the top...it might force the bulk of the energy east of the Apps w/ only a shadow pressing west. For E TN...this looks like snow to sleet to ice to rain. For now(like right this minute), it looks like 8-12 hours of frozen precip at TRI. Downslope effects are showing up on some models for sure in the Foothills - so, usual caveats apply. When I see big changes on modeling(MT hp on the 6z Euro), that usually tells me that things are not worked out. The GFS consistency gives me some weak(very weak) hope, and the 6z GEFS snow axis actually shifted south at 6z. It is possible the Euro is too amped....but the AIFS supports the Euro as does the GEM and GEM-para. Let's see what this hurricane hunter data reveals. I think the big error(and this error has been present on modeling all winter long) is trying to hold that Baja low in place too long or string it out. More often than not, that low has been poorly modeled up until about 3-4 days before reality. We have also seen energy transfer from the Rockies back to San Francisco on modeling(not this particular setup to clarify) - didn't happen. The Southwest has been a big problem for modeling this winters. Oddly, that Baja bias has been across modeling. One thing I will add...any time a Baja low is involved, the Euro struggles...until it doesn't. Then, it locks in.