January 7 - 8 ULL potential

[PowellVolz]

Green line is the DP...Red line is the temp...as the line moves up the chart, it's like a weather balloon rising...shows how each layer is responding. Chart on top is 18z...as you follow the green/red line up, they separate (shows the dry air just off the ground). 0z the entire column is moist (green/red line together). The temp at the start of the lines when comparing the two, shows how if we can get the moisture feed right...dynamic cooling (snow melting as it falls, cooling the lower atmosphere near the ground) helping the lower elevation be 4° colder. The 0 line is the dark blue faint line running up the chart at an angle...if the temp line stays below or right at that line on its entire path, then you get snow...if it goes to the right of the 0 line (warmer temps) then you get mix or rain.

With the temps so close wouldn’t a little dry air help us wet bulb?


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[Jed33]

RGEM also coming back some from 18z. Looks better for NE TN. Also better for the plateau and some other areas of eastern Middle TN

[TellicoWx]

6 minutes ago, PowellVolz said:

With the temps so close wouldn’t a little dry air help us wet bulb?


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If the dry air is right at the surface then yes..but on 18z, the bigger separation is in the mid levels..if this was more woundup surface low vs ULL then the available moisture would be more plentiful. Since it's an ULL..we need the mid levels to be as moist as possible at the start. There has been hints at modeling on a slight downslope component as well..that would be another issue the ULL would have to fight off..but that's more track dependent and angle the winds are coming across the mtns from NC side.

[TellicoWx]

3 minutes ago, Jed33 said:

RGEM also coming back some from 18z. Looks better for NE TN.

Yeah, looking at the sounding for here..looks like it has the moisture like the NAM..but it doesn't generate quite the same dynamic cooling right at the valley floor..that 1-2° makes the world of difference for the central/southern valley.

[John1122]

The GFS will probably shift back West some like the NAM did. Though it may wait until tomorrow. 

[Jed33]

3 minutes ago, John1122 said:

The GFS will probably shift back West some like the NAM did. Though it may wait until tomorrow. 

Knowing the GFS, it probably won’t find it tonight. For the Christmas storm, it literally took it till the day before, before it began to understand what was going to happen. Really it was 12z the day of the event before it fully caught on. That model struggles so bad these days!

[TellicoWx]

0z RGEM,...columns moist/ no dry air issue. Difference with the surface temp is the precip rate isn't as high on the RGEM vs NAM (central/southern valley)

[Icy Hot]

Thanks so much. That was really educational!

[John1122]

My RGEM sounding best guess is freezing drizzle/rain though the model output shows about 2 inches of snow in the prior 3 hours. I assume it's the dry air above 700mb causing a lack of moisture in the dendritic growth area (-10c temps)  

 

[John1122]

As expected, the GFS is still very Eastern edge on modeling. Could still be right but it's very much on a island right now. 

[TellicoWx]

I can't figure the GFS out...has almost the same RH/PWAT/UVV as the RGEM/NAM...just never produces the precip

[TellicoWx]

4 minutes ago, John1122 said:

My RGEM sounding best guess is freezing drizzle/rain though the model output shows about 2 inches of snow in the prior 3 hours. I assume it's the dry air above 700mb causing a lack of moisture in the dendritic growth area (-10c temps)  

 

Yeah noticed that too..wonder if it's just due to the snapshot at that particular hour and the dendrite zone is beginning to dry out.

[John1122]

Big run from the Canadian. I do wonder if it's seeing downsloping in NETN. 

[John1122]

30 minutes ago, TellicoWx said:

Yeah noticed that too..wonder if it's just due to the snapshot at that particular hour and the dendrite zone is beginning to dry out.

It's possible but it shows it snowing here for 3 more hours after that. 

[John1122]

UKIE backed off a bit and is still extremely elevation dependent. But it is temp issues and not precip that is the problem on it. 

 

 

[John1122]

MRX put this out around 10pm. 

[John1122]

Euro is doing work on the Plateau and Eastern Rim back towards Nashville. 

[John1122]

Sharp cutoff in the Eastern Valley that run. 

[Holston_River_Rambler]

Looks like the RGEM has decided it doesn't like Sullivan county, but favors Washington and Carter counties:

 

Meanwhile the 12km NAM seems to have decided to just do its own thing and keeps the snow going for most of its run from Nashville east:

You be you NAM:

 

3km NAM has some nice bands:

 

 

Here is the RAP, for fun, since we are entering its long range now:

 

It will be interesting to see what synoptic evolution the WRF-ARW core shows when it gets in range later this evening. That core has been spitting out some big solutions on the SREFs

[Coach B]

I like that the 12k and 3k NAM as well as the RGEM are actually bringing some accums to even some non Plateau areas back into middle TN. This one is going to be close. Pretty good spread on the boom or bust potential for the southeastern half of mid TN and the valley locations in east TN. 

[Holston_River_Rambler]

6z Euro:

 

[Holston_River_Rambler]

Here is also how the 6z Euro sees it unfolding:

[Holston_River_Rambler]

Ye Olden NAM suite looks a little south at 12z. I, of course, like it, on account of the southern Morgan County dollop. 

 

It's track looks like just a classic one for TN, but with an air mass just a little too marginal for everyone.

[tnweathernut]

At this point, modeling seems to indicate an opportunity for snow winners, but keeps moving the goal posts around from model to model and from run to run.  My guess is (outside of the mountains) we will just end up having to nowcast this one to determine where the lollipops of snow will end up falling.  There's a reason they say, upper level low......... weather man's woe.

[fountainguy97]

The 15z RAP is just absolutely absurd.  Dumping wet paste like crazy.  We know it wont verify but wow check it out.

 

[Math/Met]

I'm not suggestion that the RAP will be correct, but it has a more defined inverted trough centered over the mountains in the early part of that loop. There are many factors to consider, but that's what caught my attention. If that happens, then that could be a major help for some areas.

[Windspeed]

Model evolution / trends over the past 12 hours are definitely swinging towards a major snowstorm event for the eastern half of the Valley. Also extreme SWVA to Roanoke looking better to avoid a potential sharp cuttoff around KTRI. I suppose I am cautiously optimistic now. See if this can hold serve on the next few runs to gain confidence.

 

[Daniel Boone]

Riding the Rap...maybe my possible big dog bold prediction trigger will be pulled shortly, lol

[Carvers Gap]

Just wanted to stop in and say that the the gif work land map sharing in this sub-forum is outstanding.   

[John1122]

This one will either end power for some or go down as a moderate to large bust. Maybe even both. That is the nature of ULL snows and their path will likely not be correctly modeled even going off short term now cast models like the HRRR and RAP for more than a few hours in advance. In this scenario bet on elevations but expect your bet to possibly bust wildly even in valley floors.