December 8-10 Storm Discussion

[Carvers Gap]

Well, the Euro stopped the southward jog. W the 10" line, it moved a layer of counties to the northwest.  So, basically all models jogged north at 12z - I think.  Big improvements over SW VA and NE TN.

 

[nrgjeff]

The ECMWF, ICON, and GFS-Para all halted the poor overnight trends.

Preliminary only: I'm going 1-3 inches from northwest TN, across KY/TN border and 1-2 counties each side. Upper Plateau (Tenn) Southeast KY into TRI could get 2-6 inches but the temp profile still, could be less could be much more. Southwest VA 4-8 inches seems reasonable. Of course the mountains will be hammered; 10-15 seems reasonable, esp NC side. It really is about elevation, as usual before Dec. 15.

Likely be a little ice during transition(s) to snow. Some risk of wet snow on top of ice. However it does not look awful, nor like a snow show stopper (where it snows).

This is my last forecast post of the event, with a busy weekend at home, but I will keep reading. Good luck to All! 

I really think it'll turn out OK despite the NAM and GFS-Op.

[Carvers Gap]

Here is the clown map.

 

[Blue Ridge]

Here is the clown map.

 

Cut in half and that’s a solid 4-6” event across NE TN. That fits well with my current thinking.

[Windspeed]

Not surprised at the NW back building in the 12z ECMWF. At any rate, we're in hirez meso range. Global modeling at this point probably won't change or matter much with the 850-600 mb level weakness against the HP within 24 hours of event in progress. It's hirez meso time from this point forward folks. Good luck to everyone! Hope this event verifies for everyone and the sharp cutoff stays NW enough to produce for those posting/lurking.

[Carvers Gap]

Dec 9th of last year we drove to Roan Mountain, almost a year to the day of this event.  I checked my obs from last year.  They had 10-12".   We went sledding and had a blast.  Another storm on the same date?  Pretty cool.

[AMZ8990]

4 minutes ago, Carvers Gap said:

Dec 9th of last year we drove to Roan Mountain, almost a year to the day of this event.  I checked my obs from last year.  They had 10-12".   We went sledding and had a blast.  Another storm on the same date?  Pretty cool.

    I just made a post in the long range thread that kinda falls under this category.  Seems like we’ve see a trend with these systems over the past year or so.  

[Blue Ridge]

On a somewhat lighthearted note, who would like to place a friendly (hypothetical) bet on what MRX does during the PM update? I see three possible scenarios:

1) Hold serve with watches, no new areas or upgrades. (IE, passing the buck to night shift.)
2) Overcorrect based upon 0z and 6z suites, cutting all but the mountains out of the WSW area and giving the valley a WWA.
3) Push the chips all in, with WSWs extending several counties west of the current watch area. 12-16” in the mountains, 6-12” in TRI, 3-6” toward TYS. John doesn’t receive so much as a WWA, all but guaranteeing warning criteria snowfall in his backyard.

[TellicoWx]

1 minute ago, Blue Ridge said:

On a somewhat lighthearted note, who would like to place a friendly (hypothetical) bet on what MRX does during the PM update? I see three possible scenarios:

1) Hold serve with watches, no new areas or upgrades. (IE, passing the buck to night shift.)
2) Overcorrect based upon 0z and 6z suites, cutting all but the mountains out of the WSW area and giving the valley a WWA.
3) Push the chips all in, with WSWs extending several counties west of the current watch area. 12-16” in the mountains, 6-12” in TRI, 3-6” toward TYS. John doesn’t receive so much as a WWA, all but guaranteeing warning criteria snowfall in his backyard.
 

1...except upgrade current watch w/o extending it lol

[Blue Ridge]

1...except upgrade current watch w/o extending it lol

No fun, that’s the obvious choice. Lol.

[BlunderStorm]

8 minutes ago, Blue Ridge said:

On a somewhat lighthearted note, who would like to place a friendly (hypothetical) bet on what MRX does during the PM update? I see three possible scenarios:

1) Hold serve with watches, no new areas or upgrades. (IE, passing the buck to night shift.)
2) Overcorrect based upon 0z and 6z suites, cutting all but the mountains out of the WSW area and giving the valley a WWA.
3) Push the chips all in, with WSWs extending several counties west of the current watch area. 12-16” in the mountains, 6-12” in TRI, 3-6” toward TYS. John doesn’t receive so much as a WWA, all but guaranteeing warning criteria snowfall in his backyard.
 

1) with the exception of warnings being declared in NE TN mountain counties.

[TellicoWx]

12z EPS 

[TellicoWx]

Kind of crazy to have this many different outcomes this close to an event lol

[Blue Ridge]

I caution against placing much weight in the EPS or other ensembles at this stage. As@windspeed noted above, we are moving out of the wheelhouse of global models - ensembles in particular - and entering meso-model and nowcast time.

[Holston_River_Rambler]

I'll take #1 @Blue Ridge

extra points if the phrases: "poor run-to-run consistency" or "conferring with surrounding offices" or "hard to predict deformation axis" are included in the write up

[TellicoWx]

Ensembles lose wieght at this range, due to the general area doesn't change as much and we can look at the meso models to pinpoint more localized points within that area...members can't even figure where that area is within 48 hrs.

[EastKnox]

3 minutes ago, TellicoWx said:

Ensembles lose wieght at this range, due to the general area doesn't change as much and we can look at the meso models to pinpoint more localized points within that area...members can't even figure where that area is within 48 hrs.

Are we in no mans land at the moment?  The snowfall I'm looking at is happening at 48-72 hours and many agree that the mesoscale models at that range are unreliable.

[Carvers Gap]

Basically, I use ensembles to see if the op matches up or is close.  If the operational is an outlier to its own suite, then you have to decide if it is leading the way or needs to be tossed.  I usually look to see if it makes sense and that it has support from other models.  I use global models all of the way up to the event.  Just look for trends.  The short range models just give better resolution and focus and allow for accurate forecasts in tighter grids...less smoothing at low resolution that looks wonky.

[Reb]

NAM coming in much wetter than 12z

[Windspeed]

8 minutes ago, Reb said:

NAM coming in much wetter than 12z

Much wetter with a stronger and more pronounced surface low over the panhandle at 42 hr. 850-700 mb weakness is cranking back over ETN by 42 hr and a thumping in progress by 48 hr.

[TellicoWx]

41 minutes ago, EastKnox said:

Are we in no mans land at the moment?  The snowfall I'm looking at is happening at 48-72 hours and many agree that the mesoscale models at that range are unreliable.

Ensembles basically use chaos theory and test it by change one initial very small variable in the atmosphere to see the effect on a system. The wide range on the EPS members show how dynamic and changing this system still is. From ECMWF:

  "In a chaotic system, a slight change in the input conditions can lead to a significant change in the output forecast. In a non-chaotic system, small differences in initial conditions only give small differences in output. Hence, it is important in weather forecasting to investigate how sensitive the atmosphere is at any stage to initial conditions. Ensemble forecasting does this by looking at a spread of possible outcomes."

 

[Runman292]

The 18z NAM.....

 

[TellicoWx]

[John1122]

This happens often. This is from a February storm thread a few years ago. It's 48 hours from the event happening. I think Todd was using the NAM or GFS here. As they often do, models started out showing a state wide event. Then trended east more and more until at 48-72 hours out they had dried out all but extreme eastern sections.

 

This was the RGEM

Ground reality was 8 inches for my area, 6 inches back to Crossville. 8-12 in the Central Valley. This was also the infamous downslope event that stopped snow in NE Tn at around 5.5 inches, but the next morning a secondary low dropped into the area and gave 1-3 more inches of snow to most of East Tennessee. Any of this sound familiar? The biggest difference is that there were no mixing or rain issues. It was much colder and February is more favorable for snow than early December. But the precip field was much larger. Several inches of snow fell in Kentucky during this event as well.

[Blue Ridge]

SHORT TERM...(Tonight and Saturday)...

Mostly cloudy to overcast skies will continue to fill in and lower

in height as we move into the weekend. Precipitation will begin to

move into the Chattanooga area tonight and continue to spread

northeastward across the Valley. We will likely see mostly rain in

the Valley locations early tomorrow, but temperatures around the

800-900 mb range will be near freezing... This means we could see

a wintry mix of rain, sleet, and possibly snow in some Valley

locations for a brief period. Any winter precipitation would be

very light and accumulation should be minimal (if any occur).

Higher up in the mountains temperatures should remain low enough

that any precipitation falls in the form of mainly snow. In the

higher terrain we could see some accumulations beginning overnight

into tomorrow morning.

 

During the daytime hours tomorrow, temperatures will warm up into

the upper 30`s in the Valley, but the mountainous areas will stay

closer to freezing, leading to more winter weather through most

of the day.

 

.LONG TERM...(Saturday night through Friday)...

Difficult forecast this afternoon in regard to potential wintry

precipitation this weekend and early next week. Different Model

solutions and Model inconsistency creates a low confidence forecast.

Lets begin...

 

For Saturday night...upper trough moves into the southern

Appalachians. The NAM has the strongest jet dynamic forcing over the

forecast area with the GFS showing the best dynamics over western

North Caroline. These differences are a key why the vertical

temperature profile (850mb temperatures) are much cooler for the NAM

than the GFS, thus producing more snowfall, especially in the valley.

 

After coordination with WPC and surrounding WFOs, will trend with

highest confidence and amounts over the far eastern Tennessee

Mountains where we will upgrade to a winter storm warning. For the

southwest Virginia, northeast Tennessee and Plateau, will

continue with near warning criteria snowfall amounts up to 4-5

inches, but confidence is too low to upgrade and will continue a

watch there.

 

For the central valley, a trace to 2 inches, higher ridges, are

expected.

 

for Sunday, wintry mix will continue in the morning, but diminishes

in the afternoon to mainly drizzle, except snow showers/flurries

higher elevations.

 

For Sunday night and Monday, again differences in the models in how

much precipitation will be around for the southern valley and

southwest North Carolina with upper trough axis. Rain/snow showers

or flurries is expected. Will need to watch this potential closer

later for later this weekend.

 

[Runman292]

Well, at least MRX finally acknowledged the Plateau. Still didnt extend the watch to our neck of the woods, John. Definitely some encouraging trends this afternoon in the models.

[Windspeed]

18z 32k NAM resolved a much better defined 850 vort over N. AL/GA and 700 mb weakness up into Kentucky versus 12z. No surprise at greater forcing and crushing snowfall rates over ETN this round. You can follow the 304dm and 145dm contours respectively.

[Blue Ridge]

Updated zones have yet to load, but it looks like a MRX torch across NE TN per the grids. Sunday is mostly liquid, even in the WSW area.

[John1122]

5 minutes ago, Runman292 said:

Well, at least MRX finally acknowledged the Plateau. Still didnt extend the watch to our neck of the woods, John. Definitely some encouraging trends this afternoon in the models.

It doesn't really matter if they put Campbell in or not, but I truly have no idea what they are seeing that makes them include Claiborne and not the rest of the Northern Plateau. Basically the models that hit there hit here, and the ones that don't miss us both by a wide margin.

[John1122]

2 minutes ago, Blue Ridge said:

Updated zones have yet to load, but it looks like a MRX torch across NE TN per the grids. Sunday is mostly liquid, even in the WSW area.

I'm almost certain the  point and click are generated almost directly off the GFS.