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John1122

12/10-11/19 Potential Valley Wide Snow Event.

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We've been RGEM'd

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NAM suite has backed off from its wild runs, but it seems like this is because A. it s the NAM and B. it has the energy passing south of us flatter. 12km NAM flattens it, but 3km NAM brings out a second piece. Looks like it depends on exactly how that energy behaves:

RGEM:

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NAM 12km:

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It should be getting sampled pretty well as of 10z, its right over a major sounding site in souther CA:

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6z Euro continues to struggle with the southern energy, but does seem to be picking out a very quick moving northern stream pulse (almost like a clipper) that sort of cuts the head off the southern energy but also provides a bit of extra lift as it races toward the TN Valley:

0z Euro:

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6z Euro:

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On both runs the energy races in and sweeps out the northern part of the souther wave, but the 6z run keeps it more consolidated toward the Gulf coast. Not sure that would help us at all, but interesting to see that there's still variability even 36 hours out. 

6z Euro precip. result

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You can actually see the second piece of energy producing precip and  racing down the front range of the Rockies in the early frames. 

I think the 6z RGEM did a better job pulling more of the energy north and in the RGEMs version of events, the second piece of energy does more to help than hurt. But right now it is on its own with that evolution. 

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52 minutes ago, Holston_River_Rambler said:

Snowfall looks pretty realistic on the 6z Euro:

giphy.gif

But the RGEM has won a few battles with storm evolution in the past. 

I think it is now kind of a watch and see now, but then again, I'm sure I'll be looking at the SREFs in 45 min. lol

   Latest Euro a downer, for sure. One thing it does have a bit of a problem with is micro precision. As far as the great Valley,  it forecasts as if it runs due NE all the way into SWVA . The Valley takes more of an ENE turn once you get to Morristown/ Rogersville TN area.

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12 NAM is much weaker with post frontal moisture. 3km NAM has a stronger backend as it moves thru. Interested to see RGEM and other models with regards to QPF behind the front.

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Looks good Middle Tennessee. Looks boring southeast Tenn.

I've been surprised at ECMWF snowfall output. Euro seems to be accumulating too soon with the warm ground and questionable 2m temps. Plus the old fashioned thickness/QPF chart ends precip so quickly right after thicknesses drop. Yes, the 06Z is more realistic with the Valley of Death black hole.

NAM might be more realistic with lighter totals. Interestingly, the NAM shows temps cold enough for a few hours during light QPF. Its thickness/QPF chart meshes well with its snow output, which adds confidence over the Euro. While my lighter model choice may sound bearish, at least it accumulates a little - an improvement over my thoughts yesterday.

Plateau and parts of Middle Tennessee lower elevations appear to have a good shot at sticking snow. Might only be an inch or so but it should be really pretty sticking to the trees and stuff. Mountains of course will get theirs later, but that's pretty much a given. Forecast fun is at lower elevations. A more vigorous last piece of energy could drop a couple inches west of the Plateau with 3+ on the Plateau. More likely scenario is less than an inch lower elevations (Mid-Tenn) to a couple inches Plateau. 

Mid-South edit: Gut says it gets going east of Memphis and North Mississippi. However that area is infamous for surprises with rushing cold fronts. We'll see, but I doubt it.

I'm not expecting anything around Chattanooga. Maybe Signal Mountain dusting, iff the more vigorous case above. I've doubts about Huntsville and Knoxville, but we'll see. Feel better about Tri Cities, maybe even Morristown. Those two can do OK with lingering moisture. Still think the best correctly timed true lift will be Middle Tenn.

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Looks to me like the overall 12z suite, excluding the Canadian suite, has backed off somewhat.  May just be simple ebb and flow OR that models are getting a better handle on things.  Euro has backed-off quite a bit over several runs.

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Check that.  GFS looks good in west TN.  The NAM has backed off considerably.  For middle and west TN, looks like a decent chance...models ranging from and inch or two...to warning criteria for some locals.  I think for E TN...the further east one goes, tougher it gets.  When it starts to get down to and inch or two of accumulation...that IMHO begins to get close to model tolerances - meaning it is just tough to be accurate as really one is just splitting hairs between a tenth of an inch of water.

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The Euro has definitely back off from the past few days, but not much change to me from 6z as far as I can tell. Of course as you said it does depend on where you're at. 

The push from the second piece of energy that Jeff mentioned seems to be key now. Euro doesn't seem to have it as much as the 3k NAM and RGEM. HRRR, waaayyy out in its la la land has it too. If that materializes there will probably be a narrow band somewhere of more accumulation. 

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The Euro tends to really mess around with pieces of energy coming out of the southwest, and that messes up its solution in cases like these where the tail end of the front is lagging to the southwest.  The GFS, which is usually more progressive, is more amped and thus has more accumulation.  For west and middle TN, I think a blend of the two is probably a good idea.  The faster this thing goes, the better it is for the eastern valley.  IMHO, if all of this slows down, then the system just dissipates.  Looks like a notable event for west and middle w/ the Plateau as well.  I have slim hopes for NE TN.  Just looks like the moisture zips out of here before meaningful cold arrives.  I think west and middle might see 1-3" w the eastern valley seeing accums close to the Plaeau.  Not sure this makes it to the eastern mountains though that seems counterintuitive. 

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I'm getting suspicious now that the whole first half, if I can call it that, may fizzle out and a lot will just depend on timing that second wave. 

The HRRR shows this idea. Not saying it will be right this far out, more to show a type of progression that might occur and it would fit with your reasoning quite well:

giphy.gif

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Yeah, if the area of weak low pressure is going to hang that far back...might as well just slow down and come out as a Miller A.  LOL.   This time frame is the one where I thought there might have a shot at an overrunning even this week - just didn't pan out that way.  That said, that HRRR map is very close to that, but the HP is just too weak.  Now, let that energy hang back there about twelve more hours...nah, but fun to think about.

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AFD with some interesting tidbits:

"

Temperatures will be on the decline through the day on Tuesday as
the front moves through and colder air from the northwest begins to
funnel into the area. With a non-diurnal temperature trend the
"daytime high" temperatures on Tuesday will actually come early in
the morning as temperatures only decrease through the daytime hours.
The transition to wintry weather will begin in the Cumberland
Plateau during the afternoon hours. Biggest questions at this point
will be the warm nose and how quickly surface temperatures drop
below freezing. In the plateau counties these may combine to bring a
period of rain/snow/freezing rain/ice pellets mix before finally
transitioning to all snow during the evening hours. All models are
showing this general trend, but some of the details between models
are still up in the air. One parameter to keep an eye on is the
dendritic growth zone on the plateau. The NAM wants to keep the -12
to -18 C zone drier, longer than the GFS. This is the region of the
atmosphere that needs to be saturated to create good dendritic
growth. A drier dendritic growth zone would mean a delayed
transition to snow, and possibly and longer duration of the wintry
mix. The GFS keeps this dendritic growth zone saturated much quicker
meaning that the transition to snow would be much faster. Have gone
with a top down methodology across the area that is a general blend
of GFS/NAM/HREF. The end result is rain, then wintry mix, then snow
in the Cumberland Plateau. In the Valley and Southern Appalachian
Mountains, expect there to be a much quicker transition to snow as
the colder air at the surface gets caught along the Cumberland
Plateau and has trouble making it further east, until after the warm
nose has dissipated. That being said, am continuing with most of the
area transitioned to snow during the overnight hours, which then
leads to a snowfall accumulation forecast.

Not many major changes have been made to the snowfall accumulation
forecast, with the highest amounts expected in Northeast
Tennessee/Southwest Virginia and the nearby higher elevations.
Expect snowfall to struggle to accumulate on the roads, due to
temperatures recently being in the 50/60`s. So most accumulations
will occur on grassy and elevated surfaces, hopefully keeping travel
impacts somewhat isolated.
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1 minute ago, John1122 said:

OHX disco is basically, several factors suggest heavy snow possible, several factors suggest less snow possible, we're not sure what is going to happen, so we're going down the middle.

Its really hard to fault anything mentioned in the afternoon AFDs. HUN's disco was a little more informative:

Quote
Taking a look at cross sections and determining deformation zones,
there is a period of time where enhanced lift at 700mb occurs
overhead, and therefore may enhance snowfall rates tomorrow evening.
Though this is a small ~3 hour window, this may lead to higher
snowfall rates for a few select locations across the area. Overall,
snowfall totals look to be around a dusting to half an inch in NW AL
and Srn Middle TN, dwindling as you head to the South/East. A few
spots in Nrn areas may receive up to 1", which is Winter Weather
Advisory Criteria for our office. For now, will not officially be
issuing that just yet, and will allow one more run of the models to
come in for the midnight shift to officially decide to pull the
trigger. Confidence is not high enough at this point today to issue
that product this far out.

Assuming we get a little deformation banding setting up,
realistically, we could see 1-1.5" in some spots, if not a little
higher. Some short-term models have gone with MUCH higher amounts,
but am not biting on that given the current atmospheric profiles in
place.

 

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RGEM and GFS hit big, NAM lesser amounts for reasons mentioned in AFD. 
What's hard for me is usually this is the window I trust the RGEM the most, right around the 48 hour window, I don't know I trust it with this storm. Either it has lost its way over the years in the 48 hour window or it's going to score a big coup.

Sent from my SM-G965U using Tapatalk

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Just now, ShawnEastTN said:

What's hard for me is usually this is the window I trust the RGEM the most, right around the 48 hour window, I don't know I trust it with this storm. Either it has lost its way over the years in the 48 hour window or it's going to score a big coup.

Sent from my SM-G965U using Tapatalk
 

Each model seems to have their day or their bust. Right now the GGEM, RGEM, and GFS are in the same camp. If one were lining up a model hall of fame, those probably wouldn't all be included. That said, the OHX afd does a good job of explaining what each model shows and why their are different possibilities on the table in the area. Not read MRX yet but am about to.

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MRX posted above, similar to other WFO, model differences are resulting in big differences in precip type and amount that can fall as frozen.  MRX must think the Plateau is going to massively hold up the cold. OHX southern Plateau counties are forecast to be at 33 by 5 pm with frozen.  30 miles east MRX has the transition at 2-4 am.

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The 21z 39 hour RAP is pretty healthy for Northern Mississippi and then Plateau, north of 40 and SWVA. Not alot going on middle and west Tennessee on it. Granted, that's at the far edge of it's range to say the least except for the part happening in Mississippi.

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Decided to tweak the gif above to make the cold show up sharper.

giphy.gif

Blue line is the approximate 32 degree line based on surface temps. 

I tried to estimate the arrival time, based on the above, but my calculations put it in Nashville in 5 hours, lol, so back to the drawing board. 

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22 minutes ago, Holston_River_Rambler said:

Decided to tweak the gif above to make the cold show up sharper.

giphy.gif

Blue line is the approximate 32 degree line based on surface temps. 

I tried to estimate the arrival time, based on the above, but my calculations put it in Nashville in 5 hours, lol, so back to the drawing board. 

I assume it may slow a bit as it makes it's way south but it does look like its moving quickly on that gif. 

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19 minutes ago, John1122 said:

but it does look like its moving quickly on that gif. 

I had it clocked at 110 mph, but couldn't believe it. 

But like you said, our terrain and distance from what's driving south will have an effect here. 

That's a two hour loop 

 

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4 minutes ago, Holston_River_Rambler said:

I had it clocked at 110 mph, but couldn't believe it. 

But like you said, our terrain and distance from what's driving south will have an effect here. 

The cold can rush down the plains and even into West Tennessee pretty fast.  But the system to the south will slow it down. I'm honestly not sure if 110mph is fast for an air mass in the plains or not. 

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