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John1122

12/18/16 Arctic Front/Severe Storms/Ice/Sleet/Snow event.

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The models keep showing this as a possible threat. Jeff mentions his reasoning for the ice potential in the winter pattern thread. A very strong cold front will sweep through the area overnight Saturday into Sunday causing crashing temps. Most models agree there will be moisture of some amount behind the front. There could also be freezing issues due to the heavy amounts of water left by the rain. 

In addition to the totals of ice below. Models also show some degree of sleet or snow. I believe the Euro showed an inch or so in a good portion of the area. The NAM shows some snow in parts of East Tennessee. The GFS had a couple of runs showing it too. I believe some of their maps may be incorporating sleet into their snow total maps.

The worst case scenario is the Canadian, crippling ice storm for a good portion of the Valley on the 12z run. The Canadian was the first to show this possibility and hasn't wavered from it for days on end.

zr_acc.us_ov.png

 

The GFS is much more localized with freezing rain totals.

zr_acc.us_ov.png

 

The NAM splits the difference.

 

zr_acc.us_ov.png

 

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This shifts east through the day. First changing to zr along the Plateau then to sleet. Not sure why it's showing any green behind the initial red along the Plateau at this point. Temps are in the 20s from extreme SEKY to Northern Alabama by this time.

 

refcmp_ptype.us_ov.png

refcmp_ptype.us_ov.png

 

GFS also shows the heavy sleet band. Which is what holds down the zr totals on it's map.

prateptype_cat.us_ov.png

 

The Canadian is the most widespread with the zr/sleet. Also has the longest duration of it but never moves the freezing line off the Plateau while moisture is in place. Still shows light freezing rain over the Plateau 6 hours after that lower map with rain in the Valley and points East. I think the front will clear those areas and the precip won't linger as long as the Canadian shows.

prateptype.us_ov.png

prateptype.us_ov.png

 

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The temperature gradient will be something to behold with this one. Jeff mentioned this would be like a plains cold front with extremely fast drops behind it. 30-35 degree drops in a few hours, especially middle and western areas. As the front goes east and heads to the Apps it slows down.

sfct.us_ov.png

sfct.us_ov.png

 

 

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Oh goody, our first thread.  We had to have one sooner or later.  Since nothing in the future is guaranteed I am glad you started this.  It will be interesting to see how this plays out.  Cold rarely works it's way in before the precip moves out, but this high means business.  I used to remember times when rain would change to snow behind strong cold fronts so maybe this has legs for our brothers and sisters to the west.  Not feeling it for upper east TN, but the enhancement in middle TN to the plateau has my attention.  

Interestingly enough, the NWS for upper east TN (Morristown) mentions possible snow as the warm front lifts through and then a possible changeover with  light accumulation Sunday on the backside of the front.  The NWS in Nashville is running with no mention of a possible accumulation (ice/snow), which I also find interesting.  I think their chance is greater than our chance.  They probably don't consider it a real threat at this point and just want to watch a couple of additional cycles in the modeling.

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8 minutes ago, tnweathernut said:

Oh goody, our first thread.  We had to have one sooner or later.  Since nothing in the future is guaranteed I am glad you started this.  It will be interesting to see how this plays out.  Cold rarely works it's way in before the precip moves out, but this high means business.  I used to remember times when rain would change to snow behind strong cold fronts so maybe this has legs for our brothers and sisters to the west.  Not feeling it for upper east TN, but the enhancement in middle TN to the plateau has my attention.  

Interestingly enough, the NWS for upper east TN (Morristown) mentions possible snow as the warm front lifts through and then a possible changeover with  light accumulation Sunday on the backside of the front.  The NWS in Nashville is running with no mention of a possible accumulation (ice/snow), which I also find interesting.  I think their chance is greater than our chance.  They probably don't consider it a real threat at this point and just want to watch a couple of additional cycles in the modeling.

The NAM is showing a several hour window of frozen precip Friday morning, but I think it could be virga.

NWS offices, as we know, are very reluctant to mention winter possibilities until they absolutely have to do so.

I guess the part I consider most odd is that they are all very confident it's going to be snow. 

NWS Nashville simply says thunderstorms with a low of 31 for Jamestown Saturday night, then Snow Likely with a high of 31 for Sunday. MRX current forecast basically says rain most of the day Sunday then switching to snow showers. 

 

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Just read the updated OHX discussion, they now say no snow likely and mentioned mixed precip. Up to .10 for their area. Seem to be focusing on the GFS fast exit to moisture but mention the NAM and Euro have much more significant precip behind the front.

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1 hour ago, John1122 said:

The models keep showing this as a possible threat. Jeff mentions his reasoning for the ice potential in the winter pattern thread. A very strong cold front will sweep through the area overnight Saturday into Sunday causing crashing temps. Most models agree there will be moisture of some amount behind the front. There could also be freezing issues due to the heavy amounts of water left by the rain. 

In addition to the totals of ice below. Models also show some degree of sleet or snow. I believe the Euro showed an inch or so in a good portion of the area. The NAM shows some snow in parts of East Tennessee. The GFS had a couple of runs showing it too. I believe some of their maps may be incorporating sleet into their snow total maps.

The worst case scenario is the Canadian, crippling ice storm for a good portion of the Valley on the 12z run. The Canadian was the first to show this possibility and hasn't wavered from it for days on end.

zr_acc.us_ov.png

 

The GFS is much more localized with freezing rain totals.

zr_acc.us_ov.png

 

The NAM splits the difference.

 

zr_acc.us_ov.png

 

As someone that has resided in middle TN all of my life I have seen situations before where the NWS, their tendency to err on the very conservative side, has come back to bite them. It is concerning to me living north of Nashville, seeing the Canadian stay with the idea of icing and to hear Nashville talk it down. Perhaps by tomorrow they will be able to put a finer tune on things.

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I think the NAM is showing sleet as heavy snow on there. Any sleet total of more than 1/2 inches is extremely slick and pretty impressive. Especially with it falling onto frozen surfaces as there will be some likelihood of freezing rain on the edge, plus flash freezing in areas. The temps fall pretty rapidly into the 20s behind the front.

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Yes I agree that is sleet not snow. However Middle Tennessee could see some light snow at the very end. Looks like the tri-fecta of winter wx. Freezing rain is hopefully brief as the arctic air first arrives. Arctic air should become deep and cold enough fast enough to flip it to sleet. Perhaps it ends as very brief light snow. Looks like the sounding starts to dry out. Sets up a race between evaporative cooling for snow, and just shutting off precip. I lean toward the latter after a few flakes.

Back to the sleet period: If enough moisture keeps riding over the cold dome it could be an interesting sleet event...

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One thing is certain, this cold front does fall into the "making front's great again" category.  Modeling suggests Nashville is around 66 at 1am, falling to around 30 by 4am and into the mid 20's by sunrise!!  Flash freeze potential.  

Will be interesting to see how much sleet can fall and if it can be topped off by a little snow.  Either way, Tennesseans will get smacked with a great winter front.  Too bad it won't happen while most are awake.  I lived through a couple of these growing up in Gallatin as a kid and remember sitting and watching in amazement as the thermometer dropped like a rock!

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I think the NAM is showing virga like it did with the beginning of this system last night when it was looking at today. Not sure why, as I don't think it's supposed to do that. The precip in 72 hours isn't showing up on any other models right now that I can see.

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day1otlk_1630.gif?1481993373982

 

 

Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1030 AM CST Sat Dec 17 2016

   Valid 171630Z - 181200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF THE
   LOWER MISSISSIPPI VALLEY AND MID SOUTH REGION...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM THE OHIO
   VALLEY SOUTHWARD INTO THE LOWER MISSISSIPPI VALLEY...

   ...SUMMARY...
   Severe storms are possible late this afternoon into tonight across
   the Mid South and ArkLaMiss.  Damaging wind gusts will be the
   primary threat, but a couple of tornadoes and isolated large hail
   will also be possible this evening.

   ...Mid South and Lower Mississippi Valley...
   A progressive shortwave trough is noted in water vapor imagery
   moving across NM into the Southern Plains.  This feature will
   strengthen southerly low-level winds over the Gulf Coast Region and
   TN/OH Valleys today, leading to rapid warm/moist advection.  Surface
   moisture is streaming into the region, with dewpoints now in the mid
   50s to mid 60s throughout the slight risk area.  Present indications
   are that scattered thunderstorms will form along/ahead of an
   approaching cold front by late afternoon - mainly from western KY
   into eastern AR, then spreading into west TN and northern MS. 
   Visible satellite imagery suggests a few breaks in the clouds in
   this region, which may help to weaken the cap and lead to a few
   discrete cells for a few hours before congealing into a more
   organized squall line after dark.  Forecast soundings in this region
   suggest the potential for supercells capable of damaging winds,
   hail, and isolated tornadoes.  Even after dark, the intense
   low-level shear may be sufficient for an occasional QLCS tornado
   spin-up, but damaging winds should become the dominant threat.  The
   slight risk area has been extended northward into southern KY for
   this update to reflect 12z model solutions showing more persistent
   and robust convection on the northern end of the squall line. 
   Uncertainty exists how far southeast the threat will persist
   overnight into AL, but will not make any changes to this area at
   this time.

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RGENT - WINTER WEATHER MESSAGE
NATIONAL WEATHER SERVICE NASHVILLE TN
1123 AM CST SAT DEC 17 2016

...WINTER WEATHER WILL AFFECT MIDDLE TENNESSEE...

.COLD AIR WILL BE QUICKLY USHERED IN BEHIND THE COLD FRONT STARTING
LATE TONIGHT. THIS WILL CAUSE THE RAIN TO QUICKLY CHANGE OVER TO
SLEET...FREEZING RAIN AND POSSIBLE SNOW. FREEZING RAIN WILL BE THE
MAIN CONCERN WITH A TENTH TO A FIFTEENTH OF AN INCH OF
ACCUMULATIONS ON ELEVATED AND USUALLY COLD SURFACES POSSIBLE.
TEMPERATURES WILL NOT RISE MUCH ON SUNDAY.

TNZ005>009-023>029-056>060-062-093-180300-
/O.NEW.KOHX.WW.Y.0008.161218T0600Z-161218T1200Z/
STEWART-MONTGOMERY-ROBERTSON-SUMNER-MACON-HOUSTON-HUMPHREYS-
DICKSON-CHEATHAM-DAVIDSON-WILSON-TROUSDALE-PERRY-HICKMAN-LEWIS-
WILLIAMSON-MAURY-RUTHERFORD-WAYNE-
INCLUDING THE CITIES OF...DOVER...CLARKSVILLE...SPRINGFIELD...
GALLATIN...LAFAYETTE...ERIN...WAVERLY...DICKSON...ASHLAND CITY...
NASHVILLE...LEBANON...MOUNT JULIET...HARTSVILLE...LOBELVILLE...
CENTERVILLE...HOHENWALD...FRANKLIN...BRENTWOOD...COLUMBIA...
MURFREESBORO...WAYNESBORO
1123 AM CST SAT DEC 17 2016

...WINTER WEATHER ADVISORY IN EFFECT FROM MIDNIGHT TONIGHT TO
6 AM CST SUNDAY...

THE NATIONAL WEATHER SERVICE IN NASHVILLE HAS ISSUED A WINTER
WEATHER ADVISORY FOR FREEZING RAIN...WHICH IS IN EFFECT FROM
MIDNIGHT TONIGHT TO 6 AM CST SUNDAY.

* TIMING...MIDNIGHT TONIGHT THROUGH 6 AM SUNDAY MORNING.

* MAIN IMPACT...A TENTH OF AN INCH OF FREEZING RAIN ACCUMULATION
  ON ELEVATED AND MAINLY COLD SURFACES. SOME SLEET ACCUMULATION
  POSSIBLE TOO.

* OTHER IMPACTS...THE FREEZING RAIN AND SLEET COULD CHANGE OVER TO
  SNOW BRIEFLY IN THE MORNING. A LIGHT DUSTING MAY BE POSSIBLE ON
  MAINLY ROOFTOPS AND GRASSY SURFACES.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

A WINTER WEATHER ADVISORY MEANS THAT PERIODS OF SNOW...SLEET...OR
FREEZING RAIN WILL CAUSE TRAVEL DIFFICULTIES. BE PREPARED FOR
SLIPPERY ROADS AND LIMITED VISIBILITIES...AND USE CAUTION WHILE
DRIVING.


&&

$$


 

Hazardous Weather Outlook

HAZARDOUS WEATHER OUTLOOK
NATIONAL WEATHER SERVICE NASHVILLE TN
501 AM CST SAT DEC 17 2016

TNZ005>011-023>034-056>066-075-077>080-093>095-181100-
STEWART-MONTGOMERY-ROBERTSON-SUMNER-MACON-CLAY-PICKETT-HOUSTON-
HUMPHREYS-DICKSON-CHEATHAM-DAVIDSON-WILSON-TROUSDALE-SMITH-
JACKSON-PUTNAM-OVERTON-FENTRESS-PERRY-HICKMAN-LEWIS-WILLIAMSON-
MAURY-MARSHALL-RUTHERFORD-CANNON-DE KALB-WHITE-CUMBERLAND-BEDFORD-
COFFEE-WARREN-GRUNDY-VAN BUREN-WAYNE-LAWRENCE-GILES-
501 AM CST SAT DEC 17 2016

THIS HAZARDOUS WEATHER OUTLOOK IS FOR PORTIONS OF MIDDLE
TENNESSEE.

.DAY ONE...TODAY AND TONIGHT

SOME STRONG TO SEVERE THUNDERSTORMS WILL BE POSSIBLE LATE THIS
AFTERNOON THROUGH THIS EVENING. MAIN CONCERNS WITH SEVERE
THUNDERSTORMS THAT DO DEVELOP WILL BE DAMAGING WINDS...ALTHOUGH
ISOLATED LARGE HAIL AND THE FORMATION OF A TORNADO CAN NOT TOTALLY
BE RULED OUT.

LATE TONIGHT A CHANGE OVER TO A WINTRY MIX OF LIGHT FREEZING
RAIN AND LIGHT SLEET...EVENTUALLY TURNING OVER TO LIGHT SNOWFALL
IS EXPECTED ACROSS NORTHWESTERN PORTIONS OF THE MID STATE REGION.
ALTHOUGH NO SLEET OR SNOWFALL ACCUMULATIONS ARE EXPECTED...ICE
ACCUMULATIONS ARE EXPECTED TO BE GENERALLY LESS THAN ONE TENTH OF
AN INCH. A FREEZING RAIN ADVISORY MAY BE ISSUED LATER TODAY FOR
TONIGHT IF EXPECTED WEATHER CONDITIONS WARRANT.

.DAYS TWO THROUGH SEVEN...SUNDAY THROUGH FRIDAY

A WINTRY MIX OF LIGHT FREEZING RAIN AND LIGHT SLEET...EVENTUALLY
TURING OVER TO LIGHT SNOWFALL IS EXPECTED ACROSS MIDDLE TENNESSEE
DURING THE MORNING HOURS ON SUNDAY. ALTHOUGH ONLY MINOR
ACCUMULATIONS OF SNOW AND SLEET ARE EXPECTED...ICE ACCUMULATIONS
OF UP TO AROUND ONE TENTH OF AN INCH WILL BE POSSIBLE ACROSS
EASTERN PORTIONS OF THE MID STATE...ESPECIALLY THE CUMBERLAND
PLATEAU REGION. A FREEZING RAIN ADVISORY MAY NEED TO BE ISSUED
LATER TODAY FOR SUNDAY MORNING IF EXPECTED WEATHER CONDITIONS
WARRANT.

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MD 1890 graphic

 

 Mesoscale Discussion 1890
   NWS Storm Prediction Center Norman OK
   0100 PM CST Sat Dec 17 2016

   Areas affected...Northeastern AR...western and middle TN...western
   and central KY

   Concerning...Severe potential...Tornado Watch likely 

   Valid 171900Z - 172100Z

   Probability of Watch Issuance...95 percent

   SUMMARY...A tornado watch will likely be needed as buoyancy
   continues to increase and sustained deep/convective initiation
   occurs over the next few hours.

   DISCUSSION...Visible-satellite imagery shows cloud breaks early this
   afternoon across the AR/TN portion of the MS valley northeastward
   into western KY.  Temperatures have warmed into the lower to middle
   70s deg F over the MS valley and are warming into the middle 60s
   over the Kentuckiana region.  Along with surface heating, increasing
   low-level moisture is contributing to weak destabilization as SBCAPE
   and MLCAPE range from 1250 J/kg near Memphis to around 250 J/kg near
   Louisville.  Additional destabilization is expected over the next
   couple of hours near the Louisville area.  Very strong deep-layer
   wind profiles will support storm organization/rotation once
   developing updrafts can become strong enough to become sustained. 
   Area WSR-88D VAD data show 0-1 km SRH in the 200-400 m2/s2 range. 
   Given weak capping and the continued warm-air-advection regime
   aiding in moistening/lifting parcels to their LFC, gradual storm
   development and intensification are expected.  The magnitude of
   low-level shear will conditionally support an appreciable risk for
   low-level mesocyclones capable perhaps of a few tornadoes, in
   addition to isolated damaging wind and marginally severe hail with
   the stronger storms.

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