Winter Storm in Georgia, December 8 to 9, 2017

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1250 AM EST Fri Dec 8 2017

.SYNOPSIS...
A moist frontal zone will linger just to our southeast through
tonight with light wintry precipitation for many areas north of
Interstate 85.  A cold upper level trof will then re-amplify over
the area later on Saturday and bring a shot of snow showers to the
North Carolina mountains. Cool temperatures will persist through
much of next week.

&&

.NEAR TERM /THROUGH TODAY/...
As of 1230 AM EST Friday: No changes. Will address the near term
with the new fcst package. Suffice to say the wheels have not
fallen off...yet.

Otherwise...have expanded the Advisory farther into the NC
mountains and NW NC Piedmont, and upgraded to a Winter Storm
Warning for Habersham and Rabun. To the details...event unfolding
in TX is either at or above some of the predicted levels, and HRRR
is actually doing a pretty good job of nowcasting the event, so
would like to think that the rest of the HRRR forecast is in line.
Guidance continues to bump up QPF. Atmosphere is slowly saturating
so evaporational cooling will of course have a limit, but concern
with incoming CAA will be the dynamic cooling and diabatic effects,
specifically the cooling of the surface layer from the latent
heat of melting, which will serve to continue to lower the snow
layers without a source of low-level WAA (and there really isn't
one). KFFC 00z sounding had a bit of a warm nose just above 800mb
but that should erode by the time precip gets this far north. 00z
NAM bufr soundings show a deep near-freezing isothermal layer with
gradually dropping snow levels here at GSP through the night (using
the thickness nomogram is a bit misleading with the near-freezing
layer so deep). So even with the increased QPF, think snow ratios
might not be quite as high and so have not increased snow amounts
quite as much as the QPF would suggest. In any case, since WSW
criteria for GA is 2", Rabun should easily get it, and northern
Habersham as well. Most of the SC mountains should easily get 2",
but WSW criteria for SC is 3" (4" for NC), so for now have kept SC
and NC to advisories. Would not be surprised to see the rest of the
NC mountains added to the advisory or expanded east more into the
Piedmont (right now Burke/Cleveland/Rutherford Counties are really
only included because of the Southern Mountains). Graham and Swain
actually do not currently have advisory-criteria snow amounts,
but for consistency's sake, made sense to go ahead and include them.

Otherwise, guidance is coming into better agreement regarding the
precip potential for tonight and Friday. Deep moisture moves in
this evening with decent upper divergence from the right entrance
region of the upper jet. Short waves move over as the upper trough
deepens to the west. Low level isentropic lift develops as well,
but remains relatively weak with abundant moisture. Despite an
overall cooling trend in forecast temps and surface wet bulb
values, they remain above freezing through the night outside of
the mountains, but do fall below freezing across the mountains. Of
course, precip chances are lower where temps are colder since those
locations are farther away from the better forcing. Therefore,
have snow developing in the colder locations with mainly rain
elsewhere. Forecast soundings show the freezing level dropping
low enough for snow to mix in north of I- 85 with a change over
possible across the Northern Foothills. QPF is light but could be
enough for around half an inch of snow along and near the NC/GA
to NC/SC border mountain locations. Snow would be less across the
rest of the mountains.

Categorical precip chances continue generally along and south of I-
85 Friday with likely chances north of there across the foothills
to along the Blue Ridge. Good chance PoP west of there. Temps and
surface wet bulbs remain cold enough for the precip to fall as snow
across the mountains with a transition zone across the NC Foothills.
Outside of these areas, freezing levels may fall low enough for snow
to mix or possibly even briefly change over early in the morning
mainly along and north of the I-85 corridor. QPF has increased
on some of the guidance runs which increases the potential for
advisory level snow across the NE GA and Upstate mountains,
along with the Southern Mountains and possibly portions of the
NC foothills. After final national guidance is in, we may need
to update to post an advisory for those locations. While these
accums would develop in grassy and elevated areas, it is still
questionable how much accumulates on roads given the recent warmth
and sunshine. Locations where the precipitation will be mixed or
only a brief change over, no significant accums are expected with
any small amounts limited to grassy or elevated areas.

&&

.SHORT TERM /TONIGHT THROUGH SUNDAY/...
As of 230 PM Thu: We will remain slightly on the cold northwest
side of a surface frontal boundary through early Saturday. Models
are now more supportive of a resurgence in precipitation over the
CWFA Friday night, as the sharp shortwave moves into the central
Gulf states, and the jet streak strengthens over the southern
Appalachians and adjacent Piedmont. GFS and EC hold the thermal
gradient relatively steady over the area thru the night; wet-bulb
temps do gradually drop across the southeast half of the CWFA,
which should permit snow to mix in with rain if not totally change
over. However, the area with wet-bulb values at or below freezing
looks likely to expand little. This minimizes the accumulating
snowfall in the greater I-85 corridor.

QPF trends overall have been upward this cycle for the Friday night
period, particularly over the Blue Ridge and adjacent mountain
ranges (namely the Balsams); compare this to earlier suggestions
that all precip may have ended there by 00z Sat. In collaboration
with our neighboring offices, we will wait another cycle to see if
this trend continues before reflecting the higher numbers in the
forecast for these mountain areas. It is possible an additional
couple of inches could result and headlines still could be needed.

Another minor yet potentially significant trend in models is for
Saturday. As before, consensus suggests strong DPVA pushing thru
the area during the day along the trough axis. The difference is
that several sources depict the moisture departing more slowly,
and hence the vort max generates some QPF.  By the time this
precip develops it could have warmed up enough to fall as rain,
particularly in the east. However this is something for us to
watch closely.  A small amount of SBCAPE is depicted in parts of
the eastern CWFA to boot; however it slightly lags the forcing and
colder temps. For now PoPs will be allowed to linger thru Saturday
morning over our eastern zones; temp trends suggest this will be
a rain-snow mix changing back over to rain as it ends.

Winds veer Saturday afternoon as stronger cold advection sets
in across the area. The final phase of precip in this period
thus will be northwest-flow driven snow showers along the Tenn
border, developing by late afternoon, and continuing through early
Sunday. Thermo profiles look reasonably good in terms of saturation
at snow-growth temperatures, which given the very cold air spilling
into the area will be in the upslope layer. Current expectation
is for about another inch to fall Saturday night along the spine
of the Appalachians. Brisk and gusty winds in this same timeframe
may bring wind chills below zero on the higher mtn ridges.

Settled weather is expected Sunday as dry high pressure builds
across the South. Despite abundant sunshine, the very cold nature
of the airmass will be reflected in max temps 11-13 degrees
below normal.

&&

.LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/...
As of 210 PM EST Thursday: the extended fcst picks up at 00Z on
Monday with the broad upper trof lifting NE and over New England
while steep upper ridging persists over the Western CONUS. By early
Tuesday, the long range models re-amplify the upper trof again over
the eastern half of the CONUS bringing more widespread cold air to
the region and keeping temperatures well below normal for most of
the period. By early Wed, the trof axis is expected to be moving
over the fcst area or lifting just NE of it. Beyond this point,
the trof broadens and flattens, but still remains in place over
the entire Eastern CONUS. At the sfc, another progressive low will
surge SE and across the Great Lakes on Monday and bring another cold
front to our doorstep by late Monday/early Tuesday. The front is
expected to move thru the CWFA quickly and be SE of the fcst area
by late Tues/early Wed with high pressure moving back over the area
to end the period.

&&

.AVIATION /06Z FRIDAY THROUGH TUESDAY/...
At KCLT and elsewhere: Grim-looking aviation weather as flight
conditions are expected to deteriorate gradually early this
morning. Once precip moves in from the south, we should see a
steady progression down through MVFR with the ceiling. The guidance
suggests that IFR ceiling will prevail after about 13Z and see no
reason yet to argue. Wind should come around to NE by that time
and stay there for the rest of the period. All sites should be
IFR after 00Z Saturday. From that point onward, we will have to
look at the possibility of a mix/changeover to snow. There was
not the time to address that situation with this fcst, but it will
be handled over the next few hours as we have more time to digest
the new model guidance. Expect it to be fine tuned on the 08z or
10Z amendment. Elsewhere...suffice to say, as elevation increases,
snow is more likely, so KAVL should expect some accum.

Outlook: Restrictions will be likely as well as mixed/wintry precip
through Saturday morning. Precip and associated restrictions will
decrease during the day Saturday, with dry conditions expected at
all sites into early next week. The possible exceptions would be
KAVL/KHKY with potential for northwest flow snow Saturday night.

Confidence Table...

            15-21Z        21-03Z        03-09Z        09-12Z
KCLT       High  81%     High  82%     Med   68%     Low   56%
KGSP       Med   70%     Med   75%     High  83%     Med   64%
KAVL       High  94%     High  85%     Med   70%     High  87%
KHKY       High 100%     High  83%     Med   66%     High 100%
KGMU       Med   76%     High  86%     High  85%     Med   63%
KAND       Med   76%     Med   65%     High  90%     Med   61%

The percentage reflects the number of guidance members agreeing
with the scheduled TAF issuance flight rule category. Complete hourly
experimental aviation forecast consistency tables and ensemble forecasts
are available at the following link:

www.weather.gov/gsp/aviation

&&

.GSP WATCHES/WARNINGS/ADVISORIES...
GA...Winter Storm Warning from 5 AM early this morning to 7 AM EST
     Saturday for GAZ010-017.
NC...Winter Weather Advisory from 5 AM early this morning to 7 AM
     EST Saturday for NCZ051>053-058-059-062>065-068-501>510.
SC...Winter Weather Advisory from 5 AM early this morning to 7 AM
     EST Saturday for SCZ001>003.

&&

SYNOPSIS...JPT
NEAR TERM...PM/RWH/TDP
SHORT TERM...Wimberley
LONG TERM...JPT
AVIATION...PM

Hazardous Weather Outlook
National Weather Service Greenville-Spartanburg SC
1047 PM EST Thu Dec 7 2017

NCZ051>053-058-059-501>503-505-090400-
Swain-Haywood-Buncombe-Graham-Northern Jackson-Caldwell Mountains-
Greater Caldwell-Burke Mountains-McDowell Mountains-
1047 PM EST Thu Dec 7 2017

...WINTER WEATHER ADVISORY IN EFFECT FROM 5 AM FRIDAY TO 7 AM EST
SATURDAY...

This Hazardous Weather Outlook is for western North Carolina.

.DAY ONE...Tonight.

Please listen to NOAA Weather Radio or go to weather.gov on the
Internet for more information about the following hazards.

   Winter Weather Advisory.

.DAYS TWO THROUGH SEVEN...Friday through Wednesday.

..Friday...Winter weather likely.
..Saturday...Winter weather likely. Any snowfall is expected to
taper off during the morning. As winds turn to the northwest and
become gusty, snow showers may redevelop Saturday evening along the
Tennessee border.
..Sunday...No hazardous weather is expected at this time.
..Monday...No hazardous weather is expected at this time.
..Tuesday...No hazardous weather is expected at this time.
..Wednesday...No hazardous weather is expected at this time.

.SPOTTER INFORMATION STATEMENT...

None.

NCZ033-035-048>050-090400-
Avery-Alexander-Madison-Yancey-Mitchell-
1047 PM EST Thu Dec 7 2017

This Hazardous Weather Outlook is for piedmont North Carolina and
western North Carolina.

.DAY ONE...Tonight.

Snow may develop overnight tonight. The snow may cause reduced
visibility on area roadways. Where temperatures are especially cold,
elevated surfaces such as bridges may see a trace of accumulation.
This may make for hazardous road conditions in high elevations, if
the snow develops.

.DAYS TWO THROUGH SEVEN...Friday through Wednesday.

..Friday...Winter weather possible. A chance of snow will persist
through the day. This could result in accumulations of around an
inch in some locations.
..Saturday...Winter weather possible. Any snowfall is expected to
taper off during the morning. As winds turn to the northwest and
become gusty, snow showers may redevelop Saturday evening along the
Tennessee border.
..Sunday...No hazardous weather is expected at this time.
..Monday...No hazardous weather is expected at this time.
..Tuesday...No hazardous weather is expected at this time.
..Wednesday...No hazardous weather is expected at this time.

.SPOTTER INFORMATION STATEMENT...

None.

GAZ018-026-028-029-NCZ036-037-056-057-069>072-082-SCZ004>014-019-
090400-
Stephens-Franklin-Hart-Elbert-Iredell-Davie-Catawba-Rowan-Lincoln-
Gaston-Mecklenburg-Cabarrus-Union-Greater Oconee-Greater Pickens-
Greater Greenville-Spartanburg-Cherokee-York-Anderson-Abbeville-
Laurens-Chester-Greenwood-
1047 PM EST Thu Dec 7 2017

This Hazardous Weather Outlook is for northeast Georgia, piedmont
North Carolina and upstate South Carolina.

.DAY ONE...Tonight.

Hazardous weather is not expected at this time.

.DAYS TWO THROUGH SEVEN...Friday through Wednesday.

..Friday...Winter weather possible. Rain may change over to snow
Friday night. Light snow accumulation up to an inch is possible
north and west of I-85.
..Saturday...No hazardous weather is expected at this time.
..Sunday...No hazardous weather is expected at this time.
..Monday...No hazardous weather is expected at this time.
..Tuesday...No hazardous weather is expected at this time.
..Wednesday...No hazardous weather is expected at this time.

.SPOTTER INFORMATION STATEMENT...

None.

NCZ062>065-068-504-506>510-SCZ001>003-090400-
Macon-Southern Jackson-Transylvania-Henderson-Cleveland-
Greater Burke-Eastern McDowell-Rutherford Mountains-
Greater Rutherford-Polk Mountains-Eastern Polk-Oconee Mountains-
Pickens Mountains-Greenville Mountains-
1047 PM EST Thu Dec 7 2017

...WINTER WEATHER ADVISORY IN EFFECT FROM 5 AM FRIDAY TO 7 AM EST
SATURDAY...

This Hazardous Weather Outlook is for piedmont North Carolina,
western North Carolina and upstate South Carolina.

.DAY ONE...Tonight.

Please listen to NOAA Weather Radio or go to weather.gov on the
Internet for more information about the following hazards.

   Winter Weather Advisory.

.DAYS TWO THROUGH SEVEN...Friday through Wednesday.

..Friday...Winter weather likely.
..Saturday...No hazardous weather is expected at this time.
..Sunday...No hazardous weather is expected at this time.
..Monday...No hazardous weather is expected at this time.
..Tuesday...No hazardous weather is expected at this time.
..Wednesday...No hazardous weather is expected at this time.

.SPOTTER INFORMATION STATEMENT...

None.

GAZ010-017-090400-
Rabun-Habersham-
1047 PM EST Thu Dec 7 2017

...WINTER STORM WARNING IN EFFECT FROM 5 AM FRIDAY TO 7 AM EST
SATURDAY...

This Hazardous Weather Outlook is for northeast Georgia.

.DAY ONE...Tonight.

Please listen to NOAA Weather Radio or go to weather.gov on the
Internet for more information about the following hazards.

   Winter Storm Warning.

.DAYS TWO THROUGH SEVEN...Friday through Wednesday.

..Friday...Winter storm likely.
..Saturday...No hazardous weather is expected at this time.
..Sunday...No hazardous weather is expected at this time.
..Monday...No hazardous weather is expected at this time.
..Tuesday...No hazardous weather is expected at this time.
..Wednesday...No hazardous weather is expected at this time.

.SPOTTER INFORMATION STATEMENT...

None.