Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 051142
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
642 AM EST Mon Dec 5 2016

.SYNOPSIS...
High pressure quickly builds back in from the north today while
moist air returns associated with a low pressure system moving up
from the western Gulf of Mexico. Expect a soaking rain tonight and
early Tuesday followed by dry high pressure returning Wednesday. A
strong cold front crosses from the northwest early Thursday bringing
the coldest air of the season for the end of the week.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 630 AM EST Monday: Precipitation has mostly moved out of the
area (a little earlier than anticipated) so have adjusted pops
accordingly. Also made some adjustments to hourly T/Td trends
through the morning. No other significant changes, but will have to
monitor QPF for late tonight/early Tuesday. 06z runs add an element
of confusion to the mix (ha, like there isn`t already confusion
inherent in a forecast) with the NAM increasing QPFs over the NC/TN
state line and the GFS increasing QPF across extreme SE zones.

Otherwise, surface high pressure over the Ohio Valley will begin to
cross the central Appalachians today, with damming onset beginning
between 21z-00z this evening. In the meantime however, as the
initial shortwave lifts farther northeast and the attendant moisture
exits, we may see a bit of sunshine today across the northern tier,
specifically the northern NC Piedmont. This really complicates high
temperatures, even before onset of the damming, as southern zones
will remain solidly under cloud cover with no isolation and as a
result will likely end up in bizarro world with cooler highs for
those extreme southern zones than we`ll see in the northern NC
Piedmont.

We have been talking about the NW Mexico cutoff upper low for
several days now and it has started lifting northeast (per H20v
imagery) and will lift into TX today as it begins to fill and damp.
Pacific moisture will combine with fetch off the Gulf as
cyclogenesis is induced off the TX Gulf Coast this morning, with
moisture lifting across the South through the day. The former cutoff
will eventually morph into an open shortwave just beyond the end of
the period, with the primary surface low lifting into the Tennessee
Valley late today and tonight but with a secondary low developing on
the south side of the damming wedge in a Miller B-type pattern. The
heaviest rainfall will follow the tracks of the lows with the area
solidly in the wedge as a bit of a minimum. For our forecast area,
the heaviest rain will be along and SE of I-85, with the heaviest
rain still progged across those areas between 06-12z Tuesday. It`s
all relative because we`re looking at less than 3/4" during those
six hours, with storm total after 12z (note this does NOT include
what fell yesterday or is falling now) generally 1.25-1.75". Not
much concern about impacts from this so will likely remove mention
of heavy rain from the HWO.

Damming signature overnight is pretty impressive and northeast flow
picks up as the surface low approaches, with breezy conditions in
the Piedmont as a result. Again not particularly impactful, with
sustained 10-15mph and gusts up to 20-25mph or so. In the mountains,
above 5kft, this is a slightly different story with stronger winds
out of the SE, but we really only meet wind advisory criteria on the
very highest peaks and even that is very brief, so no wind products
will be issued. (However, on the TN side, could see some decent
winds coming off those higher peaks.)

&&

.SHORT TERM /TUESDAY THROUGH WEDNESDAY NIGHT/...
As of 300 AM EST Monday...the Short Term will start out at 12Z
Tuesday, with an area of strong deep-layer DPVA atop the CWFA,
associating with a strong shortwave lifting NE out of the Southern
Plains. This forcing will be accompanied by coupled upper jet
divergence and strong low-level isentropic lift. At the surface, a
low pressure system will "jump" from the TN Valley to the Outer
Banks by 00z in Miller-B fashion. So Tuesday should start out with
widespread stratiform rain Tuesday morning, and hence, categorical
PoPs. By early afternoon, a dry slot will start to work in from the
west, as the low shifts east. So PoPs will quickly ramp down during
the afternoon hours. Total additional precip on Tuesday should range
from 0.25-0.60", highest east. Forecast soundings show that while
flow will veer to westerly with mid-level drying, the low levels may
struggle to clear out, with a strong inversion still in place. So
skies will likely remain cloudy and keep temps below normal.

Tuesday night thru Wednesday, a trailing cold front will push in
from the NW and stall roughly along the I-20 corridor by midday.
Low-level moisture will linger with the front, especially along the
TN border with upslope flow keeping clouds and spotty showers around
thru late Tuesday evening. Temps look too warm for any wintry p-type
concerns with the NW flow showers. By Wednesday morning, the next
shortwave trough will be approaching from the west, as a broad
longwave trough persists across Great Plains. The deterministic
models all generally agree on 850 mb flow quickly backing out of the
SW with WAA commencing atop the stalled frontal zone. This may keep
a fair amount of clouds around thru the day on Wednesday, but the
moisture should be too shallow for any precip. Temps will be
slightly above normal Tuesday night, and near normal for highs on
Wednesday.

Wednesday night, the next front will be approaching from the west.
It will not be able to tap into any Gulf moisture and such only
spotty showers are expected along and ahead of the boundary. Models
have trended drier overall, and I have cut PoPs back to the NC
mountains and foothills in the slight chc to low-end chc range thru
12z Thursday. Temps will be slightly above normal under WAA and
mostly cloudy skies.

&&

.LONG TERM /THURSDAY THROUGH SUNDAY/...
As of 120 AM EST Monday: The strong cold front will quickly cross
our region on Thursday with the GFS being the fastest and driest
while the ECMWF is a little slower and more moisture. The GFS is so
dry that only widely scattered showers occur east of the mountains
as the front crosses then all precip moving east of I-77 after 18Z
Thursday. The EC has frontal showers reaching the NC mtns Thursday
morning and less coverage elsewhere over foothills and piedmont
during the day then NW flow snow lingering over NC mtns until late
Thursday night. Compromising between the models results in our
forecast. Definitely the coldest air of the season so far from
Thursday night through Saturday. After the frontal precip and any
brief NC snow showers, very dry under cold high pressure which will
be centered over our area early Saturday. Below zero wind chills at
the highest elevations late Thursday night and again late Friday
night. Temperatures 10 to 20 degrees below normal across the western
Carolinas and NE GA Thursday night through Saturday. Much of the NC
mountains will remain below freezing from Thursday evening until
Saturday afternoon.

The cold high will move out over the Atlantic Sunday with wind from
a southerly direction and moderating temperatures with some chances
of precip by Monday. The GFS is the driest model with no precip
reaching the NC Mtns until around 12Z Monday.  The EC has precip in
the NC Mtns Sunday night and increasing into Monday morning. Since
this is a week away is subject to much change, we are just going
with a mix of rain and snow with the initial period of precip.

&&

.AVIATION /12Z MONDAY THROUGH FRIDAY/...
At KCLT and elsewhere: First round of moisture has exited the area
with a patchwork of restrictions across the TAFs. Have seen
improvement for most sites this morning except KHKY which has gone
VLIFR in cigs. Expect VFR no later than noon for all but KAND (where
moisture should keep MVFR cigs in), but with a lowering trend
everywhere again this evening as the second round of moisture moves
in. Expect widespread RA to develop with IFR cigs sometime after
00z, with sites dropping to LIFR toward the end of the period. For
all but KAVL, NNE winds will shift more NE and ENE through the
period, increasing to over 10kt late in the period with low-end
gusts. KAVL will see N up-valley winds shifting SE down-valley this
afternoon.

Outlook: Rain will be on the decrease through the day Tuesday with
drying expected through Wednesday. Another front will approach on
Thursday with some restrictions possible. Dry and much colder
through the weekend.

Confidence Table...

            11-17Z        17-23Z        23-05Z        05-06Z
KCLT       Low   55%     High 100%     High 100%     High 100%
KGSP       Med   61%     High  95%     High  80%     High  81%
KAVL       Med   61%     Med   70%     Med   66%     High  94%
KHKY       Low   47%     High  95%     Med   73%     High 100%
KGMU       Low   55%     High  95%     Low   46%     High  81%
KAND       Low   55%     Med   70%     Med   61%     High  87%

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...None.
NC...None.
SC...None.

&&

$$
SYNOPSIS...DEO
NEAR TERM...TDP
SHORT TERM...ARK
LONG TERM...DEO
AVIATION...TDP


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