Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 051754
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1254 PM 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 1230 pM EST Monday...we are in a lull in-between systems.
Latest model guidance suggests that next round of precip will not
arrive until later in the afternoon, so precip chances have been
temporarily lowered across the southern zones. Meanwhile, sky cover
has thinned out nicely across the NC Foothills and wrn Piedmont,
which should give us the warmest temps this afternoon in those
locations. The fcst will be updated accordingly.

Otherwise, 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 /18Z MONDAY THROUGH SATURDAY/...
At KCLT and elsewhere: Have seen improvement early this afternoon
and we appear to be in the lull between weather systems at the
outset. Already we can see the next round of restrictions coming
over the SW horizon, so the VFR period will not last much beyond
sunset in many locations. Expect widespread RA to develop with IFR
cigs sometime after 00z, with some sites possibly dropping to LIFR
in the early morning hours. For all but KAVL, NNE winds will shift
more NE and ENE through the period, increasing to over 10kt with
low-end gusts. KAVL will see N up-valley winds shifting SE
down-valley this afternoon. Once in place, the IFR restriction will
probably not lift until midday Tuesday, but these details were
omitted for brevity.

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

Confidence Table...

            18-24Z        00-06Z        06-12Z        12-18Z
KCLT       High  93%     Low   55%     High  81%     High  89%
KGSP       High  81%     Med   75%     Med   79%     High  94%
KAVL       High  86%     Low   52%     Low   57%     High  93%
KHKY       Med   72%     Med   69%     Med   77%     High  92%
KGMU       Med   73%     Med   77%     Med   79%     High  91%
KAND       Med   65%     Med   72%     Med   77%     High  89%

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...PM/TDP
SHORT TERM...ARK
LONG TERM...DEO
AVIATION...PM



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