Area Forecast Discussion
Issued by NWS Greer, SC

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750
FXUS62 KGSP 280236
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1036 PM EDT Tue Sep 27 2016

.SYNOPSIS...
A stationary front will remain over the Midlands of South Carolina
and the Sandhills of North Carolina through mid week. An upper level
low will drop south into the Ohio Valley on Wednesday and linger
there through the end of the week.

&&

.NEAR TERM /THROUGH WEDNESDAY/...
As of 1025 PM, convection has been continuously redeveloping within
an area of convergence across the upper Savannah River Valley and
moving over portions of Elbert, Abbeville, and especially Greenwood
Counties over the past 2-3 hours. Amounts haven`t been too scary
thus far, with maximum 3-hr amounts in the 1.5-2+ inch range noted
over parts of Greenwood County. In fact, folks are probably enjoying
the rare rainfall. However, the situation may get a little
concerning if the engine doesn`t shut off in the next hour or so
(and it is indeed beginning to show signs of doing so). Pops have
been reoriented to high chance/likely across the southern zones for
the next couple of hours. With the front still more or less stalled
from north GA into far western NC, chances remain for additional
spotty development across the area tonight, and slight chances will
be maintained through the overnight in most areas. Min temps will be
more than 5 degrees above climo once again.

In the big picture, the western Carolinas and north Georgia should
remain in the cyclonic flow aloft through Wednesday as a large upper
low over L Superior today drops almost due S over the next 24 hours,
keeping the trof axis to our west. This development should keep the
front moving only slowly east across the fcst area overnight. The
front will probably still be hung up across the foothills/wrn
Piedmont on Wednesday. Synoptically, we should retain much of what
we have today, that is weak upper divergence and a few weak spokes
of vorticity rotating around the upper low. Combined with the low
level focus of the sfc boundary, expect that new storms will fire
over the Piedmont/Upstate during the late morning and afternoon.
There should be enough shear and buoyancy along/east of the front to
provide for a few strong/severe storms, although the
shear/instability combo will be better just to the east. The Day 2
has the wrn Piedmont in the Marginal Risk and again this will not be
debated. Will not be surprised to see it expand west or get an
upgrade in later issuances. Leaned toward the warmer guidance for
for highs on Wednesday with the front not all the way through the
area.

&&

.SHORT TERM /WEDNESDAY NIGHT THROUGH FRIDAY/...
As of 100 PM Tue: The region will still be in the process of pattern
change Wednesday night, as a cold front pushes into central NC and
the SC Midlands, and a deep cutoff low moves south into the lower
Ohio Valley. Consensus of NAM, GFS and SREF suggests convective
activity will linger over our lower Piedmont zones thru the evening
along/ahead of the front. Meanwhile, CAA sets up over East Tennessee
overnight into Thursday morning, concurrent with enough moisture to
warrant steady PoPs in our Tenn border zones. Min temps Thu mrng will
be back around normal.

The proximity of the upper low will permit deep mixing and sustain
good lapse rates aloft over much of the CWFA thru Thursday.
Deterministic models do not respond to this pattern with appreciable
QPF, so the higher PoP output from some MOS is assumed to reflect the
association of the pattern with abundant if not shallow showers.
Though the low remains centered in essentially the same position
Friday, heights are higher and profiles more stable over our area, so
we can advertise a dry forecast then. Temps will be refreshingly
cooler Thursday, with maxes near normal. These trend still cooler for
Thu night and Fri over most of the area, with the additional sunshine
seemingly offsetting the cooler air in the north and mountains. They
should top out generally 2-4 degrees below normal.

&&

.LONG TERM /FRIDAY NIGHT THROUGH TUESDAY/...
As of 155 PM EDT Tuesday...Starting at 00Z Saturday the cut off
upper low will be centered near Louisville KY. This low will slowly
lift north and fill as it becomes absorbed in the westerly upper
flow near the eastern Great Lakes Sunday night into Monday.  Models
still have a surface high pressure over eastern Canada to start the
week ridging southward to the Carolinas. A high amplitude upper
ridge will be over the East during the first of the week as a trough
sweeps across the northern Rockies and plains states.  Ridgetop
convection becoming more favorable in the PM of Day 7 for the NC
Mtns.

Concern will be building next week as a pending hurricane moving
north out of the Caribbean.  The 06Z GFS has it crossing Haiti at
12Z Monday then just east of Cape Hatteras as a powerful hurricane
at 18Z Wednesday as it heads toward Long Island. The old ECMWF has
it crossing eastern Cuba at 18Z Wednesday so this model is much
slower.  The new 12Z GFS has a track farther to the east with still
a powerful hurricane at 12Z Thursday halfway between NC and Bermuda.
All progression of this tropical system must be watched very closely.

Temperatures are expected to be a few degrees above normal from
Saturday through the first half of next week after the cut off low
has departed and we are under the upper ridge. Although Min Temps at
first will be near or a little below Saturday.

&&

.AVIATION /03Z WEDNESDAY THROUGH SUNDAY/...
At KCLT: Chances for convection, while not 0 (hence a VCSH is
continued through 02Z), are rapidly diminishing, as activity upstream
of the terminal has dissipated. Winds are expected to turn to light
SW in short order. The main concern for this TAF update is therefore
the potential for fog and/or stratus development late tonight, as
low level moisture will remain plentiful in advance of slow-moving/
essentially stalled frontal boundary. Guidance sources are (as
usual) all over the place regarding the potential, but based upon
the latest observational trends, and the fact that it`s looking
highly likely that no rain will fall the the terminal this evening,
do not see any reason who conditions should be radically different
Wed morning vs Tue morning. Development of MVFR cigs/fog is
therefore advertised along with SCT007 by around 10Z. Confidence
however is below normal. Otherwise, convection is expected to
develop across the Piedmont once again Wed, warranting PROB30s for
TSRA during the afternoon.

Elsewhere: Similar to KCLT, with convection apparently done for the
evening and a fog/low stratus concern later tonight. Only KAND
received significant rainfall today, so restrictions are not the
slam dunk that they were at KAVL/KHKY last night. However, based
upon current obs trends, with support for guidance, will forecast
recurrence of IFR conditions at KHKY overnight, but later and not as
low as Mon night/early Tue. KAVL will be kind of tricky, as the
frontal boundary will be in the area, and there is a decent amount
of dry air in the wake of the front. However, I don`t expect the dry
air to make substantial progress toward the Blue Ridge, and will
therefore forecast development of IFR/LIFR conditions there late
tonight. Elsewhere, expect mainly MVFR conditions, but would not
rule out something lower, esp at KAND, where rain did fall this
afternoon. Otherwise, convection is expected to develop east of the
terminals tomorrow, with the possible exception of KHKY, where a
PROB30 is advertised during the afternoon.

Outlook: Dry high pressure will build in slowly behind the front
Wednesday into Thursday, which should bring VFR conditions in most
places through the end of the week, although some instability could
lead to showers over the NC part of the area on Thursday afternoon.

Confidence Table...

            02-08Z        08-14Z        14-20Z        20-00Z
KCLT       High 100%     Low   38%     High  91%     High 100%
KGSP       High 100%     Low   45%     High 100%     High 100%
KAVL       High  98%     Low   35%     High 100%     High 100%
KHKY       High 100%     Med   62%     High  83%     High 100%
KGMU       High 100%     Med   62%     High 100%     High 100%
KAND       High  95%     High  88%     High  98%     High 100%

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...JDL/PM
SHORT TERM...Wimberley
LONG TERM...DEO
AVIATION...JDL



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