Area Forecast Discussion
Issued by NWS Greer, SC

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000
FXUS62 KGSP 282342
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
742 PM EDT Tue Mar 28 2017

.SYNOPSIS...
A cold front will push through this evening and settle southeast of
the area for Wednesday. Drier weather will develop as high pressure
builds southward from eastern Canada along the east coast in the
wake of the front. Moisture will gradually return on Thursday,
with abundant moisture in place by Friday as a strong low pressure
system moves east from the Mississippi River Valley. Drier weather
will return for the weekend, but unsettled weather could return
early next week.

&&

.NEAR TERM /THROUGH WEDNESDAY/...
As of 225 PM EDT: A few showers popping up over the Upper Savannah
River valley at the moment. Latest CAM guidance shows if any
convection gets going it will be in the next couple of hours along
and south of I-85 then move east of the area by 22Z. Instability is
weak with best mid level lapse rates to our north near the cold core
of the upper low passing by to our north. Moderate wind shear is
over the area though, so if a storm can develop, it will bear
watching. That said, chance is low and diminishing. Isolated showers
will also be possible over the NC mountains this afternoon the the
moist NW flow.

This evening, drier high pressure builds into the area at the
surface as upper ridging moves in from the east. Northerly winds
develop and a slightly cooler air mass moves in. Still, lows will be
around 10 degrees above normal. Low level moisture begins to move
back into the area in the Wednesday in the northeasterly flow. There
may be an increase in cloudiness, especially over the NC Blue Ridge
and Foothills. The air mass is only slightly cooler, so expect highs
to be around 10 degrees above normal.

&&

.SHORT TERM /WEDNESDAY NIGHT THROUGH FRIDAY/...
As of 230 PM EDT Tuesday: A progressive long wave trough will cross
the mid and lower MS Valley, while a downstream ridge slides off the
East Coast thru the Short Term. As the upper trough approaches the
forecast area, southerly upglide flow will increase atop a wedge
that will set up east of the mountains by early Thursday. It still
looks like the models may be overdone on PoPs and QPF, as forecast
soundings show fairly shallow RH and low-level lift (while
persistent) is not supported much by mid or upper forcing thru at
least 00z Friday. In the very least, spotty showers and persistent
low clouds should keep temps down in the 50s to mid 60s across most
of the usual CAD areas, with upper 60s to mid 70s around the edges
in the southwest NC mountains and Upper Savannah Valley.

Thursday night thru Friday, a surface low will track from Ozark
Plateau to the southern Great Lakes. Increasing moisture and
deep-layer forcing should spread better shower coverage overnight,
especially along the south-facing slopes of the blue ridge
escarpment. Some areas may receive 1-2" of QPF, as mid-level flow
remains parallel to the mountains, helping train convection. Lows
will be well above normal. During the day on Friday, the wedge front
should begin to lift north and uncover a fairly unstable warm sector
across at least the I-85 corridor, if not further north. Models are
in good agreement on a slug of very strong deep-layer Q-vector
convergence around 18z Friday, in line with timing of a cold front.
Shear and helicity looks to support supercell or otherwise organized
convection within the warm sector, and confidence is increasing in a
severe weather threat. The SPC Day 4 convective outlook does have a
slight risk for the much of the Carolinas and GA. So I plan to add a
mention of severe tstms to the HWO. Temps should be able to get into
the upper 60s to mid 70s where the wedge erodes.

&&

.LONG TERM /FRIDAY NIGHT THROUGH TUESDAY/...
As of 200 PM EDT Tuesday:  The medium range fcst period kicks off
on Friday evening amidst passage of an upper shortwave, while an
upper ridge axis amplifies across the Plains.  At the surface,
a cold front will be approaching the region from the northwest
as broad high pressure digs out of the Upper Midwest into the
Ohio Valley.  Weak upper support associated with the passing
H5 wave will warrant at least mid chance pops initially, before
pops begin to taper from the west into Saturday morning, however
holding a bit longer along the TN line where weak nw upsloping is
possible.  By mid morning Saturday guidance favors rising heights
aloft as the surface ridge intrudes from the north with the cold
front clearing the I20 corridor, all leading to a drier fcst into
Saturday evening/night.  The surface anticyclone will shift sharply
to the southeast into/through Sunday while llv flow becomes more
veered, thus moist upglide atop the periphery of the wedge-like
pattern as well as weak upsloping potential will warrant low end
pops across the escarpment regions.  Meanwhile to the west, another
possibly closed H5 cyclone will be marching through the southern
Plains into the Midsouth region.  Pattern evolution from this
point forward is a bit unclear as operational guidance displays
some discontinuities regarding timing/placement of the Midsouth
cyclone, as well as the demise of the surface wedge pattern.
Given a more southern track of this system as displayed via the
ECMWF, the potential for strong/severe convection and heavy rainfall
is present via hslc fields as well as abundant upslope potential
along the Blue Ridge.  However at this point it is way to early
to lock in on such details, therefore for now the fcst will just
reflect increasing pops Monday night, holding at widespread chances
into/through Tuesday.  Temperatures through the medium range will
remain above normal levels.

&&

.AVIATION /00Z WEDNESDAY THROUGH SUNDAY/...
At KCLT and elsewhere (except KAVL/mtn valley): Weakening W/NW winds
will continue to bring gradual drying to the area tonight, and VFR
conditions are expected through the period. Wind are expected to
become NE at most terminals by late morning. However, developing lee
trough may play havoc with the directions a bit at the Upstate SC
terminals, where light south winds could become established during
the afternoon.

Elsewhere: Although drier air is slowly filtering into the area this
evening, moisture levels should remain sufficient to pose a threat
for valley fog and/or low stratus toward daybreak, especially if
winds become calm. While this is expected to be the case for the
terminals west of KAVL, it`s not clear that KAVL will completely
lose their up-valley flow before sunrise. We are advertising
categorical 5SM beginning at 09Z, and also included a tempo for
3SM/FEW003 to hint at the potential for something more dire.

Outlook: Moisture returning above a developing cool surface wedge
could result in low clouds and light rain across the area on
Thursday. The approach of a strong storm system will bring continued
chances of restrictions as well as a potential for heavy rain
showers and thunderstorms Thu night and Friday. Drier conditions
will return again for the weekend.

Confidence Table...

            23-05Z        05-11Z        11-17Z        17-18Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       High 100%     High 100%     Med   66%     High 100%
KHKY       High 100%     High 100%     High 100%     High 100%
KGMU       High 100%     High 100%     High 100%     High 100%
KAND       High 100%     High 100%     High 100%     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...Wimberley
NEAR TERM...JDL/RWH
SHORT TERM...ARK
LONG TERM...CDG
AVIATION...JDL



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