Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 221756
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
156 PM EDT Tue Aug 22 2017

.SYNOPSIS...
Summer like conditions will prevail into Wednesday when a cold front
sweeps through the region from the west allowing for increasing
precipitation chances.  High pressure will build into the region
late in the week yielding drier and cooler conditions through the
weekend.  Ongoing tropical activity will make for a variable
forecast leading into next week.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 1045 AM EDT: Similar conditions across the area to yesterday,
with only minor tweaks needed to the temp and pop trends.

Otherwise, the upper ridge will weaken a bit over the area as a
broad upper trof moves over the Great Lakes. This should help
promote a broad area of upper lvl divergence to our north and favor
convection across the higher terrain based on a diurnal trend.
SBCAPE is expected to be around 1500-2500 J/kg with weak shear.
Storms will struggle to propagate into the foothills and piedmont
later in the day/evening, due to weak steering flow and waning CAPE.
High temperatures will be a few degrees above normal.

Convection associated with an approaching cold front will push
across the Ohio Valley this evening, but is expected to weaken as it
reaches NC mountains before daybreak. CHC PoPs linger along the TN
line to take this into account. Lows tonight above normal.

&&

.SHORT TERM /WEDNESDAY THROUGH FRIDAY/...
As of 3:15 AM Tuesday: Frontal boundary and perhaps associated
convection should be moving into the forecast area at the beginning
of the period, before the front "jumps" into the lee trough position
by afternoon. Convective trends Wed afternoon will depend largely on
how much convection and attendant cloud cover is impacting the
forecast area Wed morning. Since that is the subject of much
uncertainty, model guidance (both the traditional operational
guidance and convection-allowing models) are at odds as to how
trends will evolve during the afternoon, with some guidance
suggesting that limited heating/destabilization will preclude much
in the way of initiation outside the mtns Wed afternoon.
Nevertheless, the Piedmont should see sufficient heating to realize
moderate instability during the afternoon, with the front/lee trough
acting as a focus for at least scattered convection, and most of the
forecast area receives 50-60 pops on Wed. Increasing height gradient
aloft will result in improving shear profiles (albeit still less
than 30 kts in the 0-6km layer). The combination of modest shear and
modest instability will yield a modest severe weather threat,
especially across the Piedmont, and the SPC Day2 marginal risk
across our far eastern areas appears well-placed.

Drier/more stable post-frontal air will gradually chase the
convection out of the area Wed night/early Thursday. After some
disagreement among yesterday`s model guidance regarding the
southward penetration of the front, and/or how quickly a moist
easterly flow would develop north of the boundary, there is now a
strong consensus that the period from Thu morning through Fri
morning will be dry, with temperatures averaging a few degrees below
climo.

&&

.LONG TERM /FRIDAY NIGHT THROUGH TUESDAY/...
As of 145 PM EDT Tuesday: The medium range fcst period kicks off on
Friday evening amidst a broad/deep yet departing longwave upper trof
across the eastern CONUS, while a sharp ridge builds westward atop
the Plains.  At the surface, high pressure centered beneath max
confluence on the descending branch of the aforementioned upper trof
will extend across much of the OH/TN Valleys, eastward atop the
Appalachians and into the mid Atlantic seaboard.  With that,
cooler/drier air looks to slide into the region by way of a damming
wedge on Saturday morning with below climo pops favored, the highest
of which residing over the terrain where subsidence is weakest.  At
that point much uncertainty arises in the fcst given two distinct
systems of tropical origin.  As of now, the first of these systems
is the remnants of TC Harley which is expected to rapidly intensify
over the western Gulf, possibly regaining TS/HC status by Saturday
with various model sources favoring landfall along the TX coast.
The second system is currently a weak area of low pressure off the
southeastern FL shores, which is also expected to slowly gain
strength with time, possibly moving northward and approaching
northeast FL by Saturday.

Sunday into midweek, the latest GFS seems to have come into form
with the 00z ECMWF as 06z GFS runs favored additional tropical
moisture advection into the region leading to broad scale isentropic
lift and thus showers, which combined with improving easterly llv
could lead to upsloping as well.  However, these two solutions now
tend to favor intensification of the Atlantic system as it slides
northward along the GA and Carolinas coastlines Monday through
Tuesday.  Meanwhile, the TC Harvey looks to have slowed along the
TX/LA coastline all the while potentially producing significant
rainfall, however with little/no impact across CWFA through the
period.  All said, given the expected track of the Atlantic system,
would expect the high pressure wedge to be reinforced during this
time frame keeping PWATs rather low, generally under 1.5 inches.
Thus, will carry token slight/chance pops through the remainder of
the period, increasing to widespread chances Tuesday evening into
Wednesday.  Temperatures through the period will remain generally a
few degrees below normal across the high terrain, possibly as much
as 6-10 degrees below normal over the low terrain where wedging and
thus abundant sky cover could prevail.

&&

.AVIATION /18Z TUESDAY THROUGH SUNDAY/...
At KCLT and Elsewhere: Scattered showers are developing across the
southern Upstate and just south of KCLT. There are a few isolated
showers in the NC mountains and NW piedmont near KHKY. A few of
these cells may get deep enough for TS, but otherwise, mainly VCSH
for KAVL/KHKY/KCLT this afternoon. The piedmont activity is east of
the Upstate TAF sites and is drifting east. So I opted to keep no
mention at those sites. Later tonight, a line of showers and tstms
may reach the the NC/TN border before daybreak Wed, but is expected
to weaken as it approaches. Confidence is too low to have an early
morning TS/SH mention at KAVL. KAVL could also see another round of
fog and/or low stratus late tonight, so have some restrictions in
the 9-13z. Winds will favor a SW direction this AFTN, becoming light
this evening. As the front approaches, they will veer slightly to
WSW, but a shift to NW is expected to be after 18z.

Outlook: The highest chances for convection will be on Wednesday as
a cold front pushes into the area. The front should push thru and
allow drier air to work in from the north. So the probability of
restrictions and diurnal convection should diminish Thursday and
Friday.

Confidence Table...

            18-24Z        00-06Z        06-12Z        12-18Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       High 100%     High 100%     Low   58%     High  91%
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...CDG
NEAR TERM...ARK/JPT
SHORT TERM...JDL
LONG TERM...CDG
AVIATION...ARK


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