Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 172355
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
755 PM EDT Thu Aug 17 2017

.SYNOPSIS...
Moist and unsettled weather will return late week and into the early
weekend, with hot and muggy conditions remaining over the area. Some
drying is favored by Sunday with a return to moist conditions by the
middle of next week.

&&

.NEAR TERM /THROUGH FRIDAY/...
As of 730 PM EDT: Convection continues to favor the NC Foothills and
NW Piedmont and outflow boundaries should keep the best chances over
the NC part of the fcst area E of the Blue Ridge Escarpment this
eve. Elsewhere...we have retained CAPE values well into the moderate
range in some locations, so even the quiet areas such as Upstate SC
will keep a small chance going into the mid evening. DCAPE values
would support strong downbursts east of the mtns, so a few strong to
severe storms are possible into mid evening. PW values are very
high, but cells are moving. Excessive rain potential remains low,
but could increase if training cells develop.

Otherwise, although convection dissipates later this evening,
showers may return to the mountains ahead of an approaching front.
Debris clouds and clouds associated with the approaching front may
help limit fog overnight, but some fog should develop again in the
mountain valleys and I-40 corridor. Lows will be around 5 degrees
above normal.

Heights fall from the west Friday as an upper trough approaches.
This will push a cold front into the area. The synoptic scale
guidance is not overly impressed with precip chances, but the CAM
guidance is. They do agree that the best chances for convection will
be across NC with lower chances across the Upstate and NE GA. Expect
a line of convection to develop across the mountains and spread SE
across the area during the afternoon. Instability and DCAPE will
again be moderate. Shear, while not strong, will be higher than
previous days. Therefore, isolated strong to severe storms will
again be possible. Have followed these trends for the PoP forecast.
Highs should be a couple of degrees cooler, but dew points will
remain high. This should keep heat index values below 105, but they
may go above 100 along and south of the I-85 corridor.

&&

.SHORT TERM /FRIDAY NIGHT THROUGH SUNDAY/...
As of midday Thursday: Cold front should exit the CWFA to the
east Friday night as its parent low works its way northward
thru Quebec. At 00z the wind-shift line should be across the
upper Piedmont, and both NAM and GFS show as much as 2000 J/kg of
MUCAPE persisting ahead of it as low-level convergence peaks during
the evening. Both models develop some convection in the I-77 and
eastern I-85 corridors after 00z as a result. Deep layer shear
is expected to be less than 20 kt, suggesting little organization
along the front, though dry vertical profiles imply some damaging
wind risk may linger. Furthermore the NAMNest and HiRes Windows
are a bit earlier with the fropa and would lend confidence to an
evening forecast free of convection. At any rate, chance PoPs at
the start of the period will taper off as the front departs.

A weak surface high will fill in behind the front and remain
overhead through Sunday. A shortwave swinging across the Ohio Valley
Saturday will briefly maintain the mean eastern trough, but heights
rise again that night, leaving nearly zonal flow overhead. While the
US operational models keep us dry Saturday, a few SREF members as
well as last night`s run of the EC permit some convection over the
Blue Ridge as well as the lower Piedmont. While subsidence should
be enough to preclude the former, it seems plausible to expect the
front will stall close enough to our southeast border to include
a small diurnal PoP there. The case is more or less the same on
Sunday, though with low-level flow having veered to the south again,
I will allow PoPs to return to the northern mountains/foothills.

Max temps only look to drop a degree or two following the front,
though dewpoints will return to about normal, keeping heat
indices below 100. For Sunday the temps will rise a bit further
under rebounding thicknesses but excessive heat concerns are not
anticipated then either.

&&

.LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/...
As of 200 PM EDT Thursday: Subtle changes in the medium range
with this forecast package that continue a slight drying trend. The
extended period picks up Sunday night with a subtropical ridge
building into the southeast, both at the surface and aloft. Major
models are now coming into better agreement that this ridge will be
able to spread wel into the southeastern CONUS, and the intensity of
the ridge (as well as possible subsidence and convective
suppression) has been trending slightly stronger over the past few
model cycles. Subtle H5 shortwave ridging will also be in place over
the Carolinas Monday afternoon (there is a surprising amount of
model agreement on this), which would also bode well for some
convective suppression. Southerly surface flow with a generally Gulf
fetch will encourage a slight increase in moisture Monday, but model
RHs have come down slightly since the last forecast package, again
encouraging for the possibility of slightly less cloud cover. The
last (and probably most significant) factor will likely be the
surface stationary front stalled over the lowcountry of SC, and
whether or not it is able to pivot northwards enough to provide an
axis for moisture convergence in our area, and therefore increased
rain chances and cloud cover. The trend is toward a more optimistic
solution for Monday with the front getting hung up in the Midlands,
but it should again be noted that there is still a good amount of
uncertainty with the forecast for Monday at this point.

An upper shortwave and surface front will begin to approach the
forecast area Tuesday afternoon, increasing chances of shower and
thunderstorm activity with a diurnal peak. Longwave troughing
deepens over the eastern CONUS as a surface front continues its
advance through the forecast area. Profiles will be quite moist
Wednesday and Thursday, so while there doesn`t appear to be much of
a severe threat, heavy rainfall may be an issue that bears watching.
Temperatures will be just a few degrees above average through the
period.

&&

.AVIATION /00Z FRIDAY THROUGH TUESDAY/...
At KCLT: VFR expected to prevail for the evening hours, outside
of deeper convection. As long as storms remain to the west on
the radar, there is the possibility of a thunderstorm reaching
the airfield, which was shown by some of the convection-allowing
models. This will be kept in the TAF with a TEMPO group for the
01Z to 03Z time frame. In the mean time, think wind direction will
favor SW ahead of an approaching outflow boundary. The boundary
should move in during the 01Z hour and could flip the wind direction
around to NW for a few hours or until after the thunderstorm threat
has ended. Winds become light S overnight with only a low chance
of any fog. Scattered cumulus develops by late morning on Friday
with increasing SW wind. Convection should initiate around 18Z,
so a PROB30 group was employed.

Elsewhere: We begin with a thunderstorm near KAVL but moving away,
and scattered convection moving past KHKY. These conditions should
come to an end by 02Z or so, after which only convective debris is
expected. The rain that fell at KAVL should spell another round of
patchy dense fog, which was handled with a TEMPO for IFR visibility
around daybreak. This could easily end up VLIFR again. Meanwhile,
the upstate SC TAF sites should be relatively quiet. Wind will be
variable and light. On Friday, expect scattered thunderstorms to
develop once again.

Outlook: The transition to a more typical late summer pattern
continues through early next week. Expect scattered afternoon
convection and morning low stratus and patchy fog each day,
especially in the mountain valleys and in locations seeing
appreciable rainfall.

Confidence Table...

            00-06Z        06-12Z        12-18Z        18-00Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       High  97%     High  96%     High  88%     High 100%
KHKY       High 100%     High  94%     High  91%     High 100%
KGMU       High 100%     High 100%     High 100%     High 100%
KAND       High 100%     High  97%     High  94%     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...Carroll
NEAR TERM...PM/RWH
SHORT TERM...Wimberley
LONG TERM...Carroll
AVIATION...PM


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