Area Forecast Discussion
Issued by NWS Greer, SC

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000
FXUS62 KGSP 252133
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
533 PM EDT Tue Jul 25 2017

.SYNOPSIS...
High pressure moving by to the north is expected to provide cooler
temperatures on Wednesday. Another front will move in from the
northwest late in the week, with cooler and drier conditions
arriving behind it for the weekend.

&&

.NEAR TERM /THROUGH WEDNESDAY/...
As of 5:30pm EDT Tuesday:  A few widely scattered showers have
popped-up over the mountains mostly, with some lightning.  Storms are
not expected to become intense due to capping, a lack of forcing,
and limited instability; and will decline around sunset.

Not much change in the synoptic setup over the last 12 hours as an
H5 trof ejects over the northeast CONUS, and a large anticyclone
dominates westward across the plains. At the surface, a
quasistationary frontal axis remains draped in the vicinity of the
I85 corridor.

Profiles are a bit more unstable on Wednesday with deep dry air
aloft, therefore think convective chances will be slightly higher if
convective temps can be achieved, therefore pops will reflect
widespread low/mid sct levels, tapering to iso levels along the TN
line. Temperatures on Wednesday will be a bit cooler than today
thanks to the aforementioned mid/late morning stratus potential, yet
should still top out within 1-2 degrees of normal across the
piedmont/fthills, and at normal levels across the higher terrain.

&&

.SHORT TERM /WEDNESDAY NIGHT THROUGH FRIDAY NIGHT/...
As of 200 PM EDT Tuesday: Upper-level heights will begin falling
from the Great Lakes to the Ohio River Valley Wednesday night
through Thursday. At low levels, Light southerly upslope flow could
permit isolated to scattered showers, with a thunderstorm or two, to
persist into the nightttime hours early Thursday across the southern
mountains. Otherwise, anticipate slightly better diurnal convective
coverage across the NC mountains Thursday afternoon and evening
ahead of the approaching upper support as surface-based instability
increases a bit over previous days. Improving westerly steering flow
will also likely bring scattered convection farther east of the Blue
Ridge along the I-40 corrdor into the western NC foothills/piedmont
into Thursday evening. Surface to 6 km bulk shear magnitude may rise
to 25 to 30 kt across the northern tier by Thursday afternoon as
these westerlies improve. Thus, anticipate a gradual change in the
character of the convection to more multi-cell types by late
Thursday.

The upstream shortwave energy will cross the forecast area from the
northwest Thursday night through Friday ahead of much stronger
heights falls carving out a more prominent trough from the Great
Lakes to the Appalachians. The best moisture and forcing will occur
around the passing cold front, but the numerical models still
exhibit quite a bit of spread on the timing. The faster NAM/ECM
camps bring clouds and moisture into the region very early Friday
and thus limit instability, while the GFS timing is much later in
the day. Indeed, the slower GFS would keep precipitation going well
into Friday night across the southeast half of the area. This will
play havoc will instability around the fropa and also shear
profiles, but an uptick in storm organization is certainly possible
Friday afternoon and evening. For now, will blend the timing
slightly toward the faster solutions which is preferred given the
push from the rapidly falling upstream heights.

&&

.LONG TERM /SATURDAY THROUGH TUESDAY/...
As of 200 PM EDT Tuesday: A trend toward drier and cooler conditions
seen in the previous forecast was continued with this forecast
package. By Saturday morning, a surface cold front should have moved
through our area and settled to our south. The GFS/ECMWF remain in
agreement on maintaining an amplified pattern and sweeping this
front through towards the Gulf coast, with an upper longwave trough
deepening over the eastern CONUS over the weekend and into early
next week. There is also better agreement in the global models with
respect to drier air arriving behind the front...and with basically
all convection remaining suppressed Saturday and Sunday over our
area in yet another guidance cycle, have lowered pops even more to
below climo over the weekend. The Bermuda high reasserts itself a
bit more over the southeast early next week, and gradual moistening
of the column will lead to pops returning closer to average by
Tuesday and Wednesday.

With the above-mentioned upper trough expected to deepen
dramatically over the eastern CONUS over the weekend and into early
next week, maintained the downward trend in temperatures through the
period. Large adjustments weren`t made with this package, but
temperatures more than 5 degrees below climo and dew points in the
mid to low 60s could make for some...dare I say it...rather pleasant
conditions for late July.

&&

.AVIATION /22Z TUESDAY THROUGH SUNDAY/...
At KCLT and Elsewhere:  VFR through the vast majority of the
taf cycle before guidance favors mid lvl MVFR stratus by morning.
A stalled frontal axis laid out in the vicinity of the I85 corridor
looks to be lifting northward at this time as a few sites across
the Upstate have shown glimpses of sly flow, albeit mostly vrb.
Therefore, will go ahead and carry lgt sly winds at the SC sites,
while favoring vrb at KCLT through 21z-22z, then sly.  Otherwise,
low vfr cu will prevail beneath high cirrus cigs, with pops being
too low to mention and convection at any site this afternoon,
which looks reasonable given the latest visb cu field trends.
Beyond that, guidance continues to favor low stratus development
overnight across the NC Piedmont, eventually working its way south
and west to affect KCLT/KHKY with an MVFR cig, possibly as far south
as KGSP/KGMU, however only included a sct MVFR layer at those sites
as confidence is rather low.  Lastly, did include a tsra prob30 at
KCLT for the last two hours of the taf cycle as instability looks to
be more impressive on Wednesday, building first along/south of I85.

Outlook: Scattered diurnal convection can be expected each day
thru Friday across the region. Then drier air will filter in from
the north behind a passing cold front this weekend. Chances for
morning fog and stratus will also be possible each day in the
mountain valleys.

Confidence Table...

            21-03z        03-09Z        09-15Z        15-18Z
KCLT       High 100%     High 100%     High  88%     Med   69%
KGSP       High 100%     High 100%     High  88%     Med   69%
KAVL       High 100%     High  94%     High  83%     High  93%
KHKY       High 100%     High 100%     High  88%     High  80%
KGMU       High 100%     High 100%     High  87%     Med   69%
KAND       High 100%     Low   55%     High  85%     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...CDG/WJM
SHORT TERM...HG
LONG TERM...Carroll
AVIATION...CDG


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