Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 251813

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

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.


As of 200 PM EDT Tuesday:  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.  Currently, mid/high cirrus
dominates the skies, yet with improving cu as instability builds
with heating.  The latest SPC Mesoanalysis indicates a gradient of
nearly 1500-2000j/kg sbcape across the Upstate, decreasing northward
where dewpoints remains slightly lower in general.  Cam guidance
continues to favor a few rogue showers outside of the mtns this
afternoon during peak heating, however concentrating the highest
probs over the sw NC and ne GA high terrain.  Therefore pops will
reflect such with low end iso pops highlighted along I85, while
sct pops are mentioned over the aforementioned regions of the
high terrain.  As for convective hazards, only expecting general
thunderstorms with gusty winds and some lightning.

Any residual shra/tsra will erode with heating loss tonight,
all ahead of increasing llv cloudiness per guidance, specifically
across the NC/SC piedmont by daybreak as moisture advects in from
the east.  Said sky cover will likely remain through much of the
post dawn hours of Wednesday morning as the BL remains rather
moist, yet sct`ing somewhat towards midday.  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.


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

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.


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.


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...

            18-24Z        00-06Z        06-12Z        12-18Z
KCLT       High 100%     High 100%     High  94%     Med   71%
KGSP       High 100%     High 100%     High  93%     Med   70%
KAVL       High 100%     High  97%     High  83%     Med   74%
KHKY       High 100%     High 100%     High  93%     Med   74%
KGMU       High 100%     High 100%     High  93%     Med   68%
KAND       High 100%     Med   78%     High  92%     High  82%

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:




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