Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 170845
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
445 AM EDT Thu Aug 17 2017

.SYNOPSIS...
High pressure will gradually erode over the next few days, though
warmer than normal conditions will remain. Moist and unsettled
weather will return late week and into the weekend. Some drying
is favored by Sunday with a return to moist conditions to start
the new work week.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 445 AM EDT: Isolated showers along and east of the I-77
corridor early this morning will dissipate or move east before
sunrise. The potential for fog development continues to look a bit
better than observed Wednesday morning, with surface condensation
pressure deficits likely 5 mb or less by daybreak over the
mountains, foothills, and NW NC piedmont. Will advertise patchy to
areas of fog at this point, but any patchy dense fog should occur
mainly in the mountain river valleys through 13Z. Some slightly
deeper moisture may start arriving in the western NC mountains
around daybreak, but with any PoP for showers confined to isolated
coverage.

The convection-allowing models today have mountain showers and
thunderstorms developing from 18Z to 20Z this afternoon in a region
of better low level wind field convergence over the high terrain.
This activity should move east off the Blue Ridge around 20Z to 21Z.
A secondary axis of low level convergence should impact piedmont
sections east of the forecast area, so piedmont coverage will depend
on the mountain activity surviving eastward. 850 to 500 mb lapse
rates appear a slight tick higher this afternoon than yesterday,
with sbCAPE values at 2000+ J/kg, so an isolated severe storm cannot
be ruled out. Precipitable water values remain above climo, but
storms should move along steadily from west to east to keep any
excessive rain/flooding problems isolated in nature. Heat will be an
issue this afternoon with many piedmont temperatures in the low to
mid 90s, with dewpoints in the lower 70s. Will likely post an SPS
today for piedmont Heat Index values in excess of 100 degrees at
times.

The shortwave ridge over the region late today will move east
tonight, with heights falling once again from the west overnight.
Precipitation chances will increase along the spine of the southern
Appalachians during the early morning hours as deeper moisture
begins to push back in. Expect continued mild and muggy min temps
early Friday, with mountain valley fog and low stratus redeveloping.

&&

.SHORT TERM /FRIDAY THROUGH SATURDAY NIGHT/...
As of 245 AM EDT Thursday:  The short term fcst period kicks off on
Friday morning amidst a departing shortwave ridge in lee of the
southern Appalachians while a region of height falls associated with
a longwave trof approaches from the west.  At the surface, a cold
front tied to a surface low will extended southward beneath said
upper trof into the OH/TN Valleys as a prefrontal lee trof extends
across northeast GA and the western Carolinas.  Pattern evolution
through the short term will feature the intrusion/passage of the
cold front on Friday afternoon/evening with modest cape in excess of
1.5/2.0k j/kg with shear generally maximized along/ahead of the
front over western NC in the 25kts range at 0-6km.  With that, the
fcst will feature low/mid chance pops along/south of I85 where shear
will be weakest, while favoring mid/high chances with a few areas of
likely pops across the NC high terrain eastward.  Given the
abundance of dry air evident in the profiles, wouldn`t be shocked if
a few storms became severe with damaging winds and large hail being
the primary threats.

Moving into Saturday, the cold front will sweep through remainder of
the fcst area overnight into the mid morning hours as cooler/drier
high pressure spills in from the northwest.  Fortunately given the
drier nature of the airmass present over northeast GA and the
western Carolinas behind the front, conditions look rather stable
with a dry fcst to prevail across the vast majority of the region
with the only mentionable pops for Saturday afternoon being
along/south of I85 in closer proximity to any residual frontal
convection.  Temperatures on Friday will top out a few degrees above
normal with heat indices reaching near the century mark across the
far southern zones.  By Saturday temperatures will top out near
normal with dewpoints being a few degrees cooler, thus keeping any
excessive heat concerns at bay.

&&

.LONG TERM /SUNDAY THROUGH WEDNESDAY/...
As of 300 AM EDT Thursday:  The medium range fcst period initializes
on Sunday morning amidst the passage of a dry upper trof, with
shortwave ridging building back across the southeast states thanks
to a strengthening H5 anticyclone along the Gulf coast.  At the
surface dry high pressure will be in control once again across
northeast GA and the western Carolinas effectively keeping the fcst
dry to round out the weekend amidst slightly above normal
temperatures.

As for Monday (Eclipse day), conditions do look to be improving for
our region as guidance still favors the resurgence of a stationary
front, however at a slower rate than previous model runs on both the
GFS and ECMWF.  With that, the front slowly surges northward out of
the Low Country and Midlands, yet now not reaching the I85 corridor
until mid/late afternoon, most likely after totality.  In addition,
low level RH fields have improved as well with values ranging in the
45-55% percent range as opposed to nearly 10% higher in days past.
All in all, think viewing conditions are looking better.  That said,
by the late afternoon hours the front is expected to slide into the
region increasing chances for showers and thunderstorms along/south
of I85.  Thus pops on Monday will be mostly dry, with the exception
of slight chances over the higher peaks given ridgetop convection,
as well adjacent to the approaching front south of I85 with some
chance pops highlighted as well.  Tuesday looks to return to more of
a diurnal convective trend as moisture streams into the region from
the south as the front lifts well northward, while another cold
front approaches the region from the west on Wednesday.  Pops both
days will be at or slightly above climo levels regionwide.
Temperatures through midweek look hold at near normal levels.

&&

.AVIATION /09Z THURSDAY THROUGH MONDAY/...
At KCLT: A few showers may linger in the vicinity of the I-77
corridor early this morning, but chances of precipitation at the
airfield continue to look rather small. Cannot rule out some low
cloud and fog restrictions toward daybreak, but a brief window of
MVFR should be the best forecast. Anticipate slowly building cumulus
today, with late day chances of convection from the west as
thunderstorms build east off the Blue Ridge. Light winds will become
steady SW through the day.

Elsewhere: KAVL will see the best chance of low stratus and fog
problems this morning with very low condensation pressure deficits
over the mountains. The foothills may see patchy fog and low clouds
as well, with KHKY having the next best chance of daybreak
restrictions. Otherwise, cumulus will build today with mountain
showers and thunderstorms developing 18Z to 20Z, with TEMPO TSRA
most likely from KAVL to KHKY, and PROB30 after 19Z for the Upstate
SC TAFs. Expect light winds to toggle southwest through the day, but
remain less than 10 kt. Any late day thunderstorms should move east
this evening with just lingering debris clouds around.

Outlook: The transition to a more typical late summer pattern
continues Friday 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...

            08-14Z        14-20Z        20-02Z        02-06Z
KCLT       High  93%     High 100%     High 100%     High 100%
KGSP       Med   75%     High 100%     High 100%     High 100%
KAVL       Low   36%     High 100%     High 100%     High 100%
KHKY       Low   53%     High 100%     High 100%     High 100%
KGMU       High  93%     High 100%     High 100%     High 100%
KAND       Med   75%     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...HG
SHORT TERM...CDG
LONG TERM...CDG
AVIATION...HG


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